Home Battery Backup Sizing Calculator
Size a home battery backup system from essential load wattage and desired backup hours.
Shows required kWh capacity and equivalent battery bank size.
Battery backup systems range from a small UPS for a router and modem ($50-100, 100-300 Wh) to a whole-home system that can run essentials for days (Tesla Powerwall: 13.5 kWh, $9,000+ installed).
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 essentials running” load is 500-1,000 W average draw. At 1,000 W for 24 hours, you need 24 kWh at 80% DoD — almost two Powerwall 3 units (13.5 kWh each).
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.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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