I have a modest set of solar panels on an entirely ordinary house in suburban London.

On average they generate about 3,800kWh per year. We also use about 3,800kWh of electricity each year. Obviously, we can’t use all the power produced over summer and we need to buy power in winter. So here’s my question:

How big a battery would we need in order to be completely self-sufficient?

Background

Let’s take a look at a typical summer’s day. The graph is a little complex, so I’ll explain it.

The yellow line shows solar production. It starts shortly after sunrise, peaks at midday, and gradually drops until sunset.

The red line shows how much electricity our home is using. As you can see, there’s a large peak about 19:00 when we cook dinner.

The blue line shows how much electricity we draw or export from the grid. From midnight until sunrise we import because the sun isn’t shining. Once the sun has risen we’re able to power our house and export to our neighbours. When we cook, we draw from the grid and our battery – which is why the evening grid peak is lower than the household use dip.

The CSV of the data looks something like this:

It’s fairly trivial to sum both columns and subtract one from the other. That shows either the excess or deficit in solar power for the household.

On that day, the house used 9.7kWh and generated 19.6kWh. I’d need a 9.9kWh battery to store the excess right? Wrong!

Because my usage doesn’t track the sun, I’d actually need a 13kWh battery. That’s the peak amount of excess electricity I’ve generated in that one day.

What I want to do is find out what the maximum size battery I would need in order to store all of summer’s electricity for use in winter.

Luckily, I have several years of real data to go off! Let’s get started!

Disclaimer

This is based on data generated by my home battery. It has probes to measure solar output and grid flow. It is not 100% clock-accurate compared to my solar-panels’ internal reporting nor what my smart-meter reports. I estimate a 1-2% deviation, which is good enough for these purposes.

My energy usage isn’t representative of anything other than my usage. Your household is probably different. I already have a 4.8kWh battery which changes how and when I use energy.

This doesn’t account for gas heating or hot water. We have some electric heaters and taps which increases our electricity usage.

My maths is probably right – but the code is open source, so feel free to check for yourself.

Remember, this is just a bit of fun. There’s no practical way to build domestic batteries with this capacity using the technology of 2025.

Code

We tend to start generating more electricity than we use starting in Spring. So I’ve picked the end of March 2024 to the end of March 2025.

Let’s see how big a battery we’d need to store our summer excess for winter. This finds the cumulative difference between each day’s energy production and usage:

 Python 3import os
import pandas as pd

# Load all the CSVs
filepaths = [f for f in os.listdir(".") if f.endswith('.csv')]
df = pd.concat(map(pd.read_csv, filepaths))

# Make sure they're in order
df = df.sort_values("Timestamp")
df = df.reset_index(drop=True)

# Resolution is every 5 minutes, so divide by 12 to get hourly
df["Cumulative_Difference"] = ( (df["Household_(W)"] + df["Solar_(W)"] ).cumsum() ) / 12

# kWh of battery needed
int(df["Cumulative_Difference"].max() / 1000)

## Draw a pretty graph
df.plot(kind="line", x="Local_time", y="Cumulative_Difference", xlabel="Date", ylabel="MWh", xticks=["2024-04-01", "2024-05-01", "2024-05-01", "2024-06-01", "2024-07-01", "2024-08-01", "2024-09-01", "2024-10-01", "2024-11-01", "2024-12-01", "2025-01-01", "2025-02-01", "2025-03-01", "2025-04-01"], legend=False, grid=True, fontsize=15)
plt.show()

The total is 1,068KWh – basically, a MegaWatt-hour of storage.

Here’s a quick graph to show how the storage would be used over the year.

As you can see, even in this scenario there are a few days where we’d need to import energy from the grid.

Is this sensible?

Probably not, no. It doesn’t account for increased energy use from having an electric car or moving away from gas heating / cooking. As solar panels increase in efficiency, it might be more sensible to replace the panels on my roof, or add some onto a shed.

The environmental impact of creating and storing such huge batteries could also be factored in.

A battery which is only 100% full for a few days probably isn’t an efficient design. Using wind, hydro, and other green sources from the grid might be preferable.

But, remember, this is an exercise in wishful thinking.

Is this possible?

Grid-scale batteries exist and they work brilliantly.

But if I wanted my own MegaWatt-hour of battery storage, it would probably cost me between £100k and half-a-million quid.

That doesn’t include maintenance, the land, planning permission, and a hundred other things.

But battery prices are falling fast. In the last decade lithium ion battery prices have fallen 90%. With new sodium ion batteries promising an even bigger drop – down to US$10/kWh.

If – and it is a big if – those numbers came to pass, it would probably cost around £8,000 for a domestic battery. Basically the same cost as adding solar panels in the first place.

Domestic solar works – yes, even in the rainy UK! It is relatively cheap, moves energy production as close as possible to energy consumption, reduces bill-shock, and means we don’t have endless planning arguments about whether fields should be turned into solar farms.

It is possible that, not too long in the future, every home could also have a 1 MegaWatt-hour battery. They would be able to capture all the excess solar power generated in a year.

There’s a bright and sunny future where every home can be solar-self-sufficient.