Battery Backup Sizing Calculator

The model

Each load contributes its power multiplied by the hours it actually runs. For anything thermostatically controlled — a fridge, a freezer, a well pump — the hours figure is the compressor's run time, not the number of hours it is plugged in. A fridge is plugged in for 24 hours and runs for about 8.

daily kWh = Σ (watts × hours) / 1000 net daily = daily kWh − solar recharge usable kWh = net daily × days × (1 + margin) / efficiency nameplate = usable kWh / depth of discharge

Two separate derations are at work and they are often confused. Depth of discharge is how much of the nameplate the chemistry will let you take out — roughly 90% for LFP, less for older NMC packs. Round-trip efficiency is what is lost to conversion and heat on the way in and out. Both have to be applied, which is why a "13.5 kWh" battery does not deliver 13.5 kWh of useful work.

Energy is not the only constraint

A battery has two independent limits: how much energy it stores, and how much power it can deliver at any instant. A system with plenty of kWh will still trip if you start a well pump and an air conditioner together. The continuous inverter rating shown here covers everything running simultaneously, with headroom — but motor loads draw several times their running wattage for the first second or two, so check the surge rating separately against your largest motor.

Typical running wattages

The presets are mid-range figures for common equipment. Your own appliances carry a nameplate label, and a $30 plug-in meter will tell you the truth about the ones that matter. Two loads dominate almost every real backup plan: electric resistance heat and central air conditioning. If either is on your list, that decision alone sets the size of the system.

What this leaves out

No cold-weather capacity loss (lithium delivers meaningfully less below freezing unless the pack is heated), no standby draw from the inverter itself, and no degradation over the battery's life — budget for roughly 70% of nameplate after ten years. Nor does it model partial outages, load shedding, or a generator running in parallel.

Sources

The constants and default values above come from these public sources. Every one is free to read and none of them are ours — check the figures against them.

• DOE — Estimating Appliance and Home Electronic Energy Use The method behind the watts-times-hours load table, and typical wattages for common appliances.
• NREL — Behind-the-Meter Storage Research Round-trip efficiency and degradation figures for residential lithium systems.
• EIA — Residential Energy Consumption Survey Baseline household consumption, useful for sanity-checking your own load total.
• ENERGY STAR — Appliance Product Finder Nameplate power draw for specific models, better than any generic preset.