Tariff Calculations: Octopus GO versus Octopus FLUX

Friends, I have struggled to write this article. As you may have noticed, it has taken weeks.

I started writing after I was asked on Twitter about a new electricity tariff called ‘FLUX’ offered by Octopus Energy. Would it be cheaper or more expensive than their ‘GO’ tariff?

It’s a simple question and one that is worth asking. But it is very hard to answer because it involves both hourly details, but also seasonal changes.

I could see how to get at an answer but I have struggled with my waning technical skills. Imagine if you will, an old boxer going in for one fight too many. Finding themselves on the ropes, they face the unavoidable and inevitable reality of their own decline. But bravely they struggle and finish the fight bruised and defeated, but with their dignity in tact. Similarly I have found my prowess with Excel and Visual Basic to be much diminished, but I have somehow battled through.


The Octopus GO tariff which I currently use offers 4 hours of electricity for 7.5p/kWh between 00:30 and 04:30 each day. The rest of the time the cost is 40.75p/kWh. Exports of electricity are paid for at 4.1 p/kWh.

During winter, I buy electricity cheaply at night, and then use it during the day. For most of December and January, the battery could not supply the house for the whole day and I had to purchase electricity at full price for a few hours on those days. Overall, the average price I paid was around 12p/kWh in those two months.

Click on Image for a larger version. Illustration of the variation in price through the day for electricity imports (left) and exports (right) on the Octopus Go and Octopus FLUX tariffs.

The Octopus FLUX tariff is more complicated. It has a cheap rate in the night, but only for 3 hours 02:00 to 05:00 and not so very cheap (20.4 p/kWh). But it also has a more expensive rate (47.5 p/kWh) during peak demand hours from 16:00 to 19:00 each day. The rest of the time the cost is 34 p/kWh.

Initially FLUX looks much worse than GO, but the twist is that FLUX offers much higher rates for exporting electricity: 9.4 p/kWh, 22 p/kWh, 36.5 p/kWh for the cheap medium and high rates respectively. These figures should be compared with the miserly 4.1 p/kWh on the GO tariff.

There are so many variable quantities that I really had no idea which tariff would be cheaper. The results of my calculations appear obvious in retrospect, but that didn’t make the calculations any easier! My conclusions are that:

  • For small solar PV installations (<~4,000 kWh/year), the big savings from using the night time electricity on GO outweigh the gains from exporting electricity at a good price.
  • For large solar PV installations (>~6,000 kWh/year), this situation is reversed: The savings from using the night time electricity on GO are outweighed by the gains from exporting electricity on the FLUX tariff.
  • For medium-sized solar PV installations, the two tariffs have similar costs.

Click on Image for a larger version. Estimates of the annual cost of electricity on the Octopus Go and Octopus FLUX tariffs as a function of the amount of solar generation. This applies to my household – see text for details – and assumes a 13.5 kWh storage battery. 

It turns out that, if you have the capability to export lots of solar PV, then the FLUX tariff could result in very low – and even negative – electricity bills. In retrospect, this is sort of obvious, but it was not obvious at all to me when I began.

But the spreadsheet I developed for the calculation allowed me to do the calculations for different sizes of battery and different amounts of solar PV generation, so I’ve investigated the matter a little more deeply below.

Sadly, because the spreadsheet is Macro-enabled, for security reasons I can’t link to it from this blog and many users wouldn’t be able to download it anyway. But if you really want a copy, please ask for a copy in the comments and I will send it to you somehow. But be warned that the spreadsheet is complicated and slooooow. On my computer it takes around 1 minute to evaluate the yearly calculation.

[Update: I think you can download the spreadsheet from this Dropbox Link]

Let me explain how I made the calculation and then I’ll discuss a few more details.

How to work out which tariff is cheaper

To answer this question I wrote a spreadsheet which modelled the electricity use in a household hour-by-hour for an entire year i.e. the spreadsheet has 365 x 24 = 8,760 rows.

