Should I get a home battery? A year of energy monitoring

Around twelve months ago I was getting a bit obsessed with the idea of having getting a home battery installed to capture the excess electricity we were generating from our solar panels. As a way to stop myself from reaching for the cheque book I decided to spend the last twelve months monitoring our generation and usage.

We have a 3.7kW system in London, split 50/50 between a set of east facing panels (minimal shading) and a set of south facing panels (which have a reasonable amount of shading unfortunately). We generated 2,918 kWh in the year I've been doing this analysis.

I'm the first one to admit that my measurements might not be the most scientific. I also don't really know what I'm doing. One of the challenges is that we have a hot water diverter that already stores some of our electricity in the form of hot water by turning on our immersion heater.

I started monitoring in June 2018.

Caveats given, here is what I've learnt.

How many months of the year could I be off grid with a domestic battery?
For six months of the year.

In June, July, August and September 2018 we exported more than we imported. We entered positive territory again in April and May 2019.

A 10.2 kW battery size would see us able to live completely off grid. A 7 kW battery would cover us for the vast majority of the time, but we have on some days imported between 7 - 10 kW. This generally happens on the weekend when we have the oven on a lot, are using the washing machine, mowing the grass etc....

What about if I had a really big battery?
In the good months we export a lot more than we import, so if we had a really big battery we'd be able to take the big summer surplus to see us through the winter months.

I don't know very much about district storage, the idea that there is a really big battery on your local network, but it seems to be an area of increasing research. The concept as I understand it, is that there is a large battery on the local network storing excess generation which can then be discharged when needed.

By the end of August we could have stored 574 kWh of electricity if we had a battery big enough. This would have been enough to see us through October, November and December 2018, running out sometime in early January.

I appreciate that with district storage an individual wouldn't in practice be able to get the electricity that they'd put in back three months later, but it is an interesting theoretical exercise.

What about an electric car?
Over the past year I've lost my obsession with idea of a domestic battery and replaced it with the idea of getting an EV.

We take public transport to work so the car would be sitting on the drive way during peak generation times five days a week, effectively acting as a 50 kWh battery. It would enable us to be off grid for six months of the year, but wouldn't really be big enough to help keep us off grid for another four as a district batter would.

Vehicle to Grid and Vehicle to Home technology is in its infancy in the UK, but is already proven in other countries and is supported by the latest Nissan Leaf for example.

I appreciate that it wouldn't work for everyone (e.g. if you drive to work), but for our lifestyle I don't see why you'd spend £7 - 10k on a home battery when you could put that money towards an electric car.