Suburban home — 900 W critical for 18 h × 2 days, no solar
900 × 18 = 16.2 kWh/day × 2 ÷ (0.90 × 0.90) ≈ 20 kWh battery → roughly 1.5× Tesla Powerwall 3 (13.5 kWh each).
Home battery backup calculator
How big a battery do I need for a power outage?
Add up critical loads (fridge, well pump, internet, a few lights), pick how many hours per day you need them, set a target number of backup days, and the calculator returns the battery bank size and how much solar recharge offsets the draw each day.
Home backup calculator
Outage coverage planner
Estimated coverageSolar positive
Daily load16.2 kWh
Daily solar20.7 kWh
Usable battery10.9 kWh
Target coverage161%
How the math works
How the math works
- Daily critical kWh = Critical watts × Hours per day ÷ 1000.
- Required battery kWh = Daily kWh × Backup days ÷ (Battery DoD × Inverter efficiency).
- Solar recharge per day = Solar kW × Peak sun hours × Performance factor (default 0.82, matching 18% system loss).
- Net daily draw = Daily kWh − Solar recharge. If negative the system is solar-positive and only needs enough storage for the worst-case overnight.
Worked examples
Worked examples
Same loads, 5.6 kW solar at 4.5 PSH
Solar recharge ≈ 5.6 × 4.5 × 0.82 = 20.7 kWh/day. Net daily draw is negative; one Powerwall (13.5 kWh usable) covers overnight comfortably.
Off-grid cabin essentials — 450 W for 12 h × 3 days
450 × 12 = 5.4 kWh/day × 3 ÷ (0.9 × 0.9) ≈ 20 kWh LiFePO4 stack, ideally paired with 3 kW PV.
Home battery backup FAQ
Home battery backup FAQ
How do I figure out my critical loads?
Walk the breaker panel and identify must-have circuits: refrigerator (150 W avg), well pump (1000–2000 W intermittent), furnace blower (500 W), internet (40 W), a few LED lights (50 W). Sum the steady-state watts.
Do I need a transfer switch?
Yes for whole-home backup. Tesla Powerwall, Enphase IQ, and Franklin aPower ship with smart load centers. DIY stacks need a manual or automatic transfer switch installed by a licensed electrician.
Can solar alone keep me powered?
Only with batteries. Grid-tied solar without batteries shuts down during outages to protect line workers. AC-coupled batteries or hybrid inverters are required for daytime backup.
How many Powerwalls do I need?
For partial backup (essentials + fridge) most homes use 1 Powerwall (13.5 kWh). Whole-home with AC and EV charging usually needs 2–3 stacked. Use the calculator to confirm.
What's a realistic discharge depth?
LiFePO4 systems (Powerwall 3, Enphase IQ, Franklin) cycle to 100% safely. The 90% default in the calculator reserves headroom for cold-weather de-rating and end-of-life capacity loss.
Calculator directory
Choose the right EV, solar, and battery calculator
EV charging time calculator
Enter your battery capacity, starting and target state of charge, and charger output to estimate charging time, the kWh pulled from the wall, and your cost per session — all from open, transparent formulas.
Open calculatorSolar panel calculator
Size a solar PV array from your daily electricity use, local peak sun hours, system losses, and panel wattage. Also estimates roof area and an off-grid battery bank using depth-of-discharge and round-trip efficiency.
Open calculatorBattery runtime calculator
Enter your battery capacity in watt-hours, the running load, idle standby draw, usable depth-of-discharge, and inverter efficiency to estimate practical runtime for a Jackery, EcoFlow, Bluetti, APC UPS, or any custom 12 / 24 / 48 V bank.
Open calculatorHome battery backup calculator
Add up critical loads (fridge, well pump, internet, a few lights), pick how many hours per day you need them, set a target number of backup days, and the calculator returns the battery bank size and how much solar recharge offsets the draw each day.
Open calculatorEV road trip calculator
Estimate stops, charging time, and drive time from route distance, EV range, battery capacity, departure SOC, reserve SOC, target stop SOC and DC charger power.
Open calculatorSolar payback calculator
Apply the 30% federal ITC, model 0.5%/year panel degradation and 3%/year rate inflation, and see year-by-year cumulative savings until the system breaks even.
Open calculatorNet metering calculator
Compare a solar bill under traditional 1:1 net metering vs export-only credits (e.g. CA NEM 3.0). Get monthly net cost, exported kWh, self-consumption ratio, and annualized totals.
Open calculatorHeat pump vs gas calculator
Convert your annual heating demand to kWh and therms with COP and AFUE, then compare yearly fuel cost and CO₂. Uses EPA grid average 0.39 kg CO₂/kWh and 5.3 kg CO₂/therm.
Open calculatorGenerator sizing calculator
List your running watts and the largest motor (well pump, AC compressor, fridge), apply LRA-based starting surge, and add a safety factor for fuel headroom and altitude derating.
Open calculatorSolar lease vs buy calculator
Model cash purchase (with 30% ITC) versus a 25-year lease with annual escalator. Inputs: system cost, lease payment year 1, escalator %, annual production, retail rate.
Open calculatorSolar panel tilt calculator
Get the year-round optimal tilt (≈ latitude), summer tilt (latitude − 15°) and winter tilt (latitude + 15°), plus expected production gain from seasonal adjustment.
Open calculatorSolar inverter sizing calculator
NREL and SEIA recommend a DC:AC ratio of 1.15–1.30. Higher ratios cost less per AC watt but cause midday clipping. Find the sweet spot for your location.
Open calculatorWind turbine calculator
Estimate annual kWh from rotor swept area, average wind speed, air density, capacity factor and turbine efficiency. Uses the Weibull-corrected power equation.
Open calculatorEV total cost of ownership calculator
Sum purchase price (minus federal tax credit), fuel/electricity, insurance, maintenance and registration over 5 years. Default electricity 4.0 mi/kWh, gas 28 mpg.
Open calculator