For each hour of the year I estimated:

  • The household demand: I modelled this as being the sum of a fixed amount each day (10 kWh/day) plus an amount used for heating that peaked in winter at 25 kWh/day.
  • Solar PV: Using the EU sunshine database, I downloaded hour-by-hour sunshine data for my house location from 2005 to 2016, and then averaged this to give a typical solar generation year.
  • I then worked out how to supply the household demand.
    • If Solar Power exceeded demand, then the excess was used to charge the battery, and if the battery was full, the excess was exported.
    • If Solar Power was less than demand, then the solar power offset the imported electricity.
    • During winter, the battery was fully charged during the cheap hours.
    • I estimated the battery to have a round-trip storage efficiency of 90%.

The spreadsheet and associated VB Macros took days to debug, but here are the results.

Household Demand

The modelled daily demand is shown below along with the EU sunshine database estimate of PV generation amounting to ~ 4,000 kWh/year. Basic electricity demand is ~ 10 kWh/day but peaks at 25 kWh/day in mid-winter due the heat pump, and amounts to ~ 5,000 kWh/year.

Click on Image for a larger version. Graph showing the modelled daily household demand throughout the year, and the 3-day average of solar generation. The solar data is the average of the years 2005 to 2012 estimated for my location and array size in Teddington.

The relationship between Solar PV supply and household demand is such that one needs to use two different strategies depending on the time of the year.

  • In the Winter: the battery is charged using cheap rate electricity and discharges during the day – sometimes running out at night.
  • In the Summer: there is no night time charging and the battery charges during the day and discharges during the night.

These two modes are illustrated in the graphs below.

The first graph shows a week in winter under the two different tariffs. The four hours of cheap electricity under the GO tariff allows the battery to charge to full, but the FLUX tariff only has three hours of cheap electricity so the battery only charges to around 10 kWh. The battery then discharges to run the household, and is partially supported by the weak solar generation, but typically runs out well before the end of the day.

Click on Image for a larger version. The state of charge of the battery through 7 days in winter. The upper graph shows the Octopus GO tariff which allows the battery to be fully re-charged each night. The lower graph shows the Octopus FLUX tariff which only has enough cheap hours to enable partial filling of the battery. The solar generation is also shown in yellow.

The second graph shows a week in summer. At this time of year, solar generation is enough to run the household and charge the battery during the day, with enough left over for export.

Click on Image for a larger version. The state of charge of the battery through 7 days in summer. Also shown is the solar generation is also in yellow and electricity exports in grey.

The switch between the summer and winter strategies is made on day 90 and day 270 – an arbitrary choice but one which corresponds roughly to the point where the 3-day average of solar exceeds the average household demand. The graph below shows the state of charge of the battery throughout the entire year on both tariffs.

Click on Image for a larger version. The state of charge of the battery through the entire year. The top graph shows the estimate for the GO tariff and the lower graph shows the estimate for the FLUX tariff.


The simulation runs hour-by-hour through the entire simulated year. For each hour, I estimated how much electricity was imported and exported, and then applied the appropriate tariff rate. This allowed me to summarise the situation for my home as below.

Click on Image for a larger version. Results of calculations of cost of running my household on (top) the GO tariff and (bottom) the FLUX tariff.

Both tariffs offer the possibility of running a home very cheaply: with annualised energy bills in the range £30/month to £50/month. However the GO tariff appears to be cheaper in this simulation £34/month compared with £50/month for FLUX.

The analysis shows why: being able to fill up with electricity at 7.5 p/kWh reduces the cost the electricity dramatically – £298/year versus £648/year. The improved rates for export on the FLUX tariff (£209/year versus £38/year) aren’t enough to make up for that.

Discussion#1: The effect of extra solar generation 

My conclusion is that for me, with my existing 3,800 kWh/year PV installation, the GO tariff is more economical.

