Comparing home battery specifications: the technical side
Two quotes can name batteries of the same size and still describe quite different things. The specification sheet is where the differences show, once you know which lines to read.
In short
Compare batteries on usable kWh, not the label size; check the continuous power in kW against your evening peak; read the round-trip efficiency if you will charge from the grid; work out the warranted energy; and confirm the coupling, backup scope and the location the unit is rated for. Cost comparison is a separate exercise.
The lines that matter, and what to ask
| Line on the sheet | What it tells you | What to ask |
|---|---|---|
| Nominal capacity (kWh) | The size on the label; what MCS records and official statistics use | Compare with other batteries only alongside the usable figure |
| Usable capacity (kWh) | Energy you can actually draw out | Size and compare on this |
| Continuous power (kW) | How much the battery can supply at once, for as long as it has energy | Is it enough for your evening peak? Does it change the network route? |
| Peak or surge power (kW, seconds) | A short burst above the continuous rating | Useful for motors and pumps starting up; not for long loads |
| Round-trip efficiency (%) | Share of energy put in that comes back out | Matters most if you will charge from the grid |
| Warranty: years and energy (MWh) or cycles | How long, and how much energy, the warranty covers | Which limit is reached first for your use? |
| Coupling (AC or DC) | How it joins your panels and home | Does it keep or replace your solar inverter? |
| Backup output (kW) and scope | What runs in a power cut, if anything | Which circuits, and is backup included? |
| Installation location ratings | Whether it is designed for outdoors, a garage or indoors | Does it fit the spot PAS 63100 allows in your home? |
Usable before nominal
The headline size of a battery is usually its nominal capacity. Most batteries hold back part of it, so the usable capacity, the energy you can draw out, is a little lower. MCS records nominal capacity, and DESNZ's statistics use it, which is fine for counting batteries but not for sizing one. Two batteries sold as "10 kWh" can let you use noticeably different amounts. Always ask for the usable figure in writing.
Power in kW, not only energy in kWh
The continuous power rating says how much the battery can supply at once. An evening when the oven, kettle and shower pump overlap can ask for more than that, in which case the grid supplies the rest even though the battery still has energy. For backup in a power cut the backup output rating matters in the same way. The inverter's rating also helps decide whether your system can be notified under G98 or needs a G99 application. The kW and kWh guide works through it.
Round-trip efficiency
No battery gives back everything put into it. The round-trip efficiency on the sheet is the share that comes back. It matters little for a battery that stores spare solar, and more for one charged from the grid every night, because those losses are on energy you bought. See the efficiency guide.
Warranty energy, worked in kWh
Most battery warranties have two limits: a number of years, and a total amount of energy (or a number of cycles) the battery may put through. Which? notes battery systems often come with a 10-year warranty. To compare two warranties, turn both into energy:
- If the warranty states throughput in MWh, multiply by 1,000 for kWh.
- If it states cycles, multiply the usable kWh by the cycles. That ignores the slow loss of capacity with age, so treat it as the most generous reading.
Then compare that total with your own use. A battery cycled once a day uses roughly its usable size each day; divide the warranted energy by that to see whether the energy limit or the years limit would be reached first. Lifespan and warranty goes further. Turning those kWh into a cost is a job for a price guide, not this one.
Paperwork that should come with any quote
- The installer's MCS registration showing battery storage (MIS 3012), which is separate from solar PV.
- Which network route will be used (G98 or G99) and who submits it.
- Where the battery will go, and how that meets PAS 63100.
- Confirmation that the electrical certificate will be issued under BS 7671 as amended in 2026, with a PAS 63100 statement of conformity.
Lines set by the fire-safety specification
Some of what matters is fixed by PAS 63100 rather than by the product brochure, and it usually sits in the installation instructions rather than on the front of the datasheet:
- Arc flash incident energy. The maker has to assess the risk of arc flash, fire and explosion and declare the incident energy, in cal/cm², in the installation instructions. Above 8.0 the battery is not suitable for a home at all. From 4.0 up to 8.0 it must be able to split into isolated blocks each under 4.0, sit in an enclosure rated to at least REI 60, and carry a recommendation for interlinked fire detection. From 1.2 up to 4.0, the cells must be reachable only with a tool (6.3).
- The enclosure. Non-combustible, such as steel, with the overcurrent protection inside; opened only with a tool; a top surface rated at least IP4X; and no more than 20 kWh in any one enclosure (6.1).
- Product standards. For lithium-ion, the enclosure assembly to BS EN IEC 62485-1, 62485-5 and 62933-5-2, and the cells to BS EN IEC 62281; the inverter to BS EN 62109-2. The system must also include a battery management and monitoring system (5.1, 6.2, 6.4, 7.1).
You don't have to judge these yourself. Ask for written confirmation that the proposed battery meets them, and note its declared arc flash figure, because that also bears on where the battery can go.
What MCS does and does not check
MCS keeps a directory of certified products, and under its scheme rules only products listed there may be installed, with one exception: battery storage. A battery missing from that directory therefore tells you nothing either way, and its datasheet and the installer's written confirmation carry more of the weight. The same rules say every installed product must be new and unused, and that MCS covers electrical technologies, battery storage included, up to 50 kW.
The installer's certification has a scope as well. MCS assesses battery installers against the classes in the battery installation standard, MIS 3012: limited scope covers classes 1 to 3, and full scope adds class 4, which requires a class 4 system to have been assessed. Ask which class the proposed system falls into, and check the installer's certification covers it.
Comparing two quotes side by side
Put the two specification sheets next to each other and write down, for each: usable kWh, continuous kW, backup kW and scope, round-trip efficiency, warranty years and warranted energy, coupling type, and the location the installer proposes. Most differences between "the same size" become obvious. Questions about whether one is worth more money than the other are about price; this guide stops at what each system can do.
If coupling is the open question, storage for existing panels compares AC and DC. If backup is, backup in a power cut has a runtime estimator. And if you are still working out the size, start from what size battery.
What this page is based on
- MCS Installer Operating Requirements (2026), technologies and use of the MCS mark, PDF (read 2 Nov 2026)
- Which? guide to solar panel battery storage (secondary, updated 30 Oct 2026) (read 3 Nov 2026)
- IET and BSI announcement of Amendment 4 (2026) to BS 7671, the Wiring Regulations (read 3 Nov 2026)
- PAS 63100:2024, fire protection for battery energy storage in dwellings (BSI, sponsored by DESNZ), PDF (read 3 Nov 2026)
- MCS guidance for installers on telling the network operator (G98 and G99) (read 3 Nov 2026)
- DESNZ statistics on home batteries registered with MCS in existing homes (workbook to March 2026) (read 3 Nov 2026)
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