More Tariff Calculations: GO, FLUX, COSY and more

Friends, in the previous article I estimated the annual cost of running a household with various “time-of-use” (TOU) tariffs i.e. bargains with the electricity company in which the price of a unit of electricity varies through the day.

The article seemed to strike a chord with many people asking several “But what if..?” questions. This article is an attempt to answer some of those questions.

The reason that I think this is an important issue as TOU tariffs are likely to become more popular.

Why? Because getting people to avoid consuming during peak hours is cheaper and greener than building a new power station to meet that demand. It’s also cheaper and greener than using gas in an existing power station to meet that peak demand.

And as domestic batteries and solar PV installations become ever more common, avoiding consumption at peak times has become more achievable for ‘ordinary’ people.

But it has also become all but impossible to work out which TOU tariff will be cheaper (or greener). And this puts us in danger of perpetuating the confusopoly that currently exists in energy tariffs.

The only way I know to work out which tariff is cheaper is to simulate an entire year hour-by-hour with realistic consumption, solar PV and battery storage. And that is exactly what I did: please read the previous article if you want to familiarise yourself with that.

Click on image for a larger version. The concept of a Confusopoly was invented by Scott Adams.

When I began to write this article, I planned to look at

• Different Tariffs (Octopus’s GO, COSY, and FLUX tariffs – alongside the standard variable tariff)
• The effect of solar generation
• The effect of battery storage capacity
• The effect of a small (10 kWh/day) or a large (20 kWh/day) domestic consumption
• Seasonal Strategies

However as I began this program of work I realised I had bitten off more than I could chew.

So below I will look at some of these issues, but people’s requirements are so specific that I thought the best way to be helpful was to make the spreadsheet easier to use and that is what I have tried to do.

The Excel spreadsheet can be downloaded from this link. If you are downloading this macro-enabled file on a Windows computer, then the macros will probably be blocked by default. To change this you may need to right-click on the file and select the ‘Properties’ pane. Here you should see a tick box labelled “unblock”. If you unblock the file then it should work correctly. The next article will describe how the spreadsheet is structured.

One thing I won’t be looking at is the effect of charging electric vehicles. Sorry: I don’t have experience of this.

Tariffs

In this article I will investigate four tariffs: Octopus’s so-called COSY, GO, and FLUX tariffs, and a standard variable tariff (SVT) – currently (February 2023) set at 34p/kWh. The assumed costs and times of operation are shown in the figure below.

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 FLUX and COSY tariffs. Also shown as a dotted line is current standard variable tariff (SVT).

The daily-average import prices of the FLUX and COSY tariffs are about 34p/kWh, the same as the SVT, but the average GO tariff is 37p/kWh.

The daily-average export prices of the FLUX and COSY tariffs are 22p/kWh and 15p/kWh, the same as the SVT, but the average GO export price is just 4p/kWh.

Solar PV and Demand

The standard demand that I used last time was based on my own home. It consisted of a baseload of 10 kWh/day and then a seasonally variable heating demand peaking at 15 kWh/day. The solar PV that I used previously was based on my own home generation of just under 4,000 kWh/year. But of course these are only relevant to me – and there are many combinations relevant to other people.

For example, the figure below shows four combinations of high and low demand and high and low solar generation. It’s clear that these are very different situations and that the likely cost savings will be very different in each scenario. And this does not include changing the size of the battery.

Click on Image for a larger version. Four scenarios showing the variation of daily demand and daily solar PV generation through the year. The upper graphs show high demand and the lower graphs show low demand. Solar PV generation varies from just under 2,000 kWh to just under 8,000 kWh. Also shown as a dotted line is 13.5 kWh of battery storage available in a Tesla Powerwall.

Pre-discussion

Before looking at any results it is important to understand the ways in which it is potentially possible to generate savings compared to the standard variable tariff (SVT).

• Using the battery alone it is possible to generate savings by avoiding peak tariffs.
• Using solar alone it is possible to generate savings by using solar electricity instead of drawing electricity from the grid. But solar PV doesn’t match demand minute-by-minute, sometimes oversupplying and sometimes undersupplying.
• Using battery and solar, can generate savings in several ways:
  • By avoiding peak and standard tariffs
  • By improved self-utilisation of solar electricity
  • By exporting electricity.

Bearing these factors in mind, let’s look at some results.

Charging Strategies with a 13.5 kWh Battery: No Solar PV

I compared GO, COSY and FLUX to the SVT without considering any solar PV generation, but instead I used four different charging strategies:

1. No strategy – i.e. the battery is not used at all.
2. Pre-charge the battery for an hour before the peak rate.
3. Charge the battery as much as possible in the cheap rates
4. Pre-Charge the battery before peak rate AND charge the battery during cheap rate.

