Batteries

The figure written on a battery — 5 or 12 kilowatt-hours, say — is the total energy stored in it, not the amount you can draw without shortening its life. What is genuinely available to you is that figure multiplied by the battery's permitted depth of discharge.

This alone makes two quotes with identical nameplate capacity two different things in practice. When comparing offers, ask for the permitted depth of discharge and work out the usable capacity; comparing the label figures alone is comparing two unrelated numbers.

Lead-acid costs less to buy but tolerates a shallower discharge and offers fewer cycles, so reaching the same usable capacity means buying more nameplate capacity and replacing it sooner.

Lithium iron phosphate is thermally more stable, offers far more cycles, and needs no topping up or periodic service. For a battery cycled every night that difference is decisive; in an application used a few times a year it shows up much less.

Work out two numbers separately. First: how many kilowatt-hours have to move from daylight into the evening — that comes from the bill and the household's evening usage. Second: what has to stay on during an outage and for how long — that comes from the list of critical loads.

The larger of the two is the usable capacity you need. Nameplate capacity follows from it via the depth of discharge, not the other way round. If someone proposes a capacity without seeing a bill and without asking what has to stay on, that figure is a guess.

Battery and inverter have to agree on working voltage, and for the inverter to read state of charge and cell temperature, the battery management system needs a communication protocol they share.

Without that link the system still runs, but blind: the inverter does not know the battery's charge level and estimates its protections from voltage instead. When buying battery and inverter separately, confirm this compatibility before ordering.