But having recently had extra panels installed, this conclusion may not hold. The difference in annual cost between the two tariffs is ~£193 and the typical difference between the FLUX and GO export tariffs is ~ £0.18. So if the new system could export ~1,000 kWh more in summer, then the balance could easily shift.

And indeed, that is what the simulations show. Notice that for 8,000 kWh of generation the annual cost of electricity would be negative i.e. the house would be a bona fide power station!

Click on Image for a larger version. Lower graph: estimates of the annual cost of electricity on the Octopus GO and Octopus FLUX tariffs as a function of the amount of solar generation. Upper graphs: Details of how the the import costs and export and rewards vary on the FLUX tariff (left) and GO tariff (right). [NOTE: The original graph had an erroneous curve plotted. This was updated at 23:27 on 23/2/2023]

If the newly installed panels generate as much as I hope, then the annual generation may approach 6,000 kWh and in this case, the FLUX tariff would be marginally cheaper.

Discussion#2: The effect of battery size 

Whilst I was making these calculations, I thought it would also be interesting to look at the effect of battery size. For my home – with solar PV generation of ~3,800 kWh/year – the simulations suggest that bigger batteries are better – no news there – but that above roughly 10 kWh the additional savings are minimal.

Click on Image for a larger version. Lower graph: estimates of the annual cost of electricity on the Octopus GO and Octopus FLUX tariffs as a function of battery size with ~ 3,800 kWh of solar generation. Upper graph: Details of how the the import costs and export and rewards vary on the FLUX tariff (left) and GO tariff (right).

This is a relief to me. It means that as the batteries degrade, the system itself is likely to continue to perform well for many years.

Discussion#3: Strategy 

Friends, life is complicated enough without having to consider battery management strategy. Nonetheless, this is where we are!

Observant readers may have noticed that I made no specific efforts to avoid consuming energy at peak hours because it doesn’t happen very often. But if it could be done, then on the FLUX tariff, there would be a reduction in both costs and carbon emissions during these dirtiest hours of the day.

The problem for me is that I am not sure whether the occult Tesla logic which controls my battery, is smart enough to avoid using electricity at peak times. If it could achieve this, then on days when the battery might be expected to run out early, the system might preemptively charge in the middle of the day (at 34p/kWh) and so avoid consuming grid electricity during the peak hours when the equivalent electricity would cost 47.5p/kWh.

For a load of around 1 kW for 3 hours the potential saving would be around 45p/day which over 60 days of winter might amount to ~£27/year.

Errors and Mistakes

Friends, writing this article has been very difficult, and I must warn you although I have carried out many checks, I might easily have made some errors. Sorry. Please feel free to point them out to me when you spot them. The results appear to be about right for my own situation and so I have modest confidence that the errors are not too major.

But overall, despite the fact there are errors and mistakes, I think this spreadsheet offers a tool for evaluating the complex interaction of solar generation, battery storage, and time-of-use tariffs. I hope it helps.

61 Responses to “Tariff Calculations: Octopus GO versus Octopus FLUX”

  1. Simon Duane Says:

    Hi Michael

    I don’t have solar PV, but I’ve read that the octopus flux tariff is not available to users with more than 9kWp installed.
    Perhaps you have shown why this might be?

    Best wishes

    • protonsforbreakfast Says:

      Yes, I think that explains it. If you’ve got a lot of solar it’s a money spinner!

    • Bill F Says:


      I’ll be getting a 7kWp and battery system next month so the flux tariff looks appealing. I’d appreciate a copy of your spreadsheet to do the calcs.
      I’m expecting to generate close to 9000kWh / yr and previously have used 4500kWh/yr on the house and last year did a similar amount on the EV so some careful thinking needed on how best to set it all up !

      • protonsforbreakfast Says:

        Bill F

        Sounds like an amazing system: I hope the installation goes well and that the pigeons stay clear!

