10 solar panels and a battery: what it costs and what it returns

Ten panels and a battery is one of the most commonly quoted domestic packages. This walks through the whole calculation as a template, with each assumption named and a worked example running beside it, so you can replace any figure with your own and see what moves.

Every number in the example is a placeholder for one of yours. The point is the method and the order, but the example is kept honest: each rate is cited, each cost comes from published installation data, and the result is allowed to be awkward.

The inputs you need

Write these down before starting. Six figures, with the example values in brackets:

1. System size, in kilowatts peak. Ten panels multiplied by the panel wattage on the quote, divided by a thousand. (Ten 440 watt panels: 4.4kW.)
2. Annual generation, in kilowatt hours, from a location specific estimate for your roof. (4.4kW at our model’s UK average of 850 per kilowatt, south facing: about 3,740.)
3. Annual consumption, in kilowatt hours, from your bill. (3,600.)
4. Self consumption share without a battery, from your half hourly data. (40% of consumption falls in daylight: 1,440 kilowatt hours.)
5. Usable battery capacity, in kilowatt hours. Not the nominal figure. (6 usable.)
6. Import and export rates. (The capped 26.11p import rate and Octopus Outgoing’s 12p export.)

And the price. The government’s installation cost data puts the 2025/26 median for 4 to 10kW systems at £1,697 per kilowatt, so the solar half of the example costs about £7,470. Call the battery £3,500 installed at the same visit, an illustrative figure to replace with your quote, for a package around £10,970.

If you cannot fill one of these, that is the number to go and find. Guessing it is how these calculations go wrong.

Step one: what solar alone saves

Without storage:

• Self consumed units equal generation multiplied by your daytime share, capped at your actual daytime consumption. The example hits the cap: 40% of 3,740 is 1,496, but daytime consumption is only 1,440, so 1,440 it is. The cap matters: skipping it flatters the result.
• Avoided import: 1,440 at 26.11p is about £376.
• Exported units: the remaining 2,300.
• Export income: 2,300 at 12p is about £276.

Solar only saving: roughly £650 a year. Against £7,470 that is a simple payback around 11.5 years, and it is the baseline the battery has to improve on. The method in full is the savings calculation guide.

Step two: what the battery adds

The battery earns by converting exported units into self consumed ones. Each unit it moves is worth the difference between what the import would have cost and the export you gave up, less round trip losses: at the example rates and 87% efficiency, about 10.7p per unit moved, as worked through in the battery cost guide.

To estimate the units moved, take the smallest of three daily figures:

1. Your exported surplus that day.
2. The battery’s usable capacity, 6 kilowatt hours here.
3. Your overnight consumption, about 6 kilowatt hours in this evening-demand sizing.

Do this seasonally rather than annually. The example household exports 2,300 kilowatt hours a year, but most of it arrives between April and September. Assume the battery fills and empties most days through that half of the year and finds little surplus through the other half, and it shifts perhaps 1,500 kilowatt hours annually. At 10.7p each, the battery adds about £160 a year.

That number surprises people, so it is worth stating plainly: on current rates, with a 12p export tariff softening the loss on exported units, the battery’s increment is modest. Batteries earned more when export paid almost nothing; a decent export rate is quietly a competitor to storage.

Step three: the package versus the parts

• Solar alone: £7,470 for £650 a year, payback around 11.5 years.
• Package: £10,970 for about £810 a year, payback around 13.5 years.

In this example the battery lengthens the simple payback. It is not a verdict on every battery: a household on a cheap overnight EV tariff earns more per cycled unit from grid arbitrage, a bigger evening load shifts more units, and a worse export rate widens the battery’s margin. But it is the honest shape of the arithmetic at these particular rates, and it is why the package price should never be evaluated as one blob. Price the increment against the increment.

Step four: the costs over time

Now the corrections that separate an estimate from a sales figure. Panel output degrades slowly each year. An inverter replacement belongs somewhere in the middle of the projection, at a realistic cost. Battery capacity falls with age and cycles, along the curve the warranty implies, and if your projection runs longer than the battery’s expected life, its replacement belongs in the projection too. Our calculator applies all four rather than holding year twenty equal to year one.

And keep it in today’s money. Resist compounding an assumed energy price rise across the period: it produces a larger and more impressive number that cannot be spent and cannot be compared with anything else.

Which assumption is carrying the answer

Once you have a result, change one input at a time and watch:

• Drop the self consumption share by a third.
• Halve the gap between import and export rates.
• Move the inverter replacement five years earlier.
• Reduce usable battery capacity by the difference between nominal and usable.

Whichever of these moves the payback most is the number that deserves real measurement rather than an estimate. In most households it is the self consumption share, which is exactly the one quotes leave unstated. In this example, cutting the daytime share to 27% takes the solar only saving from £650 to about £590, more than a year of payback from one assumption.

Do it with your own six numbers

The template above is the whole method. Substitute your generation estimate, your consumption, your rates and the usable capacity from the actual quote, then let the calculator run the year by year version with degradation and replacements applied. Compare the package against solar alone before you sign anything: the difference between those two lines is what the battery is really costing, and what it is really earning.