Battery Backup Calculator

How long will your battery last during an outage? Set your loads by priority — get backup hours for each tier.

Battery & inverter

Battery capacity?

kWh

Inverter efficiency?

Depth of discharge?

Essential loads

Click the priority badge to toggle: Critical → Important → Optional

Appliance	Watts	Priority
Refrigerator	W
Lights (LED x5)	W
Phone/laptop charging	W
Internet router	W
TV (50 in)	W
Sump pump	W
Window AC (5000 BTU)	W

Backup duration

1 day 3 hr (critical loads only)

Usable battery energy9,000 Wh

Critical loads total315 W → 1 day 3 hr

Critical + important loads815 W → 10 hr 29 min

All loads running1,315 W → 6 hr 30 min

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How to Use This Calculator

Enter battery capacity and inverter efficiency

Start with your total battery capacity in kWh. This is the nameplate capacity of your battery or battery bank — for multiple batteries, add them together. Enter your inverter's efficiency (typically 93-97% for quality pure sine wave inverters) and the depth of discharge you'll operate at. LiFePO4 batteries can run at 90% DoD; lead-acid should stay at 50%.

Set load priorities

The load table shows common appliances with editable wattage and a priority badge. Click the priority badge to cycle through Critical, Important, and Optional. Edit the wattage values to match your actual appliances. The calculator shows backup duration for each priority tier — this is your outage management plan.

Read the three tiers

The result shows backup time for three scenarios: critical loads only (fridge, lights, phones, router), critical + important loads, and all loads running simultaneously. Use the critical-only figure when planning for multi-day outages, and all-loads for short outages.

Use scenario buttons

The scenario buttons pre-fill realistic configurations: Essentials Only (10 kWh, minimal loads), Home Comfort (20 kWh, most loads), and Full Home (30 kWh, everything including AC).

The Formula

Usable Wh = Battery kWh × 1000 × DoD (%) Load on battery = Appliance watts ÷ Inverter efficiency Backup hours = Usable Wh ÷ Total load on battery

The inverter efficiency adjustment is important: a 500W load on a 95% efficient inverter actually draws 526W from the battery (500 ÷ 0.95). This "battery-side" draw is what depletes your stored energy. Skipping this step overestimates backup time by 5-15%.

Example

The Martinez family — Hurricane preparedness

The Martinez family in Florida wants to know how long their 13.5 kWh battery (Tesla Powerwall) will last during a hurricane outage. They have a 97% efficient inverter and operate at 90% DoD.

Battery capacity13.5 kWh

DoD90%

Usable energy12,150 Wh

Inverter efficiency97%

Load scenarios

Critical only (fridge + lights + phones)315W → 37 hours

+ sump pump + TV815W → 14.5 hours

Everything including AC1,315W → 9 hours

Running only critical loads, the Powerwall lasts 37 hours — through a typical overnight outage and well into the next day. Adding the sump pump (important during flooding) cuts it to 14.5 hours. Their hurricane strategy: run critical loads only, enable the sump pump only when it activates, skip the AC.

FAQ

How many kWh do I need for 24 hours of backup power? ▼

It depends on your loads. For a typical home running fridge, lights, and basic electronics (~400W total, ~10 kWh/day), you'd need about 11 kWh at 90% DoD. To run central AC on backup you'd need 30-50 kWh — impractical for most battery systems. The key insight: shed your highest-draw loads (AC, electric water heater, EV charger) and your backup duration multiplies dramatically.

What loads should I consider "critical" for backup power? ▼

Critical loads are things that affect health, safety, or food security: refrigerator, medical devices (CPAP, oxygen concentrator), phone/communication charging, basic lighting, sump pump if flooding risk, well pump if on well water, internet router if needed for work/emergency. Everything else — TV, AC, dishwasher, washing machine — is deferrable and should be treated as optional or important, not critical.

Does a refrigerator run at full wattage continuously? ▼

No — refrigerators cycle on and off. A 150W refrigerator might only run its compressor 40-50% of the time, so actual average draw is 60-75W. This calculator uses the running wattage for worst-case planning, which is conservative. In practice, your backup duration will be longer than calculated because cycling loads run less than 100% of the time. For a tighter estimate, enter the average watt draw instead of the nameplate running watts.

How do I size a battery for X hours of backup? ▼

Rearrange the formula: Required kWh = (Total watts × Hours) ÷ (Inverter efficiency × DoD × 1000). Example: 400W loads for 24 hours at 95% efficiency and 90% DoD = (400 × 24) ÷ (0.95 × 0.90 × 1000) = 9,600 ÷ 855 = 11.2 kWh battery needed. Always round up and add 10-20% margin for cold-weather derating and aging. Use our Solar Battery Calculator for full system sizing.

Will my solar panels recharge the battery during an outage? ▼

Yes, if you have a battery-backed solar system (not a basic grid-tie system, which shuts down during outages for safety). With a hybrid inverter or off-grid setup, your solar panels will recharge the battery during the day, potentially extending your backup indefinitely if solar production covers your daytime load. A 6-8 kW solar system can fully recharge a 10-15 kWh battery on a sunny day. Use our Days of Autonomy Calculator to plan for cloudy periods.