        All the best


      • Ioannis Says:

        Hi Bill,
        Looks like you are installing an impressive system with 9000kwh predicted annual output. Would you share the details of it? (Number of pv panels, type of inverter and also the orientation and inclination of the roof and any potential shading on the panels)

  2. Adrian R Says:


    Would it be possible to get a copy of your spreadsheet? Let me know what details you’d need from me to send it?



  3. Ross Mason Says:

    More panels?
    Clearly something moved in the de Podesta household.
    Intrigue me.

  4. Ross Mason Says:

    Thanks Michael. I can safely say Upper Hutt missed the deluge. Northland got hit once. Auckland got hit twice. Once when Gabrielle headed south, east of the North Island where parts of west and northern Auckland had raging floods, then when it stalled winds and rain changed to souwesterly and the West Coast, Karekare, Piha and Muriwai got hammered with slips. Two firefighters died when a house they were rescuing got slip hit.
    Once it reached East Cape it dumped horrendous quantities onto to roughest country in NZ that is “farmed”. After Cyclone Bola in 1988 lots of the East Coast hill country got planted in Pines. They have been harvested for the last few years. Foresters take the good part of the lower tree trunk and strip the rest and leave it lying on the ground. It takes little rain to move it down the hill, into stream, into rivers and the slash (as its called) will take out bridges and houses as if they were pea sticks. This is the third biggie through there in as many years.
    Check out “Stuff.co.nz” for some video and pictures. Stories coming out of survival draw hairs on the back of the neck. Families having to break through ceilings to get above the water. One woman died in the loft trying to escape. A horse got stuck on a roof – water so high it swam on – but collapsed the roof and died later. Truckie starting out from Gisborne in light rain and hit a sharp bend and water. Had to back up a few times as the “stream” rose metre by metre in a staggering short time but had to leave the truck. Walked 3 hours until he got picked up and arrived at a farm. Three Maori boys grabbed a Jet Boat and rescued about 70 people from roofs along the Esk Valley. They will get a medal for sure. There are still around 10 people still missing. Helo pilots claim they saw bodies being washed down rivers.
    To get comms back on a helicopter carried and laid broadband cable literally across bush and farmland as a temporary connection. Wonderful picture of a piece of Manuka branch with a Y at the top planted in the ground as a support to keep stock from damaging the line.
    On a personal side, my sister and hubby are in Waipawa. All around them was flooding, the area was evacuated including them but their house missed out of damage. Their piece had been raised that extra 500mm and was “saved”.
    etc etc.
    More solar panels up there wouldn’t have helped. No sun. No Power Lines to sell it through!!

    • protonsforbreakfast Says:


      That sounds terrible. I had read one or two stories about one valley that was devastated – but there hasn’t been much coverage over here. The Guardian story conveyed a sense of bewilderment amongst people who couldn’t quite believe what had happened. And of course the question of how wise it would be to build back.

      In my opinion it’s just a matter of time before this stuff begins to happen everywhere.

      Anyway: Sympathy and Best wishes: M

    • Dan Grey Says:

      Good grief that’s one hell of a read Ross!

      Fwiw “the rest” of the trees after logging is what’s collected and milled into pellets for the Drax, the massive biomass power station in the UK. Burns millions of tonnes of the stuff a year.

  5. alexbutcher Says:

    Hi Michael,

    I’ve started working on something similar to this, and considering making it publicly available online as a tool for interested renewables enthusiasts, if I can get the quality there… I’d love to see your version if you’d be happy to share it.



  6. Simon Loker Says:

    very detailed and well done.
    Not sure if you like spanners in the wood work.
    I thought about this as at home at the moment illness stops me working.
    Would it be possible to use the flux in summer and swap for Go in the winter as the cheap rate better utilised charging the battery?

    • protonsforbreakfast Says:

      Good Thinking.

      But typically they ask you to sign a contract for year – I think you can change but I am not sure about constantly switching back and forth.

