Battery Day: One month on…

Click for larger version. The graph shows daily electricity drawn from the grid (kWh). Before the solar panels were installed average usage was 10.9 kWh/day. After the solar panels were installed this fell by a couple of kWh/day. After an increase over Christmas when our son returned, we were back to normal. In the last month the solar electricity generated on the lengthening days have reduced the electricity drawn from the grid. Since the battery was installed a month ago, we have not drawn any electricity from the grid, but daily usage has been unchanged.

Friends, I have been so busy I have been forgetting to blog! So I thought I’d just post a quick update on the Powerwall Installation.

One month on now and we are still “off grid”: we haven’t used a unit of grid electricity for a month. But as the graph above shows, our usage has continued unchanged.

My experience has convinced me that solar installations should all be accompanied by a battery of some sort.

As I have mentioned before, from a national perspective, local batteries are a bad idea: they consume energy.

But from from a user’s perspective, they allow me to benefit from my investment.

And the prospect of not using any grid electricity until maybe September leaves me giddy.

Longer term goals

Click for a larger version. Summary performance of the battery system on 17th April 2021. The battery supplies the house overnight, and is then re-charged by solar generation in the morning. When the battery is full (around 1 p.m.) solar generation is exported to the grid. In the evening the battery takes over again as the solar generation dwindles.

We haven’t experienced a summer of solar power yet, but the longer days are amazing – even in April!

On sunny days the panels generate over 20 kWh of electricity, and the battery is full in the middle of the day.

Once the battery is full, the system exports electricity to the grid in the afternoon and we don’t need to use the battery until perhaps 7 p.m.

These exports are an important part of our plan.

In the winter, we will need to buy electricity from the grid – especially as next winter we will be using a heat pump for heating and hot water.

But ideally we hope the exports of electricity in the summer should match imports in the winter.

Looking at the cumulative generation and export from the system (below), and remembering that exports should be larger in the summer, it looks like yearly exports might just reach 1000 kWh – much more than I had expected.

This would be enough to ‘balance’ 100 days of 10 kWh per day in the winter, assuming no solar power. But even in mid-winter there is typically 2 or 3 kWh per day and so we might just be able to achieve year-round balance.

Click for a larger version. Cumulative generation and export of solar electricity. The dotted green line was my initial guess for generation through the year.

4 Responses to “Battery Day: One month on…”

  1. abc Says:

    Impressive (to say the least)!

  2. carsort Says:

    Me again.

    Actually, I don’t understand the graph below.

    You installed your solar panels in November. They didn’t reduce your energy bill by much – because it was winter is what I first thought.

    When you installed the battery in March there was a dramatic drop. Where is the battery getting the energy from if the solar panels are still not producing much? If your house is using more energy than the solar produces in a day, shouldn’t the battery be drawing from the grid to fill itself up?

    Or is that your electric company was just taking most of the solar power you were generating (pre-battery) because your house couldn’t use it over a short period of time? My electric company subtracts the energy I produce from what I use even if it’s going into the grid.



    • protonsforbreakfast Says:

      “My electric company subtracts the energy I produce from what I use even if it’s going into the grid.”

      This practice is called ‘net metering’ and is common in the US. With net metering all the kWh’s your solar panels generate are either (a) used by you or (b) reduce your bill. ‘Net metering’ is not popular with utilities because they effectively have buy your electricity at the retail rate – whereas they normally make money by buying at a wholesale rate and selling at a higher price.

      In the ‘UK’ net metering does not exist. I pay ÂŁ0.25 per kWh for electricity in the day, but EDF will only pay me ÂŁ0.015 per kWh – I don’t even sign up for that.

      What the graph shows is that installing the solar panels last November made a small difference to our daily electricity usage. Mostly the panels generated a few hundred watts during the dull winter days. As spring came, we saved a bit more, but now the Sun intermittently generated kilowatts and if we weren’t using it, it was exported.

      What the battery does is capture this excess solar so at the end of the day we have a full battery to last us through the night. During the day we occasionally draw from the battery when we have a high demand. For you – the grid is essentially your battery and you get it for free! You charge it during the day and draw on it at night.

      Just about the time we had the battery installed, our average daily solar generation exceeded our average demand (~10 kWh) and so now we just operate from solar and the battery.

Leave a Reply

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

You are commenting using your 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: