12V Battery Runtime Calculator
A 100 Ah, 12 V lead-acid battery runs a 100 W load through an inverter for about 5 h 6 min, using only the 50% of its capacity that is safe to use. Runtime in hours equals Ah × volts × usable depth of discharge × inverter efficiency ÷ load watts; for a DC load set the efficiency to one.
How this result was calculated
100 Ah × 12 V × 50% usable × 85% inverter efficiency = 510 Wh usable. 510 Wh ÷ 100 W = 5.1 hours, about 5 h 6 min.
Key facts
- Runtime of a 12 V battery in hours = Ah × 12 V × usable depth of discharge × inverter efficiency ÷ load in watts.
- A 100 Ah, 12 V lead-acid battery stores about 510 Wh of usable energy through an inverter, which runs a 100 W load for about 5 h 6 min or a 1,000 W load for about 31 min.
- Lead-acid, AGM and gel batteries should be used to about 50% of their rating; lithium (LiFePO4) batteries to about 90%, so a lithium battery of the same Ah runs almost twice as long.
- An inverter running at about 85% efficiency turns the rest of the battery energy into heat; a DC load wired straight to the battery has no inverter loss.
Formula
t = (Ah × V × DoD × η) ÷ P
- t = runtime in hours
- Ah = battery capacity in amp hours
- V = battery voltage (nominal)
- DoD = usable depth of discharge
- η = inverter efficiency (1 for a DC load)
- P = load in watts
How long will a 12 V battery run a 100 W load?
A 100 Ah, 12 V lead-acid battery runs a 100 W AC load for about 5 h 6 min through an inverter. The same load wired as DC, with no inverter, runs about one sixth longer because the inverter loss of about 85% efficiency disappears. A 100 Ah lithium battery at 12.8 V stores about 979 Wh usable, so it runs the same 100 W load for roughly twice as long.
The result does not depend on your mains voltage. Whether the inverter puts out 230 V as in Pakistan, the UK and Europe, or 120 V as in the US and Canada, the battery side is the same 12 V and the watts drawn are the same.
100 Ah battery runtime calculator: lead-acid against lithium
The label Ah is not the usable Ah. A 100 Ah lead-acid, AGM or gel battery should only be taken to about 50% of its rating, so only half the energy counts. A 100 Ah lithium (LiFePO4) battery can safely use about 90%, and sits at 12.8 V instead of 12 V, which is why the lithium example below gives about 3 h 16 min at 300 W while a lead-acid battery of the same Ah gives much less.
Lead-acid batteries also lose capacity when they are drained fast. The label Ah is measured over about twenty hours; at a load that empties the battery in an hour the real capacity can be a quarter lower. The calculator does not apply this drop, so treat results under about two hours as optimistic.
Worked examples
100 Ah battery with a 100 W load through an inverter
100 Ah × 12 V × 50% × 85% = 510 Wh usable. 510 Wh ÷ 100 W = about 5 h 6 min.
100 Ah battery with a 1,000 W load (microwave or kettle)
The same 510 Wh ÷ 1,000 W = about 31 min. At this rate the inverter pulls well over eighty amps from the battery, and a lead-acid battery drained this fast gives less than its label Ah, so expect even less in practice.
100 Ah lithium battery with a 300 W load
100 Ah × 12.8 V × 90% × 85% = 979 Wh usable, which runs 300 W (a TV with a few fans, or a small compressor fridge at full duty) for about 3 h 16 min.
12 V compressor fridge, 60 W average, wired as DC
A 12 V camping or car fridge draws more when the compressor runs and nothing when it rests; its average over a day is about 60 W. With inverter efficiency set to one: 100 Ah × 12 V × 50% = 600 Wh, ÷ 60 W = about 10 h.
200 Ah deep cycle battery with a 500 W load
200 Ah × 12 V × 50% × 85% = 1,020 Wh usable. 1,020 Wh ÷ 500 W = about 2 h 2 min.
Runtime of a 12 V lead-acid battery through an inverter (50% usable, 85% inverter efficiency)
| Battery | Load | Usable energy | Runtime |
|---|---|---|---|
| 100 Ah | 50 W | 510 Wh | 10 h 12 min |
| 100 Ah | 100 W | 510 Wh | 5 h 6 min |
| 100 Ah | 300 W | 510 Wh | 1 h 42 min |
| 100 Ah | 1,000 W | 510 Wh | 31 min |
| 150 Ah | 300 W | 765 Wh | 2 h 33 min |
| 150 Ah | 500 W | 765 Wh | 1 h 32 min |
| 200 Ah | 500 W | 1,020 Wh | 2 h 2 min |
| 200 Ah | 1,000 W | 1,020 Wh | 1 h 1 min |
Common mistakes
- Using the full Ah. A lead-acid battery taken below about 50% loses cycles quickly, so only that part counts as usable.
- Forgetting the inverter. An inverter turns about 85% of the battery energy into AC; leave the efficiency at one only for a true DC load.
- Using the fridge's peak watts. A fridge compressor cycles on and off; use the average draw over a day or the runtime will look far too short.
- Trusting the label Ah at high loads. Lead-acid capacity is rated over about twenty hours; a load that empties the battery in an hour or two gets noticeably less.
- Mixing up a car battery with a deep cycle battery. A car starting battery is built for short bursts and is damaged by deep discharge, so set a smaller depth of discharge for it.
Safety: Fuse the battery cable close to the battery, size the cable for the inverter current, keep lead-acid batteries ventilated, and have a licensed electrician connect any inverter to house wiring.
Frequently asked questions
How long will a 12 V battery last?
Divide the usable watt-hours by the load watts. A 100 Ah lead-acid battery has about 510 Wh usable through an inverter, so it lasts about 5 h 6 min at 100 W, about 2 h 2 min at 500 W if it is a 200 Ah battery, and about 31 min at 1,000 W.
A 100 Ah battery gives how many hours?
About 5 h 6 min at 100 W through an inverter, or about 3 h 16 min at 300 W if it is a 12.8 V lithium battery. The hours fall in proportion as the load rises.
Can a 12 V 100 Ah battery run a 1,000 W load?
Only briefly: about 31 min on paper, and less in practice because a lead-acid battery drained that fast delivers less than its label Ah. You also need an inverter rated for the load and a thick, fused battery cable for the high current.
Is this a 12 V battery life calculator?
It calculates runtime on one charge, not the life of the battery in years. Battery life depends on how deep and how often you discharge; staying within about 50% for lead-acid and about 90% for lithium is what makes a battery last.
How long will a 12 V battery run a fridge?
Use the fridge's average draw, not its peak. A 12 V compressor fridge averaging about 60 W runs about 10 h on a 100 Ah lead-acid battery wired as DC. A mains fridge through an inverter draws more and also pays the inverter loss.
What is the runtime of a 12 V deep cycle battery?
The same formula applies; deep cycle batteries are simply built to be taken to about 50% repeatedly. A 200 Ah deep cycle battery runs a 500 W load for about 2 h 2 min through an inverter.
How do I work out the discharge time of a 12 V battery?
Discharge time in hours = Ah × 12 V × usable depth of discharge × inverter efficiency ÷ load watts. For a DC load drop the inverter efficiency; for a quick check, usable Wh ÷ watts gives the hours and Ah × depth of discharge ÷ amps gives the same answer.
Sources
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