Amp-hour and battery runtime calculator

Enter your battery's amp-hours, its voltage and type, and the watts you want to run. You get the runtime in hours, the current it draws and how the runtime changes as the load goes up or down.

Inputs

Ah

Total amp-hours of the bank. Batteries in parallel add their amp-hours; batteries in series add voltage instead.

Sets how much of the battery you use, and whether fast discharge costs capacity (lead-acid only).

The average watts your devices draw. A fridge rated 150 W only draws that while its compressor runs.

Victron’s Phoenix inverters peak at 89–92% and do worse at light loads.

Battery runtime

12.3hours

12.3 hours for a 150 W load on 200 Ah at 12.8 V, using 80% of the battery.

About 12.3 hours from full to the limit for this battery type.

Usable energy
2,048 Wh
Current from the battery
13.0 A
In days
0.5
Runtime at different loads

A 150 W load on 200 Ah at 12.8 V runs for about 12.3 hours, drawing 13.0 A and using 80% of the battery. The bars show the runtime at a quarter, half, one and a half times and twice your load: 38 W lasts 2 days 1 hours; 75 W lasts 24.6 hours; 150 W lasts 12.3 hours; 225 W lasts 8.2 hours; 300 W lasts 6.1 hours.

Safety first

High-current batteries are dangerous even at 12 V. A short circuit can melt a tool, a cable or a terminal in seconds.

  • Fuse every circuit, close to the battery, and check that each fuse is no larger than the wire it protects.
  • Follow the battery and inverter makers’ instructions for maximum current, charging and connections.
  • Follow your local electrical code and the standard for your vehicle or boat.
  • Have a qualified person check the installation, especially anything above 12 V or a large inverter.

How it works

runtime (h)       = capacity (Ah) × bank voltage × share used ÷ battery watts
battery watts     = load ÷ inverter efficiency     (1.00 for a DC load)
lead-acid only    capacity = C × (C ÷ (I × 20))^(k − 1), with k = 1.25, above the 20-hour current
  • Bank voltage is 12.8 V per “12 V” lithium battery and 12 V for lead-acid, so a 24 V or 48 V bank is two or four times that.
  • Share used is 80% for lithium (the level Renogy rates 5,000 cycles at), 100% if you choose it, and 50% for AGM or gel.
  • Peukert only reduces lead-acid capacity, and only when the current is above the 20-hour rate. Below that rate the label capacity is used as it stands, as VoltCalcs does in its AGM example.

Worked example

A 200 Ah lithium battery at 12.8 V runs a 150 W AC load (a small fridge and some lights) through a 90% inverter.

  • Usable energy: 200 × 12.8 × 0.80 = 2,048 Wh.
  • Battery watts: 150 ÷ 0.90 = 167 W, which is 13 A.
  • Runtime: 2,048 ÷ 167 = 12.3 hours.

The same load direct on 12 V DC would last 13.65 hours. A 200 Ah AGM battery at 50% gives only 1,200 Wh, which is about 7 hours.

Tips from the road

  • Use average watts, not the plate rating. A compressor fridge may rate 60 W but use an eighth of that over a day because it cycles.
  • Watch for small loads. A few watts of standby use is 100 Wh a day, enough to matter on a small battery.
  • Check the battery’s discharge limit. The runtime may be long, but a battery rated for 100 A cannot run a 2,000 W inverter on 12 V.

FAQ

How long will a 100Ah battery last?

Multiply 100 Ah by the voltage and the share you use, then divide by the watts. A 100 Ah lithium battery (1,280 Wh) used to 80% runs a 100 W load for about 10 hours, or 9.2 hours if the load goes through a 90% inverter. There is no single answer until you know the watts.

How do I convert amp-hours to watt-hours?

Multiply by the battery voltage. 100 Ah at 12.8 V is 1,280 Wh; at 12 V it is 1,200 Wh; at 24 V it is 2,400 Wh. The same battery's amp-hours mean twice the energy when it is wired at twice the voltage.

Why does my lead-acid battery run down faster than the math says?

Lead-acid capacity is quoted for a slow 20-hour discharge. Drain it faster and you get less, which Peukert's law describes. Victron's own table gives an AGM battery 85% of its label at a 5-hour rate and 65% at 1 hour. The calculator applies a Peukert exponent of 1.25, Victron's default for lead-acid, when your current is above the 20-hour current. Lithium barely changes.

Should I include the inverter?

Yes, for anything that plugs into an AC outlet. The inverter uses part of the battery's energy as heat; 90% is typical and Victron's Phoenix inverters peak at 89–92%, less at light loads. Lights, pumps and fridges that run on 12 V DC skip this loss.

Can I run the battery completely flat?

Some lithium makers say you can, and Battle Born rates its batteries for 100% discharge. Lead-acid lasts far longer if you stop near 50%. The calculator's lithium option stops at 80%, the level Renogy rates for 5,000 cycles, so the answer is a conservative one.

Sources

  1. How to Calculate Battery Runtime, VoltCalcs. Runtime = capacity × voltage × depth of discharge × efficiency ÷ load; worked examples of 20.5 hours (100 Ah LiFePO4, 50 W) and 90.1 hours (200 Ah AGM, 13.32 W); inverter efficiency of 0.90.
  2. Battery Runtime Calculator, iPowerQueen. Runtime = Ah × V × DoD ÷ load power.
  3. GEL and AGM Deep Cycle batteries data sheet, Victron Energy. Capacity is quoted at the 20-hour rate; table 1 gives an AGM Deep Cycle battery 92% of it at the 10-hour rate, 85% at 5 hours and 65% at 1 hour. 600 cycles at 50% discharge.
  4. SmartShunt manual, configuration, Victron Energy. Peukert exponent 1.25 for lead-acid if unknown, with capacity entered at the 20-hour rate; 1.05 for lithium; a 50% discharge floor by default for lead-acid.
  5. Phoenix Inverter VE.Direct 250 VA to 1600 VA data sheet, Victron Energy. Maximum efficiency of 87–92% across the range (89–92% from the 12/375 up).

Formula and sources last checked September 30, 2026. How we test formulas.

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Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.