      In any case, everything is very cheap in any case, so I am pretty happy.

      Best wishes


  7. Rob Cain Photography Says:

    As someone with a recently installed 3.6kWp solar system and potentially moving to an EV later in the year I’m interested in comparisons of octopus flux and octopus go. As I as new Go customer the rate would be 12p kWh. How would that make a different to the outcomes?

    I’m in the difficult place we’re it feels like recently installing solar would be a waste if I’m limited to 4.1p SEG by having an EV. As the EV will be a company vehicle I need to consider mileage reimbursement is 5p per mile. That could be a big cost penalty at higher charging costs as I do circa 15k business mileage a year and I may struggle to charge sufficiently within the 3 hour window per day to make it viable (daily mileage varies 60-150 miles per day). It currently feels like a PHEV and flux for Solar exporting would be the better option for me.

    It just seems like flux is a good step forward but there is no tariff for solar and EV owners, when anyone with solar is more than likely to have an EV too.

    • protonsforbreakfast Says:


      Good Morning. I sympathise: it feels impossible to work out which tariff is optimal – hence the post. And having an EV just adds to the complication. Anyway:

      I have re-run the calculations for Octopus Go at 12p/kWh. It’s similar to the previous calculation but a little worse. FLUX is cheaper after about 4,000 kWh/year of Solar generation rather than 5,000 kWh/year as previously.

      People have also asked for information on the COSY tariff with 15p/kWh export. I’ll work on that today and tomorrow.

      Hopefully later today or tomorrow I finish a post comparing Octopus GO, COSY and FLUX.

      There’s also chit-chat on Twitter (@Protons4B) around this topic.

      All the best


  8. Dan Grey Says:

    That’s a great piece of work Michael! Although the first few few paragraphs certainly sent me on a guilt trip 😄. I hope you found it a satisfying experience overall?

    My one big thought which was brewing as I read (and discussion #3 touches upon it) is optimised discharge to the grid. Would I be right in thinking that your Powerwall is set to Self-Powered mode? Is Time-Based Control available to you? https://www.tesla.com/support/energy/powerwall/mobile-app/utility-rate-plans Not sure it’s available outside the US.

    Here you would tell the Powerwall the Flux tiers and I believe it would, for example, aim to discharge during the 4-7pm 36.5p/kWh period, discharging down to a Backup Reserve level you select. If used in summer, TBC with Flux might well make extra money!

    • Dan Grey Says:

      One thing I’m not sure at all about is whether TBC is predictable, e.g. it may well limit discharge in the 22p/kWh daytime period in order to maximise discharge in the 36.5p/kWh period before hitting the Backup Reserve level.

      PS do switch between Winter and Summer modes by enabling/disabling Grid Charging in the settings?

      • protonsforbreakfast Says:

        TBC is not perfectly predictable. It has settings for peak rates, shoulder rates and cheap rates so I could program in the FLUX tariff, but the COSY tariff couldn’t be obviously programmed because it has two cheap periods.

        In winter it is predictable – it charges the battery full every night. But in Spring and Autumn it tries to ~half-fill the battery in the morning to allow it to charge up the next day. I think it currently does this by pattern matching previous days perhaps with some seasonal knowledge. But the latest software update says it now has the ability to use a solar forecast.

        So in the transitional period, it isn’t possible to know what charge level it will target.

        No: I switch between summer and winter by changing from time-based control to self-powered.

        All the best: M

    • protonsforbreakfast Says:


      Don’t feel too bad, it’s my own fault really. But it was tough de-bugging as the spreadsheet took around a minute to update after an edit. I found it very de-moralising.

      Yes, I have time-based control – it’s what I use in the winter season. And that might indeed be smart enough to preemptively avoid the peak hours. But I don’t know because the actual algorithm is not public.

      I don’t think the controls have any way to force an export at a particular time – and in summer we are off-grid most of the time anyway.

