How big a battery? The sizing arithmetic
Useful battery size is capped twice over: by how much surplus there is to store on a typical day, and by how much you would otherwise buy after dark. Capacity beyond the smaller of those two does not cycle, and capacity that does not cycle earns nothing.
Useful capacity is the smaller of two numbers.
How much surplus generation there is to store on a typical day, and how much electricity you would otherwise buy between sunset and sunrise. Whichever is smaller is your ceiling — and the sensible size is below it, not at it.
The two ceilings
A battery has to be filled and it has to be emptied. Each side sets a limit, and the binding one is whichever is smaller.
The charge ceiling: how much surplus exists. You can only store generation you were not going to use anyway. Surplus is generation minus whatever the house is drawing while the sun is up.
The discharge ceiling: how much you would otherwise buy. You can only save money on electricity you were going to purchase. If the house uses four kilowatt-hours between sunset and the next morning, a battery cannot displace more than four, however large it is.
Everything above the lower of the two is capacity you have paid for that will not cycle.
Doing it with real numbers
Take the household our rooftop calculator defaults to: 2,500 kWh of consumption a year, a 4 kWp array in London.
Generation. Our own PVGIS runs give London 919 kWh per kWp a year, so the array makes about 3,676 kWh — an average of 10.1 kWh a day. That average hides most of the problem:
| Month | kWh per day, 4 kWp in London |
|---|---|
| June | 14.7 |
| September | 11.5 |
| October | 8.0 |
| November | 5.9 |
| December | 4.3 |
Daytime use. MCS’s MGD 003 puts self-consumption for this pairing at 23% without storage, which is 846 kWh a year, or about 2.3 kWh a day used directly as it is generated.
Charge ceiling. In June the surplus is roughly 14.7 − 2.3 = 12.4 kWh a day. In December it is roughly 4.3 − 2.3 = 2.0 kWh a day.
Discharge ceiling. The household buys 2,500 − 846 = 1,654 kWh a year, about 4.5 kWh a day, and not all of that falls after dark.
So the ceilings are about 4.5 kWh for most of the year — set by demand, not by sunshine — and about 2 kWh in December, set by sunshine.
In summer the surplus is far larger than the household can absorb, so extra capacity is wasted. In winter the surplus is smaller than even a modest battery, so extra capacity is wasted. There is a band in spring and autumn where the two roughly match, and that band is what sets a sensible size.
This is why “size the battery to your daily generation” is bad advice. Daily generation in June is more than three times December’s.
Why the honest answer is smaller than this arithmetic
The calculation above uses daily averages, and real days do not behave like the average.
A run of dull days in April leaves the battery part-full for a week. A bright Saturday when the household is out leaves it full by lunchtime with the surplus exported anyway. Both push the realised benefit below what an average-based model predicts, and neither shows up in the arithmetic above.
That is why MGD 003’s storage uplift figures are lower than a daily-average calculation implies, and why our battery size calculator runs the surplus month by month rather than annually. Use the arithmetic on this page to understand the shape of the problem, and the sourced method for the number.
MGD 003 also caps self-consumption at 95% however much storage is fitted, which is another way of saying the same thing: there is a ceiling, and it is not far away.
Usable capacity, not nameplate
The figure you size against is usable capacity — the portion the system will actually charge and discharge in normal use. Nameplate is the headline; usable is the product.
Manufacturers publish both. The gap between them varies by product and is not small enough to ignore. Compare quotes on usable kilowatt-hours, and get the number in writing rather than from a brochure — what a battery specification should tell you sets out the rest of the list.
Capacity is not the only number
Power rating, in kilowatts. How fast the battery can deliver. A 10 kWh battery with a 2.5 kW output cannot run a 7 kW shower from storage — it will supply 2.5 kW and import the rest. If your objection to buying electricity is strongest at exactly the moments your demand spikes, check this number, because capacity will not save you.
Round-trip efficiency. Some of what goes in does not come out. Manufacturers state it; ask for the figure and apply it to the units you expect the battery to move, because the saving is calculated on what comes out.
Depth of discharge and the warranty. Some warranties are written in cycles, some in throughput, and the limits interact with size: a small battery worked hard reaches a cycle limit sooner than a large one worked gently. Battery warranties covers what those limits actually constrain.
A rule of thumb, with its own health warning
If you want one sentence: for a typical household with solar and no time-of-use tariff, useful usable capacity is in the region of half a day’s electricity consumption, and adding capacity beyond that buys steeply less each time.
That is a starting point for a conversation with an installer, not a specification. The reason we lead with the arithmetic rather than the rule is that the arithmetic tells you why — and it tells you immediately when your household is not typical.
Sources
- MGD 003: Solar PV Self-Consumption, issue 2.0 (1 April 2022) Read locally. The storage uplift tables and the 95% cap. Cited rather than reproduced.
- PVGIS 5.3 Monthly generation figures are our own runs: 1 kWp, crystalline silicon, building-mounted, 20% system losses, 35 degrees, due south. Contains modified Joint Research Centre data.
- Review of typical domestic consumption values: decision The typical domestic consumption values used across the industry to describe a typical household's annual electricity use.
Contains public sector information licensed under the Open Government Licence v3.0.
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