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.