What size inverter do I need for a 100Ah battery?

A 12V 100Ah battery can safely power inverters up to about 1,000-1,200W for sustained loads. The limit comes from the battery's maximum discharge rate — most 12V 100Ah batteries handle 100A continuous (1,200W). Short surges up to 200A (2,400W) are usually fine for motor startups. For loads above 1,500W, step up to a 24V or 48V system to keep the current reasonable.

Can I use a car battery with a power inverter?

Technically yes, but a car battery (SLI type) is designed for short bursts of high current, not sustained discharge. Running an inverter from a car battery for more than 30-60 minutes risks deep discharge, which permanently damages car batteries. Use a deep-cycle battery — they are built for slow, steady discharge. A car battery deep-cycled regularly will last only 30-50 cycles before losing 50% or more of its capacity.

Does inverter size affect battery drain?

The inverter draws power based on the load connected to it, not its rated capacity. A 3,000W inverter running a 200W TV draws about 200W plus the inverter's own idle consumption (typically 10-30W). That said, larger inverters often have higher idle draws. If your loads are consistently under 500W, a smaller 1,000W inverter wastes less power on idle consumption than an oversized 3,000W unit.

Battery Size for Inverter Calc — Free

4 min read

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. For solar-powered setups, our solar battery count calculator adds panel sizing to the equation.

Battery bank connected to an inverter through fused cables for load sizing.

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.

Battery-to-inverter system diagram showing cable path, fuse location, and DC disconnect. — A properly wired inverter system includes a fuse at the battery, a DC disconnect switch, and short heavy-gauge cables to minimize voltage drop.

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.

Worked Examples

Sizing a Battery Bank for a Power Outage Kit

Context

You want 8 hours of backup for essential loads: fridge (100W avg), router (15W), LED lights (30W), phone charging (10W) = 155W total. Using 12V LiFePO4 batteries at 80% DoD.

Calculation

Total energy: 155 W x 8 hrs = 1,240 Wh

After inverter loss: 1,240 / 0.88 = 1,409 Wh from batteries

Battery Ah: 1,409 / (12 x 0.80) = 146.8 Ah minimum

Interpretation

A single 200Ah 12V LiFePO4 battery covers this with 25% headroom. A 100Ah battery falls short by about 50 Ah.

Takeaway

After sizing the battery, check that your inverter can handle the startup surge from the fridge compressor. Use our amps draw calculator to convert your wattage to amps on the AC side.

Battery Bank for a 3000W Inverter in a Workshop

Context

Your off-grid workshop runs power tools drawing 2,000W average for 4-hour sessions. You use a 3,000W inverter on a 24V system at 80% DoD.

Calculation

Energy: 2,000 x 4 = 8,000 Wh

After inverter loss: 8,000 / 0.90 = 8,889 Wh

At 24V, 80% DoD: 8,889 / (24 x 0.80) = 463 Ah

Interpretation

You need about 460Ah at 24V — that is two 200Ah batteries in parallel plus one more, or five 100Ah units. This is a serious battery bank.

Takeaway

For this load level, check that your cables from battery to inverter are thick enough. Verify there is no excessive voltage drop over the cable run at 2,000W on 24V.

Frequently Asked Questions

Glossary

Inverter Efficiency

The percentage of DC battery power that reaches your AC devices. Typical pure-sine inverters lose 8-15% in the conversion. A 2,000W load draws roughly 2,250W from the batteries after accounting for this loss.

Continuous vs Surge Rating

Inverters have two ratings: continuous (sustained load) and surge (brief spikes like motor starts). Always size the inverter's continuous rating above your total load, and the surge rating above any motor starting current.

Backup Runtime

The number of hours a battery bank can sustain a given load before reaching the minimum safe discharge level. The target runtime drives the entire system sizing — batteries, charger, and solar array.

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. Once you have the battery capacity nailed down, our inverter cable sizing guide walks through selecting the right wire gauge for high-current DC connections.

Last updated: March 15, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

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.