      All the best: M

  9. Dylan Johnson Says:


    This is amazing! Would it be possible to get a script/copy of your spreadsheet?

    Let me know what details you’d need from me to send it?



    • protonsforbreakfast Says:


      Hi: I’ve sent you a copy – the comments app let’s me see your e-mail.

      If it doesn’t arrive (7 Mb and contains Macros) – let me know and I’ll try to send it to you another way.


  10. Roaming Rob Says:

    Hi Michel

    Wow if only I could replicate, what a task you have completed. I think I understand! We have a greenish (part nuclear) tariff of 5p for 5hrs night use and 27p day use until March 24 so at that point we will see what provider to pay the inevitable increase too!!!! We also live in the South East, so I am sure your figures won’t be that far out for us.

    Our 12kWh battery charges over night and on some sunny days this winter we have not discharged fully but on others we have fully used all the stored energy. We also have a 3.5kW solar panel set up and have been looking to increase this, if we can raise the funds. But that will have to wait until the Heat Pump install this April. Mind you I have been contemplating rethinking the installation in the light of Octopus/ British Gas announcements of a much cheaper install.

    We currently have storage heaters in part of our home which has meant we cannot charge the car, run appliances, as well as charging all the storage heaters and the battery without exceeding my self-imposed 85amp draw on a 100 amp main supply fuse. So it’s a plan ahead job at the moment, thank goodness for retirement.

    So judging the impact of the heat pump will be interesting for us!! A significant reduction in cheap night time tariff (no storage heaters) and an increase in costly daytime use (heat pump) BUT at least NO GAS.

    By the time out current tariff runs out we will be in our 80’s – so if we have made a mistake re costs it won’t be affecting us be for that long( mind you I am not thinking of cutting our time here short!!). AND self-congratulating we will feel a warm glow for having taken some action towards carbon reduction for the Grand Children.

    Love your articles even if they are beyond my small brain. Just takes a few reads to let it settle in then a few more reads a few days later because I had forgotten – Ha Ha.

    Keep them coming and thankyou for the work you do.


    • protonsforbreakfast Says:


      Good Evening. Thank you for your kind words. It sounds like you are really making inroads on your carbon emissions too. That combination of solar and batteries sounds very useful. And if you can spare the capital for the heat pump install, then switching from storage heaters you should notice the reduction in winter electricity bills. I guess that means you will need to install new radiators and pipework too – which is pricy – but at least you can make sure the radiators and pipework are all the right size!

      In the next article I’ll be comparing Octopus COSY rates – I had never heard of this! – with GO and FLUX.

      Anyway: best wishes


  11. Harry Wallace Says:

    Hi Michael
    Lovely effort – well done. As a retired EXCEL/VBA warrior – I know just what you mean about receding skills!
    I’ve only recently had my PowerWall2/Gateway2/solarEdge installation certified and ready to export and I’d begun research on “which tariff pays back fastest”.
    I was pondering whether or not to dive back into EXCEL/VBA to do some simulation analysis of the various tariffs from Octopus, but you’ve covered a large amount of ground for me.
    Would you kindly make your workbook available to me – I’d really appreciate the jump start?
    I thinking about three lines of possible development – Solver, AI and a solar forecast approach, rather than historical irradiance data.
    In any event, thanks for all the effort and keep up the good work.
    Harry Wallace

    • protonsforbreakfast Says:

      Harry: I am happy to send teh workbook. But for some reason I can’t see your e-mail address in the comments window I am using – usually it shows it to me. Please write to me at house@depodesta.net and I will send you a copy by return.

      I think the historical radiance data is the way to go – it has statistics that I have never seen reproduced anywhere. This previous article – which I wrote before buying the battery – has more details and a workbook which does not contain macros and so can be downloaded.

      All the best


  12. Brian Hauxwell Says:

    Thanks for this amazing work, comparing the tariffs. Can I please have a copy of the spreadsheet to try and use it to evaluate my own situation and which tarriff is best in my own setup. Just adding more solar and battery’s.

