Home Battery Backup Calculator

Select the appliances you want to run during an outage and your battery setup to see how long your backup power will last.

Battery Setup

550100
1510
50%75%100%

Appliances to Power

Load: 290W

Appliance Power Usage Reference

Typical wattage for common household appliances. Actual wattage varies by model — check your appliance's nameplate label for exact ratings.

Appliance Typical Watts Daily kWh (est.) Notes
Refrigerator150W1.4Cycles on/off; ~35% duty cycle
LED Lights (10 bulbs)100W0.8~10W per bulb, 8 hrs/day
WiFi Router15W0.4Runs 24/7
Phone Charger25W0.1~2-3 hours to charge
TV120W0.755" LED TV, 6 hrs/day
Laptop65W0.58 hrs/day usage
Central AC3,500W14.0~40% duty cycle in summer
Window AC1,200W4.8~40% duty cycle
Electric Heater1,500W12.0Space heater; runs intermittently
Microwave1,200W0.2~10 min/day usage
Washing Machine500W0.5~1 hour per load
Sump Pump800W1.6Cycles during rain; ~2 hrs/day
EV Charger (Level 2)7,200W36.0240V/30A; 5 hrs for ~150 mi range
Well Pump1,000W2.0Cycles on demand; ~2 hrs/day

How This Calculator Works

1

Set Your Battery

Enter your battery capacity, number of units, and depth of discharge. Or use the comparison table to see popular battery models.

2

Select Appliances

Check the appliances you plan to run during a power outage. The calculator sums their wattage for total load.

3

See Backup Duration

Get your estimated backup time in hours and days, plus see how popular batteries compare for your specific load.

Frequently Asked Questions

How long will a Tesla Powerwall last during an outage?
A Tesla Powerwall 3 stores 13.5 kWh of usable energy. Running essential loads only (refrigerator, lights, WiFi, phone charger — about 290W total), it lasts approximately 46 hours. With a window AC unit added (1,490W total), runtime drops to about 9 hours. Running a full house with central AC (4,000W+) drains a single Powerwall in roughly 3 hours. Adding multiple Powerwalls (up to 4) extends runtime proportionally.
What appliances should I prioritize during a power outage?
Prioritize in this order: (1) Refrigerator/freezer to prevent food spoilage (~150W). (2) WiFi router for communication (~15W). (3) Phone chargers (~25W). (4) Basic lighting (~100W for 10 LED bulbs). (5) Medical equipment if needed. These essentials total about 290W and allow a 10 kWh battery to last 30+ hours. Avoid running high-draw appliances like central AC (3,500W), EV chargers (7,200W), or electric heaters (1,500W) unless you have ample battery capacity.
How do I calculate battery backup time myself?
Use this formula: Backup Hours = (Battery Capacity in kWh × Depth of Discharge × Inverter Efficiency) ÷ Total Load in kW. For example, a 13.5 kWh battery at 90% DoD with 96% inverter efficiency powering 500W of loads: (13.5 × 0.90 × 0.96) ÷ 0.5 = 23.3 hours. This calculator uses 90% DoD by default and factors in a 96% inverter efficiency for realistic estimates.
Does running solar panels during the day extend battery backup?
Yes. If your solar system is paired with a hybrid inverter, solar panels will recharge your battery during daylight hours, significantly extending backup duration. On a sunny day, a 6-8 kW solar array can produce 30-40 kWh — enough to power essential loads and fully recharge most batteries. This calculator shows battery-only runtime without solar input, so actual backup duration with solar will be longer.
Why does the calculator use 90% depth of discharge?
Most battery manufacturers recommend not fully draining the battery to 0% to preserve long-term battery health. While many modern LFP batteries support 100% DoD, a 90% setting provides a conservative, realistic estimate that accounts for inverter conversion losses (~4%) and BMS overhead. You can adjust the DoD slider to 100% if your battery (like Tesla Powerwall 3 or Enphase IQ) supports full discharge.

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