Home Battery Backup Calculator

A home battery advertised at 13.5 kilowatt-hours does not give you 13.5 kilowatt-hours of useful electricity, and the gap is larger than most buyers expect. Two separate deductions apply, and they multiply rather than add.

The first is depth of discharge. Every lithium battery is managed by electronics that refuse to take the pack below a certain state of charge, because deep discharges shorten cell life dramatically. Lithium iron phosphate chemistry, which most current home batteries use, tolerates deeper cycling and is often specified at 95–100% usable. Older nickel-manganese-cobalt packs commonly reserved 10% or more.

The second is round-trip efficiency. Pushing energy into cells and pulling it back out loses some as heat, and converting between the battery’s direct current and the house’s alternating current loses more in the inverter. A modern AC-coupled home battery returns somewhere around 86–92% of what went in. At 90% depth of discharge and 90% round-trip efficiency, a 13.5 kWh nameplate delivers 10.9 kWh — about 81% of the number on the box.

Capacity, measured in kilowatt-hours, tells you how much total energy the battery holds. The inverter’s continuous rating, measured in kilowatts, tells you how fast that energy can come out. They constrain completely different things and a system can be generously sized on one and hopeless on the other.

A 5 kW inverter can run about 5,000 watts of load at any instant, regardless of whether it is attached to a 10 kWh battery or a 40 kWh one. An electric oven at 3,000 W, a well pump at 1,500 W and a kettle at 1,500 W together exceed it, and the system will either shed load or trip — while the battery sits half full.

Motor starting makes this sharper. An induction motor draws several times its running current for the fraction of a second it takes to get spinning, so a 800 W well pump can momentarily demand well over 2,000 W. Many inverters tolerate a brief overload above their continuous rating precisely for this reason, but the margin is finite, and this is the failure people actually encounter: the battery is fine, the fridge starts, and something else drops out.

There are two ways to wire a battery into a house. Whole-home backup connects the battery behind the main panel so everything in the house can run from it. Partial-home backup — an essentials panel, sometimes called a critical loads panel — moves a chosen subset of circuits onto a small sub-panel that the battery feeds, and leaves the rest dark during an outage.

Whole-home backup sounds obviously better and is often the wrong purchase. It requires an inverter large enough for whatever anyone might switch on, which usually means several batteries in parallel rather than one, and it invites exactly the behaviour that flattens a pack in six hours: running the dryer during a blackout because nothing stopped you.

An essentials panel forces the decision at installation time instead, when it can be made calmly. The circuits that almost always belong on it are the refrigerator, some lighting, the network gear, and the furnace or boiler controls — the last of which is the one people forget, and the one that matters most in a winter outage, because a gas furnace with no electricity for its blower and ignition is just a large metal box.

A battery on its own is a fixed store of energy that runs down. A battery paired with solar that stays live during an outage is a buffer that refills every morning, and the difference is not incremental — it is the difference between a number of hours and an indefinite period.

The threshold is lower than most people assume. The example on this page draws 9.44 kWh a day, which a 3 kW array in reasonable sun will produce comfortably. Above that line the battery never empties; below it, solar simply extends the runtime by covering part of each day’s draw.

One critical caveat applies. Ordinary grid-tied solar shuts down the moment the grid goes out, by design and by law — an inverter feeding power into lines that utility crews believe are dead is a serious hazard, and anti-islanding protection prevents it. Solar keeps working during an outage only if the system was specifically designed for it, with a battery and an inverter capable of forming its own grid. Plenty of households have discovered on the first outage that their panels do nothing.

Every wattage on this page is a typical figure, and typical figures for cycling appliances are the least reliable kind. A refrigerator’s nameplate might say 600 W, but it only draws that while the compressor runs, which on a modern unit is perhaps a quarter of the time — so the number that belongs in an energy calculation is closer to 150 W.

A plug-in energy meter costing very little resolves this properly: leave it on the fridge for 24 hours, read the kilowatt-hours, and divide by 24 to get the true average watts. For hard-wired loads such as a furnace blower, the blower motor’s nameplate amperage times the supply voltage gives a reasonable upper bound, and the equipment’s installation manual usually states the actual figure.

For anyone with a whole-home energy monitor, the better approach is simply to read what the house drew overnight during a period when nothing unusual was running. That number already includes every parasitic load — the standby draw of a dozen devices nobody thinks about — which itemised estimates routinely miss and which can easily total 100 W on its own.