Home Battery Backup Sizing Calculator

Battery backup runs from a desk UPS holding up a router and modem for twenty minutes, at a few hundred watt-hours, all the way to a whole-home system carrying the essentials for days. A Tesla Powerwall 3 stores 13.5 kWh. Installed prices vary enormously by region, installer and whatever incentives are running, so get quotes rather than trusting any figure quoted online, including here.

Sizing logic

Required energy (Wh) = total_load_watts x backup_hours

Because batteries should not be discharged to 0% (it degrades them and some capacity is reserved), you need more nameplate capacity than your required energy:

Battery capacity needed (Wh) = required_energy / (depth_of_discharge / 100)

Lithium iron phosphate (LFP) batteries like the Powerwall can be discharged to 80-90% DoD regularly. Older lead-acid batteries should not exceed 50% DoD for longevity.

Essential loads to consider

Refrigerator: 100-200 W average. Chest freezer: 30-100 W average. Well pump: 750-1,500 W (intermittent). Furnace fan: 300-600 W. Internet/router: 10-30 W. LED lighting: 5-15 W per fixture. Phone chargers: 5-20 W total. Medical devices: varies — CPAP is 30-60 W.

A typical “keep the essentials alive” load averages 500 to 1,000 W. Work a full day at the top of that: 1,000 W for 24 hours is 24 kWh of energy, but at 80% depth of discharge you need 30 kWh of nameplate capacity to deliver it. Those are different numbers and mixing them up is the standard sizing mistake. Thirty kilowatt-hours is 2.2 Powerwall 3 units, so in practice three.

Halve the load to 500 W and the same three units carry you two full days, which is why trimming the essentials list is far cheaper than adding batteries.

Inverter sizing

Battery systems require an inverter rated for your peak surge load, not just the continuous average. A well pump starting surge can be 3-5x its running watts. The inverter must handle these surge loads without tripping, even if the battery average draw is much lower.