An undersized battery bank leaves you without power when you need it most. An oversized one wastes money. This calculator works backwards from your load and desired runtime to tell you exactly how many amp-hours your battery bank needs — factoring in inverter efficiency and safe depth of discharge.
How to Size a Battery Bank for Your Inverter
- List every device you will run. Write down each device and its wattage. Include items that cycle on and off (like refrigerators) at their average wattage, not peak. Add a 10% buffer for devices you forgot.
- Determine your runtime need. For power outage backup, 4-8 hours covers most grid outages. For off-grid cabins, 12-16 hours covers overnight loads. For critical medical equipment, plan for 24+ hours with a recharging source.
- Choose your battery voltage. Match the inverter input. 12V systems are simplest for under 2,000W. 24V and 48V systems are better for larger loads because they halve or quarter the current, reducing cable size and losses.
- Apply the formula. Required Ah = (Load x Runtime) / (Voltage x DoD x Efficiency). A 500W load for 4 hours on a 12V system at 80% DoD and 88% efficiency: (500 x 4) / (12 x 0.80 x 0.88) = 237Ah. Round up to the nearest available battery size — a 300Ah battery bank gives you headroom.
Example: Sizing a Home Backup Battery
A homeowner wants to keep essentials running during a 6-hour power outage using a 3,000W pure sine wave inverter on a 48V system.
Essential loads: refrigerator (150W avg), internet router (15W), LED lights (60W), phone chargers (30W), sump pump (400W average, runs 20% of the time = 80W effective). Total: 335W.
Required energy: 335W x 6 hours = 2,010Wh. Factor in inverter efficiency (90%): 2,010 / 0.90 = 2,233Wh from the battery. At 48V with 80% DoD: 2,233 / (48 x 0.80) = 58Ah at 48V.
A single 48V 100Ah LiFePO4 battery (4,800Wh total, 3,840Wh usable) handles this with plenty of room. The homeowner could actually run these loads for over 10 hours — or add a space heater for part of the outage.
Frequently Asked Questions
Need the right cable between your battery bank and inverter? Our <a href="/electrical/inverter-cable-size-calculator">inverter cable size calculator</a> ensures safe wiring at high currents.
Size your battery bank for reality, not marketing specs. The calculator gives you the raw number — round up to the next standard battery size and you will have a reliable system with headroom for aging, cold weather, and the devices you inevitably add later.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.