Battery Runtime Calculator
Find how many hours a battery or battery bank runs a load, with depth of discharge, inverter loss and the Peukert effect.
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Runtime = usable capacity ÷ battery current, with the load in watts divided by the inverter efficiency. The Peukert option is an estimate for lead-acid batteries; the battery data sheet has the real discharge curve. Temperature, age and surge loads are not included.
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How to use the Battery Runtime Calculator
- Type the capacity of one battery in “Battery capacity (each battery)” and pick Ah, Wh or kWh. A Wh or kWh rating is divided by the battery voltage to get amp-hours.
- Enter the “Battery voltage (V, each battery)”, for example 12, 24 or 48.
- For a bank, set “Batteries in series” (adds voltage) and “Batteries (or strings) in parallel” (adds amp-hours). Leave both at 1 for a single battery.
- Set the “Usable depth of discharge (%)”. Use about 50 for lead-acid and 80 to 100 for lithium (LiFePO4).
- Enter the “Load” and choose W for an AC load run through an inverter or A for current drawn straight from the battery.
- For a load in watts, set the “Inverter efficiency (%)”. Typical inverters are 85 to 95.
- Open the advanced settings for lead-acid batteries: type a “Peukert exponent (k)” of about 1.1 to 1.3 and the “Capacity rating period (hours)” printed on the battery, often 20. Leave k at 1 to turn the correction off.
- Read “Runtime (hours)”, the runtime in days, hours and minutes, “Battery current drawn”, “Discharge rate (C-rate)” and the usable energy, then the steps, warnings and the two tables.
The example buttons load a 100 Ah starter case, a 200 Ah lead-acid battery with Peukert, a 5 kWh 48 V lithium battery and a 2S2P bank. “Copy result”, “Copy link” and “Print / save as PDF” keep the answer.
How long will a 100Ah battery last?
Runtime is the usable amp-hours divided by the current the load draws from the battery. For a 100 Ah, 12 V battery at 80 % depth of discharge, the usable capacity is 80 Ah (960 Wh). A 100 W load through a 90 % inverter takes 100 ÷ 0.9 = 111.1 W from the battery, or 111.1 ÷ 12 = 9.26 A. The runtime is 80 ÷ 9.26 = 8.64 hours, or 8 h 38 min.
| 100 Ah, 12 V, 80 % DoD, 90 % inverter | Battery current | Runtime |
|---|---|---|
| 50 W | 4.63 A | 17 h 17 min |
| 100 W | 9.26 A | 8 h 38 min |
| 200 W | 18.52 A | 4 h 19 min |
| 300 W | 27.78 A | 2 h 53 min |
| 500 W | 46.30 A | 1 h 44 min |
| 1000 W | 92.59 A | 52 min |
How do you calculate battery life with depth of discharge?
Depth of discharge (DoD) is the share of the rated capacity you allow the battery to give up. Multiply the capacity by it before dividing by the current. The same 100 Ah battery at 100 W gives 5.4 hours at 50 % DoD, 8.64 hours at 80 % and 10.8 hours at 100 %. Lead-acid batteries last longer when kept above 50 % charge; lithium (LiFePO4) batteries are usually run to 80 % or more. The calculator warns above 80 %.
What is the Peukert effect?
A lead-acid battery gives less than its rated capacity when it is drawn quickly. The Peukert formula estimates this: t = H × (C × DoD ÷ (I × H))^k, where H is the rating period in hours, C the rated capacity, I the current and k the Peukert exponent. For a 100 Ah battery rated at 20 hours, 50 % DoD, a 300 W load through a 90 % inverter at 12 V draws 27.78 A. With k = 1.2 the runtime is 20 × (50 ÷ (27.78 × 20))^1.2 = 1.11 hours, not the 1.8 hours the simple division gives. Use the exponent from the data sheet when you have it; lithium batteries are close to 1.
How do series and parallel batteries change the runtime?
Series adds voltage and keeps the amp-hours. Parallel adds amp-hours and keeps the voltage. Two 12 V, 100 Ah batteries in series make 24 V and 100 Ah (2400 Wh); in parallel they make 12 V and 200 Ah (2400 Wh). The energy is the same, but at a fixed power the higher voltage draws half the current, so cables and inverter losses are smaller. A 2S2P bank of four 12 V, 100 Ah batteries is 24 V and 200 Ah (4800 Wh). At 50 % DoD, 800 W and a 90 % inverter it runs 2.7 hours.
How is this battery calculator different from a simple Ah formula?
It shows the battery current and C-rate (a rate above 1C draws more than the full capacity per hour and gives a warning), the usable energy in Wh, a runtime table for loads from 50 W to 2000 W, and a table of the same load at 50, 80 and 100 % depth of discharge. For a cable that carries the battery current, use the battery cable size calculator.
Worked examples
| Input | Result |
|---|---|
| 100 Ah, 12 V, 1S1P, DoD 80 %, 100 W | 8.64 h (8 h 38 min) |
| 200 Ah, 12 V, 1S1P, DoD 50 %, 500 W | 2.16 h (2 h 10 min) |
| 100 Ah, 12 V, 1S1P, DoD 50 %, 300 W | 1.112 h (1 h 7 min) |
| 5 kWh, 48 V, 1S1P, DoD 90 %, 1500 W | 2.76 h (2 h 46 min) |
| 100 Ah, 12 V, 2S2P, DoD 80 %, 800 W | 4.32 h (4 h 19 min) |
Calculated by the tool above; open it with the same values to see each step.
Questions, briefly answered
How long will a 100Ah 12V battery run a 100 watt load?
About 8 h 38 min at 80 % depth of discharge with a 90 % inverter (80 Ah ÷ 9.26 A = 8.64 hours). At 50 % depth of discharge it is 5.4 hours.
How do I calculate battery runtime from watts?
Divide the load in watts by the inverter efficiency, divide by the battery voltage to get amps, then divide the usable amp-hours (capacity × depth of discharge) by those amps. Wh and kWh ratings work too: usable Wh ÷ watts drawn from the battery.
Should I include the inverter efficiency?
Yes for an AC load run through an inverter. Inverters convert at about 85 to 95 %, so the battery supplies 5 to 15 % more power than the load uses. For a 12 V device wired directly, enter its current in amps and no inverter loss is added.
What depth of discharge should I use?
About 50 % for lead-acid (flooded, AGM or gel) and 80 to 100 % for LiFePO4. Going deeper shortens lead-acid life. Check the battery data sheet.
What is a good Peukert exponent?
Lead-acid batteries are about 1.1 to 1.3, lithium about 1.0 to 1.05. A value of 1 turns the correction off. The data sheet or the battery maker gives the best figure.
Does a battery bank in series or parallel last longer?
Both store the sum of the energy. Parallel doubles the amp-hours at the same voltage; series doubles the voltage at the same amp-hours. For the same load power the series bank draws half the current, which cuts cable and inverter losses.
How do I use this as a UPS calculator, and how many uninterruptible power supply hours is that?
Enter the UPS or inverter battery as the capacity (Ah at its voltage, or Wh from the label), the load it supports in watts, and the inverter efficiency, then read the hours. The same formula applies to any uninterruptible power supply: hours = capacity in Wh × depth of discharge × efficiency ÷ load in watts. Small sealed lead-acid UPS units lose extra capacity at high current, so turn on the Peukert correction for a safer figure.
Is the result exact?
No. It is an estimate. Temperature, battery age, the real discharge curve, the inverter standby draw and surge loads change the runtime. Treat it as a planning figure and add a margin.