# 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.

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.

## 5 thoughts on “One Year With A Solar Battery”

1. ### Matti Järvinen said on mastodon.social:

@Edent our housing company estimates were around 6-8 years for a 390kWp solar.

That's a lot I know but we have just over 300 apartments and all of our heating comes from a total of 1000 MW (not exact we have 3 pumps) geothermal heat pumps. Our goal was to have 4-6 months of free heating and with the company this large probably nothing will get exported anyway.