Home batteries: usable capacity, and the rest
Nameplate capacity is not what you get. Usable capacity is, and it is the number quotes are least likely to lead with — along with the fact that a battery only earns money when it cycles, which caps how big a useful one can be.
Compare on usable capacity, not nameplate — and size to what will cycle.
The specification numbers that matter:
- Usable capacity, kWh — what you can actually charge and discharge
- Continuous power, kW — how fast, which decides whether it covers an oven or a kettle
- Round-trip efficiency — what fraction survives the round trip
- Cycle or throughput warranty, and the guaranteed capacity at the end of it
- AC or DC coupled — and whether it needs a hybrid inverter
And the constraint that decides size: a battery only earns when it cycles. Capacity beyond your daily surplus, or beyond your evening demand, sits idle — the calculator works out where that ceiling is for your house.
Nameplate against usable
Manufacturers quote a total capacity and a usable capacity. The gap exists because discharging a cell fully shortens its life, so the management system reserves a portion.
Compare on usable kWh. A 10 kWh nameplate with 8 kWh usable is a smaller battery than a 9.5 kWh nameplate with 9 kWh usable, whatever the brochures say.
Ask for the figure in writing, and check whether the quoted saving was calculated on nameplate or usable.
Power, not just energy
Two different numbers:
Capacity (kWh) — how much it holds.
Continuous power (kW) — how fast it can deliver. A battery with 10 kWh and 3 kW output cannot run an oven and a kettle together; it will hold plenty and deliver slowly.
For most households the capacity constrains the daily saving and the power constrains what it can cover at peak. If you have a heat pump or an EV charger, ask about power specifically.
This is the mechanism by which a battery makes money, and MCS’s own guidance quantifies it.
For the household archetype our self-consumption page works through, MGD 003 gives:
- 23% self-consumption with no storage
- around 37% at 1.1–2.1 kWh of usable capacity
- around 45% at 2.1–3.1 kWh
- around 50% at 3.1–4.1 kWh
and it caps self-consumption at 95% however much storage is fitted.
Notice the shape: the first couple of kilowatt-hours do most of the work, and the returns diminish quickly. That is the same conclusion the sizing calculator reaches from daily surplus and evening demand, by a different route.
Round-trip efficiency
Every stored unit loses something on the way in and out. A round-trip efficiency of 90% means 10% of what you store never comes back.
It matters most when the economics are marginal: storing a unit worth 4p on export to avoid buying one at 26p is a good trade even at 85% efficiency; arbitraging a 2p price difference is not.
Warranties: cycles, throughput and capacity
Battery warranties are stated differently from panel warranties, and the units matter:
- Cycles — a number of full charge-discharge cycles
- Throughput — total energy through the battery, in MWh
- Retained capacity — the percentage guaranteed at the end of the term
Ask which of the three binds first for your usage. A household cycling once a day reaches a cycle limit far sooner than one cycling three times a week, and a throughput warranty behaves differently again.
Coupling, siting and safety
DC coupled — through a hybrid inverter. Usually cheaper when solar and storage are installed together.
AC coupled — its own inverter. Usually the route when adding to an existing system.
Siting matters more than for panels: batteries have temperature ranges, weight, and fire safety considerations. Approved Document L points to Approved Documents B and P for on-site generation and storage, and the government’s consultation response records the IET Code of Practice for Battery Storage Systems as the guidance respondents pointed to for installation. Manufacturers’ instructions govern where a unit may go.
Why there is no price on this page
Every cost figure on this site comes from a source we can cite — for solar, an official statistics series derived from MCS installation records. No equivalent series exists for domestic battery storage that we have found.
So we publish the sizing arithmetic, the mechanism by which storage earns, and the questions to ask — and we stop before the number we cannot stand behind. If you find a published UK price series for storage, tell us and we will build the payback page the same day.
Sources
- MGD 003: Solar PV Self-Consumption, issue 2.0 (1 April 2022) Read locally. The storage uplift tables and the 95% cap. MCS restricts reproduction, so we cite clauses and quote sparingly.
- MIS 3002: The Solar PV Standard, issue 6.0 Requires the datasheet for any electrical energy storage system as part of the minimum technical information.
- The Future Homes and Buildings Standards: consultation response Records the IET Code of Practice for battery storage systems as the guidance respondents pointed to.
Contains public sector information licensed under the Open Government Licence v3.0.
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