Understanding the battery on your solar quote: capacity, power and model names

A quote that says only “10kWh battery” leaves questions unanswered. Two offers with that description can provide different amounts of usable storage, different power output and different installation features.

What do kWh and kW mean?

kWh tells you how much energy is stored. kW tells you how quickly it can be supplied.

For example, an appliance drawing a steady 1kW for one hour uses 1kWh. The same appliance running for half an hour uses 0.5kWh.

For a battery, check the continuous power output: what the installed system can keep supplying, rather than a brief peak. A battery might hold plenty of energy but be unable to run several demanding appliances together without help from the grid.

Charging power can be different from output power. Ask for both if the quote depends on charging the battery within a short cheap-rate window.

A worked example: reading the battery part of a quote

Fictional example. The manufacturer, model and figures below are invented to explain how to read a quotation. They are not a product recommendation or a typical-market specification.

Manufacturer

Example Battery Co — fictional

Who makes the battery

Exact model

HomeStore 5-UK-A — fictional

The particular version being offered

Quantity

Two battery units

How many units are included

Nominal capacity

5kWh each; 10kWh total

The stated storage before the usable limit

Usable capacity

4.5kWh each; 9kWh total

The stated accessible storage, measured at the batteries before conversion for household use

Continuous output

3kW for the complete system

The combined output through the shared inverter, not 3kW from each battery

Backup

Not included

It will not supply the home during a power cut

Warranty

“10 years”; detailed terms not supplied

A headline that still needs checking

Here, two units give 9kWh of stated usable storage. They do not give 6kW of output: the quotation specifies a 3kW limit for the whole system.

On your own quotation, ask the installer to fill in missing figures. “Not stated” does not mean zero, and it does not mean a feature is included.

Nominal and usable capacity: which should you compare?

Nominal capacity is the battery's stated storage rating before any separate usable limit is taken into account. Usable capacity describes how much is available within its operating limits. They can differ because part of the storage is kept outside normal use to protect the battery.

In our fictional example, each unit has 5kWh nominal capacity but 4.5kWh usable. Compare usable figures on both quotations, rather than a nominal figure on one and a usable figure on the other.

You may see depth of discharge: how much of the battery's capacity has been used. A maximum depth-of-discharge limit sets how much may be used. Do not assume the same limit applies to every model or subtract it again from a figure already labelled usable.

Batteries don’t always deliver every bit of their stored electricity to your home. Some energy is lost when it is converted into the electricity your appliances use. When comparing two batteries, make sure the figures are measured in the same way.

A figure measured at the battery may exclude these conversion losses. A figure measured as electricity leaves the system for your home takes them into account. Datasheets may call these the DC side and AC side. Ask the installer to confirm which figure you are comparing; don’t subtract a guessed loss percentage.

Usable capacity is not a promise of exactly that much electricity every day. Temperature, age, operating settings and energy reserved for a power cut can affect what is available. A user-selected backup reserve is separate from the battery's protective operating limit.

Why the exact model matters

A brand name or product family is not enough. Ask for the full model code, including any letters or numbers at the end, and the number of units or modules included.

Find the manufacturer's own website, choose its UK documents and match the code to the datasheet. Check the document date or version and read the footnotes beside the figures. If a model is older, ask for the document that applies to that model rather than a newer replacement.

Check which version the datasheet covers

One datasheet may cover several versions or output settings. If the code contains placeholders such as “xx”, it may describe a family rather than the complete model on your quote. Ask the installer to identify the applicable version and settings. Do not assume that the highest figure in a table is what you are buying.

What happens when you add more battery units?

More compatible units can add storage. Check whether the quote gives capacity per unit or for the whole system, and whether “three modules” means three complete batteries or three parts of one battery stack.

Do not assume that doubling storage doubles power output. The inverter — the equipment that converts electricity for household use — and the approved system configuration can still limit it. Extra units must be supported by the manufacturer.

Does every battery work with every inverter?

No. Some batteries need particular compatible inverter models. Others have their own conversion equipment. A description such as “hybrid inverter” does not confirm that a particular pairing will work.

Ask the installer to confirm the exact battery/inverter combination against the manufacturer's current compatibility documents, including the proposed number of modules. Software versions and country-specific conditions may matter.

For an existing solar system, ask whether the battery works with your current inverter or needs additional equipment. Get that equipment and work included in the written price.

Will it keep the lights on during a power cut?

Only if the installed system is set up to do so. A battery can work normally while the grid is available yet provide no household backup when it fails. Some systems are sold in both backup and non-backup configurations.

Ask which circuits or sockets would work, what could run together and whether switching is automatic. Confirm that any required backup equipment and installation work are included.

How long backup lasts depends on the charge available when the cut starts and what you run. Also ask whether the solar panels can recharge the battery during the cut; that depends on the configuration.

What does a battery warranty actually cover?

“Ten-year warranty” is only the starting point. Ask for the full terms that apply to the exact model and UK installation.

Check for a promise about remaining capacity, not just faults. A warranty may allow storage capacity to fall over time. It may also limit cycles (charging and discharging the battery) or throughput (the total energy put through it), depending on how it is used. The terms should explain how any limit is counted.

Find out whether registration, internet access, particular operating conditions or an approved installer are required. Ask who handles a claim and who pays for diagnosis, labour, removal and refitting. The equipment warranty and the installer's workmanship cover are separate things to confirm.

Keep the quotation, exact equipment list and applicable warranty documents together. A long headline warranty is not proof that every repair cost is covered.

How to compare two battery offers

Here is a second fictional offer beside the first. Both usable figures are measured at the batteries, before conversion for household use.

Offer B stores more electricity. Offer A can supply electricity at a higher rate. Neither offer gives enough warranty information. “Not stated” also leaves B's backup arrangements unresolved.

There is no automatic winner. Ask how each system fits your electricity use and compare the complete installed prices and included work.

For the financial side, use Fairbeam's battery savings calculator, which uses example assumptions, or read solar panels with or without a battery.

Five checks before you sign

These questions can be copied into an email to the installer providing your quote:

1. What battery am I getting?

   “What are the full battery and inverter model codes, and how many units or modules are included? Please send the matching UK datasheets and confirm compatibility.”

2. How much electricity can I actually use?

   “What is the total usable capacity of this complete system? Where is it measured, and what reserve or operating limits affect everyday use?”

3. How much power can it supply?

   “What continuous charging and output power will my installed system have? Are the figures for the whole system or each unit?”

4. Will it work during a power cut?

   “Does this price include power-cut backup? Which circuits will work, what can run together, and can the panels recharge the battery during a cut?”

5. What does the warranty cover?

   “Please send the applicable warranties, explain any capacity or usage limits, and confirm who pays for fault-finding and replacement work. What installation work or equipment is excluded from the price?”

The common mistakes are comparing kW with kWh, multiplying a system-total figure again, treating nominal capacity as usable, and assuming backup or full repair cover is included. Clear written answers make the quotations much easier to compare.