  13. David Casale Says:

    I would be very interested in a copy of your spreadsheet calcs.

    Great blog!


    • protonsforbreakfast Says:


      Thank you for kind words: I’ve mailed you a copy of the spreadsheet. If it doesn’t arrive, please drop me a line.


  14. Mark Timothy Says:

    Hi … This. Is just stunning stuff, I am dead impressed. Would it be possible to have a copy of the spreadsheet as my tariff is slightly different and I am considering exporting over the summer months.

    • protonsforbreakfast Says:


      Hi. I’ve sent you a copy of the spreadsheet. If you don’t receive it, let me know here and I’ll figure out some other way to send it.

      All the best


  15. Ioannis Says:

    Excellent work Michael and thank you for your ongoing efforts.
    Looks like for our circumstances, the Flux will be cheaper from the beginning of March till the end of September and then switch back to Go, if still available. We might build up some credit as well to use during the winter months.
    Switching between tariffs twice a year should be straightforward with Octopus, as there are no exit fees and the only restriction I am aware of, is to stay on the new tariff for at least 30 days following the switch.

    • protonsforbreakfast Says:

      Ioannis: Switching tariff twice a year seems like a lot of trouble, but if you have teh emotional energy for it – good luck!

      All the best: Michael

  16. gaken9 Says:

    Hi Michael, an excellent piece of work, I would love to have a copy of the spreadsheet as I’m looking at sizing a large install. Thanks Graeme Kennedy

  17. Kevin Stewart Says:

    Hi, that’s a great article and shows the complexity that we are going to have to deal with to minimise carbon emissions as well as keeping household energy affordable.

    Could I please have a copy of your spreadsheet to have a play with? You could always create a Google Drive where you store your shareable files for people to download.



  18. Kevin Stewart Says:

    Unfortunately it says that there was a problem downloading the file.


  19. Rob Cannell Says:

    There is also a tariff called “Cosy Octopus” which offers two three hour cheap periods (4am – 7 am @ 20p and 1pm – 4pm @ 54p), an expensive period (4pm – 7pm) and the remainder at the standard 34p rate. While aimed at heat pump users it should work well for those with a battery – providing the opportunity to top it up in the afternoon if there is little solar available.
    The Tesla software allows you to program in three different rates so hopefully will avoid importing electricity during the highest rate.
    So far I have remained on a standard economy 7 (17p for seven hours overnight and 42p at all other times), but in the winter my Powerwall often runs out in the evening. This is my first winter with a battery so I am still learning what it will do.
    The “Go” tariff looks appealing (now @ 12p cheap rate for new users) as long as the cheaper rate is not outweighed by three extra hours at the high rate – which could often happen in my case.

    • protonsforbreakfast Says:


      Good Morning. I am just working on a comparison of COSY with GO and FLUX – it should be ready in a couple of days.

      The Tesla software might well avoid the expensive hours, but I am not sure it has the flexibility to have multiple cheap periods. I haven’t investigated.

      Leave a comment here if you would like a copy of the spreadsheet to model your own situation.

      All the best


  20. mps12 Says:

    Many thanks for this informative article. We have no gas in the village so I’m all electric. Heat pump, MVHR, EV, 1 PHEV, Zappi, 5.1Kwp Solar and a Powerwall. 3.6Kwp is on a feed in tarrif and 50% deemed export. I recently switched to intelligent Octopus to get 6hrs at 10p and therefore had to give up my deemed export payment. But the savings easily outweigh the loss. My question is would I need to give up the FIT payment to in order to get the full flex export rate as I guess essentially that’s a SEG rate? Something else to consider when working out what tariff is best. I also can’t work out whether to invest in more PV but I know my DNO had capped my export at 3.68Kw so some summer excess would likely be wasted unless I was home and the zappi could hoover it up. All food for thought….

