A solar battery stores electricity. It does not make any. Panels make it while the sun is on them; the battery holds some of that energy, or some grid energy, and gives it back later at a limited speed. If the quote you are holding does not separate “how much it holds” from “how fast it can deliver” from “what still works when the street is dark”, the rest of the brochure is hard to use.
Updated October 2026.
Kilowatt-hours are the tank
Energy Advice, a UK guide, puts it in plain units. Kilowatt-hours (kWh) are how much electricity the battery can hold. A higher number covers more of the evening, or more hours of a modest backup load. The number that matters is usable capacity. Makers leave a slice unused so the cells are not emptied or filled to the edge on every cycle. If the sheet says 10 kWh and the usable figure is 9 kWh, design around 9.
A 10 kWh usable pack can supply 1 kW for about ten hours, or 5 kW for about two hours. It cannot do both. That is the whole relationship between the tank and the speed. People get surprised because a kettle looks small and then empties an hour of storage in a few minutes.
Kilowatts are the speed
Kilowatts (kW) are how much the battery, or its inverter, can deliver at one moment. Energy Advice uses that split so a reader does not treat a large tank as a promise that every appliance can run together. A fridge, lights and a router are a small speed. An oven, a well pump or a heat pump is a large one. The tank can be big and the speed still too low for the pump.
US quotes often hide this inside a “whole home” phrase. EnergySage’s note on critical-load panels points out that a typical home uses on the order of 30 kWh a day, while many batteries store 10 to 20 kWh, and that a dryer can draw about 5 kW. The panel exists because the speed and the tank both run out if every circuit stays connected. How to choose that circuit list is the subject of what a residential storage quote is actually deciding.
A blackout is a design, not a default
This is the line UK and US pages agree on, in different words. Energy Advice says most home battery systems do not automatically provide backup in a power cut. Grid-tied solar is built to shut down when the grid does, so that it does not energise a line a crew thinks is dead. Backup, sometimes called EPS, is extra equipment and a configuration. Their three questions are the right ones to write on the quote: is backup included, which loads and for how long, and does the switch happen on its own or by hand?
PG&E describes the other case, the system that was built for backup. In an outage the house disconnects, the battery comes on, and only the circuits that were put on the backup side receive power. Both descriptions are true. They describe two designs. A battery bought to move cheap midday solar into a pricey evening can be the first design. Assume that until the paperwork says otherwise.
There is a second catch once backup exists. Solar can recharge the battery during the outage only if the islanded system is allowed to keep the panels running. At night, or under cloud, it cannot. PG&E lists “no solar charging” as one reason a backup pack runs out faster than the owner expected.
The house uses solar before it fills the battery
With panels on the roof, Energy Advice describes a usual order. The home uses solar as it is generated. Surplus fills the battery if the battery has room. What remains can export, if the connection allows it. A battery does not jump the queue and steal power the kettle is already using. It also does not create a surplus that the roof did not make. On a dark day the tank stays low unless you have chosen to charge it from the grid, which some tariffs reward and some interconnection rules limit.
Where it sits is part of the safety case
NSW tells owners to follow the manufacturer’s location rules and to keep batteries out of habitable rooms. Garages, external walls and dedicated spots are the usual answers. The chemistry still matters inside that rule: LFP and NMC are both lithium-ion, they are not the same pack, and neither is fireproof. If the sheet does not name the cathode, that lookup belongs in which solar battery chemistry is actually in the box.
Cold is a separate limit. Charging a lithium battery below 0°C plates lithium and wrecks cells, unless the pack has a heater and a controller that uses it. A garage in a freezing climate is a design input, not a surprise for February.
Five lines worth writing on the quote
- Usable kWh, and continuous kW, written apart.
- Whether backup is in this design, and whether it switches itself.
- The circuit list, and the one large load that might join it.
- Whether the panels keep charging the battery while the grid is down.
- The cathode, the allowed room, and the warranty end point (years, cycles, or throughput).
Pages behind those lines
UK units and the backup warning: Energy Advice, how home battery storage works. US critical-load arithmetic: EnergySage on critical load panels. What a backup-equipped system does in an outage: PG&E residential battery storage. NSW on chemistry labels and siting: Guide to owning a battery. Why a battery is storage and pumped hydro still dominates grid-scale storage: US Department of Energy, solar and storage basics.
