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How long a 100 Ah, 12 V battery lasts
Direct DC loads, with lithium discharged to 90% and AGM to 50% at a Peukert exponent of 1.2. The last column shows how much of the AGM battery’s 100 Ah you actually get at that load.
| Load | Battery current | Lithium | AGM | AGM capacity delivered |
|---|---|---|---|---|
| 50 W | 4.2 A | 21 h 36 min | 12 h | 100 Ah |
| 100 W | 8.3 A | 10 h 48 min | 5 h 25 min | 90 Ah |
| 300 W | 25 A | 3 h 36 min | 1 h 27 min | 72 Ah |
| 600 W | 50 A | 1 h 48 min | 38 min | 63 Ah |
| 1000 W | 83.3 A | 1 h 5 min | 21 min | 57 Ah |
Lithium time scales almost exactly with the load: double the watts, half the runtime. AGM does worse than that, which is why a lead-acid battery that runs a small load all night can fail in minutes under a heavy one. If the load goes through an inverter, expect about 15% less, plus the inverter’s idle draw.
How the runtime is calculated
- Bank voltage = battery voltage × batteries in series. Bank capacity = Ah per battery × strings in parallel.
- Battery current (A) = load watts ÷ bank voltage. For an inverter load it is (watts ÷ 0.85 + idle watts) ÷ bank voltage. If you enter amps, they are used as they are.
- Peukert: hours to empty = H × (C ÷ (I × H))k, where C is the rated Ah, H the rated hour rate, I the current and k the exponent. Delivered capacity = I × hours, and it is never allowed to exceed the rated Ah.
- Runtime = delivered capacity × usable depth of discharge ÷ current.
- Defaults: lithium 90% usable and k = 1.0, AGM 50% and k = 1.2, gel 50% and 1.15, flooded 50% and 1.25. These are typical values, not guarantees.
It does not model cold weather, battery age, charging while discharging, or the battery’s built-in protection limits.
Series and parallel: what changes
- In series the voltages add and the Ah stay the same. Two 12 V 100 Ah batteries make 24 V and 100 Ah.
- In parallel the Ah add and the voltage stays the same. The same two batteries make 12 V and 200 Ah.
- The energy is identical either way, 2,400 Wh. What changes is the current: a 24 V bank draws half the amps for the same load.
- Use batteries of the same type, age and capacity in one bank, or the weakest one limits the rest.
Using it anywhere in the world
- Ah, V and W are the same everywhere, so nothing needs converting.
- Numbers use your own region’s separators.
- The watts you enter for an inverter load are the appliance’s AC watts. Whether your outlets are 120 V or 230 V does not change the DC battery math.
Why real runtime comes up short
- Comparing batteries by Ah alone. A 100 Ah 24 V battery holds twice the energy of a 100 Ah 12 V one.
- Ignoring depth of discharge. Discharging lead-acid fully shortens its life, so only about half is usable.
- Ignoring heavy loads. High current cuts delivered capacity in lead-acid batteries.
- Forgetting the battery’s own limit. Many lithium batteries have a protection circuit that shuts off above their rated continuous current.
- Mixing old and new batteries. The weakest battery sets the bank’s capacity.
Common questions
How long will a 100 Ah battery last?
It depends on the load and the battery type. A 100 Ah, 12 V lithium battery runs a 100 W load for about 10 h 48 min and a 1,000 W load for about 1 h 5 min. A 100 Ah AGM battery runs the 100 W load for about 5 h 25 min, because only half its capacity is usable and the capacity it delivers drops as the load rises.
What is the Peukert effect?
Lead-acid batteries deliver less total capacity when they are discharged quickly. The Peukert exponent describes this, and it is usually between about 1.1 and 1.3 for lead-acid and close to 1.0 for lithium. The higher the exponent, the more capacity you lose at heavy loads.
What is the difference between Ah and Wh?
Amp-hours measure charge, and watt-hours measure energy. Multiply Ah by voltage to get Wh, so 100 Ah at 12 V is 1,200 Wh and 100 Ah at 24 V is 2,400 Wh. Always compare batteries in Wh, or make sure the voltage is the same.
Should I connect batteries in series or parallel?
Parallel keeps the voltage and adds Ah, and series adds voltage and keeps Ah. Choose the one that matches your system voltage. Series banks need fewer amps for the same power, which allows thinner cables, but every battery in the string must be matched.
Why can I only use 50% of a lead-acid battery?
Regularly discharging lead-acid batteries deeply shortens their life, so 50% is the common limit. Lithium (LiFePO4) batteries tolerate deep discharges well, so 80 to 100% is often usable. The calculator lets you change the percentage if your battery’s datasheet allows more.
What hour rate should I enter?
Enter the rate printed next to the capacity. A label such as “100 Ah at 20 h” means the capacity is the energy delivered over 20 hours. If a lead-acid battery only says “100 Ah”, 20 hours is the usual assumption. Lithium batteries are barely affected by this setting.
Related calculators
To size a whole system, use the van and RV solar and battery calculator. For a portable unit, try the power station runtime calculator, and for cables and fuses the wire gauge and fuse calculator. For water pumping, there is the solar water pump calculator.
For power banks and small gadget batteries measured in mAh, try the power bank and portable battery calculator.
To see how long solar panels take to recharge a battery, use the solar charge time calculator.
To plan backup for a home during a blackout, use the power outage calculator.
To compare battery types by lifetime cost, use the battery cost calculator.
New to the terms? See the off-grid power glossary.