One Year With A Solar Battery

To recap, we have 5,040W of solar panels, with a 3.6kW inverter, and a 4.8kWh battery.

That's a lot of (expensive) gear! What does it mean in terms of energy savings?

Over the last 12 months we have:

• 4,000 kWh generated by the solar panels.
• 1,200 kWh purchased from the grid.
• 1,200 kWh sold to the grid.
• 1,300 kWh discharged from the battery.

(Data taken from various APIs and rounded to make life easier).

It's important to note that the battery doesn't only charge from the panels. Because we have a a smart electricity tariff, it will fill up from the grid when electricity prices are negative and empty when they are expensive.

Calculations

Generation - Export = Solar Use

4,000 - 1,200 = 2,800kWh

So we directly use about 70% of what we generate. Some of that is used immediately, and the rest goes into the battery for later use.

Using the battery's API, I was able to call up the statistics for each month. For example, in June, we only paid for 10% of our electricity!

I got the data for each month, these aren't quite as precise as I'd like, but they're good enough for modelling.

Domestic Electricity use kWh by Source per Month:

Month	Storage	Grid	Solar	Total
2024-07	108	33	150	291
2024-06	102	28	148	277
2024-05	103	40	160	303
2024-04	111	72	147	330
2024-03	100	123	121	345
2024-02	103	143	76	322
2024-01	114	192	58	365
2023-12	102	188	35	324
2023-11	111	156	58	325
2023-10	73	152	87	311
2023-09	104	65	106	275
2023-08	111	44	135	290
Total	1,243	1,234	1,280	3,756
Percent	33%	33%	34%

Wow! Averaged across the entire year, we've cut our electricity bill by about ⅔rds!

Across the year, we sell back as much electricity as we buy. We are net zero!

For reference, the average UK home uses about 3,500kWh. We're a little above average - probably due to working from home.

Cost

Electricity prices have been wild in the last 12 months. The dynamic tariff means we can buy cheap energy but use it when it is expensive. So it is hard to say with any accuracy how much money we've saved. I'm going to say very roughly that electricity costs 24p/kWh and we sell it for 12p/kWh.

( (Solar Used + Battery Used) x 0.24 ) + (Electricity Sold x 0.12) = Total Savings

( (1,280 + 1,243) x 0.24 ) + ( 1,200 x 0.12 ) = £750

The cost of installing solar and a battery will vary depending on where you are, what local incentives there are, how complicated your roof and existing electrics are, and a whole host of other factors. Payback depends on your usage patterns and energy prices.

We estimate our payback period to be between 6-8 years.

Obligatory Referral Links!

If you'd like a dynamic electricity tariff, or want an API to monitor your own use, or need to sell electricity back to the grid - join Octopus Energy today and we both get £50.