The SVT for this scenario came to £1,849/year and the four strategies above differed in cost as shown below:

Click on Image for a larger version. Annual savings or extra costs compared to the £1,849 SVT when using different tariffs. This chart shows the impact of using a battery alone with no solar PV. See text for details.

• Not using the battery with COSY and GO costs several hundred more pounds per year. So don’t use one of these tariff’s if you don’t have a battery or solar!
• Pre-charging for one hour before the peak rate resulted in modest savings (~7%) compared with the SVT.
• Charging during cheap rate resulted in big savings on the GO tariff (-39%), modest savings on the FLUX tariff(-20%), but a significant increase on the COSY tariff (+17%).
• Pre-charging and Cheap Rate charging together saves a lot of money with GO, only a little (12%) with FLUX and is more expensive with COSY.

At first I thought the simulation must be in error to for COSY to cost more than SVT. But looking at the charging details I saw that in summer (when demand was lower) using the ‘charge when cheap’ rule led to overcharging on COSY because it has two cheap rates just 3 hours apart. This could be avoided with smarter programming.

My conclusion from this is that the only way to significantly save money when using a battery alone is with a tariff such as GO which offers very low prices (12p/kWh) and a large battery able to store almost a whole day’s consumption.

Charging Strategies with modest solar PV and two sizes of batteries

Next I considered a situation with a modest solar PV installation (~2000 kWh/year corresponding to ~ 6 south-facing panels) and either a small (5 kWh) or a large (13.5 kWh) battery. I then compared the different charging strategies I used in the previous section with each of the GO, COSY and FLUX tariffs to the SVT with no solar PV.

Click on Image for a larger version. Annual savings or extra costs compared to the £1,849 SVT when using different tariffs. This chart shows the impact of using a small battery (left) or large battery (right) with a small solar PV installation. See text for details.

Comparing these results with the previous ‘No Solar’ results, it is immediately obvious that – with or without a battery – even a modest solar PV installation saves a serious amount of money. The average saving from the ‘No Battery’ scenario is 33% compared to the SVT.

Pre-charging from the grid for one hour before peak rate results in extra savings on all tariffs averaging 41% independent of battery size.

Moving to a larger battery (13.5 kWh vs 5 kWh) is only really of significant benefit on the GO tariff (56% vs. 33%). For the FLUX tariff the extra battery capacity increases savings only from 33% to 42%.

Annual Variability

Next I considered the effect of annual variability. I estimated the annual cost for a scenario similar to that in my home using the GO and FLUX tariffs and solar data for each year from 2005 to 2016.

Click on Image for a larger version. Top: Annual variability in solar PV generation (kWh/year). Middle: Annual variability in estimated year cost using the GO tariff. Bottom: Annual variability in estimated year cost using the FLUX tariff. See text for details.

The year-to-year variation in solar PV yield was roughly ±10% over the years 2005 to 2016. Unsurprisingly, good solar years led to lower overall costs on both GO and FLUX and vice versa.

But there is a curious feature in the calculations. Using the FLUX tariff the average cost over the individual years 2005 to 2016 was similar to the cost calculated using the average solar data. But for the GO tariff this was not the case. Instead, the average cost over the individual years 2005 to 2016 was much higher than the cost calculated using the average solar data.

I am not sure why this is, but it may be a feature of the fact that the averaged solar data has less variability than data for any individual year.

Other scenarios 

At this point I realised that I was becoming overwhelmed. 

I realised that there was no way to systematically summarise the results of using different tariffs across all the possibilities of demand, solar PV, tariff and charging strategy. So rather than trying to calculate everything myself, I resolved to tidy up the spreadsheet and make it reasonably suitable for other people to use. The dashboard of the revised Version 3 is shown below. In the next article I will describe how to fill out the spreadsheet.

The Excel spreadsheet can be downloaded from this link. If you are downloading this macro-enabled file on a Windows computer, then the macros will probably be blocked by default. To change this you may need to right-click on the file and select the ‘Properties’ pane. Here you should see a tick box labelled “unblock”. If you unblock the file then it should work correctly.

Click on Image for a larger version. Image of the dashboard from the revised spreadsheet for evaluating the costs of different tariffs with different demand, generation, and storage scenarios.

Summary

I compared GO, COSY and FLUX tariffs to the SVT in a couple of scenarios. My conclusions are that:

• Using a battery alone is only really valuable if one uses a big battery to download very cheap electricity.
• However, even modest amounts (e.g. 2,000 kWh) of Solar PV can be very beneficial, and a battery increases the savings possible still further.
• Because the average solar data has less variability than the solar data from any individual year, some tariff’s may not generate a typical cost when using the averaged solar data.

Beyond this, I’m afraid you will need to do these calculations for yourself. More details in the next article…

 

Tags: Tariffs