    • protonsforbreakfast Says:

      Dear MPS12: Good Afternoon.

      It sounds like you have a very impressive system there: congratulations.

      I am not an expert in FIT, but my understanding from talking to people who receive FIT is that the situation is so beneficial that nothing else comes close. Also, I think some of the tariffs which pay for exports will ask you if you are on FIT and you will need to choose which you want to be on, otherwise you would be being paid twice.

      Anyway – I am working on modifying the spreadsheet to allow it to be used more flexibly – so do watch this space. All the best


      • mps12 Says:

        Thanks for your reply and kind words Michael. Have been building our setup over last 4.5yrs with the aim of becoming less reliant on the grid and susceptible to fluctuations in market prices. I got in at the end of FIT so it doesn’t pay a huge amount but a few hundred a year tax free. I always struggle to compare tariffs as my usage really depends on which tariff I’m on. If it’s cheap at night then I schedule lots then or when excess solar likely. I used to be on old Go which was 5p/14p until end Dec 2022 so I’ve had quite a jump in prices. Intelligent is now 10p/44p but 6hrs of 10p. Not sure Flux would work for us being a high user of ~10,000kWh per year. Perhaps you could kindly send me your spreadsheet so I can have a play when you get a moment, thank you.
        I’m only a month in of having the powerwall but really impressed and most of Feb we got through the peak period without needing the grid 0530-2330. Some days without solar we can’t but 20 days in Feb we managed it. I’m wondering if I’ve now hit the sweet spot or whether more PV would be beneficial. Lots lean towards an extra powerwall but I think for me that would only help in the depths of winter so the payback would be a lot longer than the first powerwall. There is room in my garden for a ground mounted PV or the other option is considering the ripple type of investment. But not got my head round that yet. Tend to prefer physical products I own rather than cooperatives etc.

      • protonsforbreakfast Says:

        Hi. I have sent you a copy of the 6.1 Mb spreadsheet.

        Let me know if it doesn’t make it through.


  21. jonnya245 Says:

    Really interesting analysis, thank you. I am paying 12pkwh at night so that makes the flux even more tempting. With 6kw of solar ,battery and an ev I think it makes sense for me. I always try and charge the battery from solar looking a day ahead, sometimes it’s wrong and I pay peak rate. The flux would allow me to charge fully each night and just get for the solar.

    • protonsforbreakfast Says:

      Good Evening,

      Yes, with 6 kW of solar and an EV, I think FLUX would likely be very suitable for you.

      If I ever get my head around EV charging, I may modify the spreadsheet – I can’t face it for now.

      All the best: M

  22. ian Says:

    I thing flux works best with always charging the batteries at much as they will take in the 3 cheap hours, then always discharge into the grid as much as is possible in the 3 super peak hours, while also using the batteries to prevent as much super peak import as possible. Only permit battery discharge before the start of super peak if it still leaves enough in the battery to run inverter at 100% in peak.

    I’m assuming useable battery capacity is more then can be charged/discharge in 3hr, if not add more batteries to inverter.

    (Assuming battery inverter is independent of PV inverter and no output limits)

    • protonsforbreakfast Says:

      Ian, Good Afternoon,

      That sounds like a sensible strategy – but I am not sure I have the programming skills to simulate it in the spreadsheet! But I’ll think about it.

      Also, I am not sure I would know how to implement that strategy in practice. As far as I know, the Tesla Powerwall doesn’t have a command to compel it to discharge into the grid.

      • Rob Says:

        Ian, Michael,
        I think the best you can do is to “encourage“ the Powerwall to discharge by changing the export and import prices at certain times to make it look attractive to export ….and hope the algorithm agrees with your logic!

  23. protonsforbreakfast Says:

    Yes, I don’t have much experience with how the Tesla Algorithm will respond – but if I switch to one of these tariffs I will try some experiments.

    All the best: M

  24. Alan Says:

    Great work Michael, great work indeed

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: