Usable vs nominal capacity: what a home battery really delivers

Battery spec sheets lead with one number: 5, 10 or 15 kWh. But manufacturers don’t all mean the same thing by it. Some quote nominal capacity (everything the cells can store), others usable capacity (what you can actually draw). Efficiency, standby consumption and power rating then decide how much of it you really benefit from. This guide explains how to compare like with like, using figures from our own database of 39 home batteries.

Nominal, usable and depth of discharge

Lithium batteries last longer if they are never completely empty or completely full, so the battery management system (BMS) keeps a buffer at the top and bottom. What’s left is the usable capacity. The share you may use is the depth of discharge (DoD).

Across our database, manufacturers handle this differently:

• 27 batteries list only usable capacity, 8 only nominal and 4 both.
• For the four with both figures, usable capacity is 94.5–97% of nominal.
• 12 models state a DoD: nine say 100%, two 90% and one 88%.

A 100% DoD rarely means the cells are run down to zero; it usually means the buffer has already been taken out of the figure the manufacturer calls “capacity”. Always compare usable with usable. On our product pages every value is labelled as nominal or usable, with its source.

Efficiency: what comes back out

Storing electricity costs energy in three places:

1. Conversion between solar or grid power and battery voltage, both ways.
2. Inside the battery, through internal resistance and heat.
3. Standby: electronics, BMS and inverter that are always on.

The Energy Saving Trust puts it simply: using stored energy is less efficient than using electricity directly from the grid or your panels, so you lose some along the way ¹. Manufacturers usually quote a round-trip efficiency. In our database 17 models do, at 90%, 94.5% and 96% – but not always measured the same way. Some figures cover the battery module only (DC), others the whole system including the inverter (AC).

Independent testing shows how wide the spread is. HTW Berlin’s 2026 Energy Storage Inspection tested 12 systems from 10 manufacturers. Battery efficiency ranged from about 88% to 97.1%. In the 10 kW class, the best system scored 97% on HTW’s System Performance Index and the weakest 89.3% – with three and a half times the losses ².

Don’t ignore standby consumption

Standby draw in the same test ranged from 4 to 64 watts ². A battery often sits idle for hours, and in winter sometimes for days. As an upper bound:

At an assumed 25p per kWh, that is a worst-case difference of roughly £130 a year. Ask for the standby figure, or look for independent measurements.

Power: how fast it charges and discharges

Capacity (kWh) is how much you can store; power (kW) is how fast. A 10 kWh battery that only delivers 2 kW won’t cover a kettle plus an induction hob in the evening – the rest comes from the grid.

• Of 22 batteries with a stated discharge power, the median is 3.5 kW, ranging from 0.8 kW (plug-in units) to 11.04 kW.
• In practice, half of them deliver at most about half their capacity per hour.

Power also matters for grid connection. In Great Britain, anything above 16 A per phase (3.68 kW on a single phase) including storage falls under G99 and needs DNO approval before installation ³ – see our guide to AC- vs DC-coupled batteries.

How big should your battery be?

A bigger battery only pays if it actually fills and empties. Two limits set the sensible size:

1. Your daytime solar surplus. On a grey December day it may be zero.
2. Your evening and overnight use. Storing more than you use before morning adds nothing for self-consumption – unless you also charge from the grid on a cheap overnight rate.

The Energy Saving Trust says a typical home system might be 10 kWh ¹, and 10 kWh is also the median in our database: 9 models store up to 5 kWh, 11 between 5 and 10 kWh, 11 between 10 and 15 kWh and 8 more than that. Many systems are modular – for 25 models the manufacturer lists a maximum expansion – so you can start small.

Comparison checklist

• Is the figure usable or nominal? Convert nominal with the DoD.
• Is the efficiency for the battery (DC) or the whole system (AC)?
• What is the standby consumption in watts?
• Do the charge and discharge power match your evening use and tariff?
• Can it be expanded, and to how many kWh?

Next: time-of-use tariffs and lifespan and warranties.