Sizing battery storage for real outages

We spend more time correcting expectations about batteries than about any other part of a solar installation. In most of those cases the system was built properly and performs exactly as designed. What went wrong happened at the quotation stage, when two different questions were allowed to collapse into one.

Power and capacity are not the same number

A storage system has a power rating in kilowatts (kW) and a capacity rating in kilowatt-hours (kWh). Power is how much you can run at the same moment. Capacity is how long you can run it. They are independent, and a system can be generous in one and thin in the other.

A system with a high power rating and a small battery will start everything you own and then stop early. A system with a modest power rating and a large battery will run through the night, but will trip if you switch on too much at once. Neither is a fault. Both are sizing decisions that should have been made deliberately.

When you compare quotations, compare both numbers. A proposal that advertises only one of them is not describing the system you will actually live with.

Start from what must stay on

The useful question is not how big a battery you can afford. It is which circuits you refuse to lose, and for how long. That list is shorter than most people assume, and writing it down usually reduces the system size rather than increasing it.

• Lighting and socket circuits for the rooms you actually occupy during an outage — often a fraction of the house.
• Refrigeration, which draws little on average but must never be interrupted for long.
• Internet router, security system and cameras. Small loads, and the ones people are angriest to lose.
• Water pump, if you have no gravity-fed storage. Brief but heavy demand.
• Air conditioning, which deserves its own decision and is treated below.

Circuits left off this list are not abandoned. They run normally whenever the grid is up. They simply do not draw on the battery, which is what keeps the system honestly sized.

Air conditioning is the decision point

Air conditioning is usually what separates a modest installation from an expensive one. A compressor draws heavily and, on older units, draws a large surge at start-up that the inverter must absorb without tripping.

Inverter-driven air conditioners change this arithmetic substantially. They ramp rather than switching hard on and off, so the surge is far smaller and the running draw settles well below the nameplate figure. If air conditioning must run on battery, replacing an old fixed-speed unit is frequently cheaper than sizing storage to tolerate it.

Usable capacity is smaller than the label

A battery is not run flat. Lithium iron phosphate systems typically make most of their nameplate capacity available, while lead-acid chemistries are conventionally limited to around half of theirs to reach an acceptable service life. Two batteries with the same figure printed on the case can therefore deliver very different evenings.

Ask for usable capacity at the manufacturer's stated depth of discharge, not the nameplate figure. It is the only number that describes what you will get.

Heat shortens battery life faster than cycling does

Cycle-life figures are quoted at moderate temperatures. Sustained heat degrades cells faster than ordinary daily use, which is why the installation location deserves more attention than it usually receives. A sealed cupboard, a roof space, or an exterior wall in afternoon sun will all shorten the life of an otherwise good battery.

Specify a shaded, ventilated location with room to work around the enclosure. It costs nothing at design stage and is disruptive to correct afterwards.