Battery runtime calculator
How long a bank actually lasts under a given load, or how big a bank you need for a given runtime. Depth of discharge, inverter efficiency and Peukert's law are all taken out, so the answer is the one you get in practice.
Inputs
Result
The same bank, one appliance at a time
| Appliance | Typical draw | Amps from the bank | Result |
|---|
Typical draws, for comparison. Check the plate on your own equipment, and remember a fridge cycles on and off rather than running flat out, so its average over a day is well under the figure shown.
Why the nameplate is not what you get
A 100 Ah battery at 12 V holds 1,200 Wh on paper. Three things stand between that and the energy reaching your appliance, and they multiply rather than add.
Depth of discharge comes first. Taking a flooded lead-acid battery below about half its capacity shortens its life sharply, so half the nameplate is not really yours. Lithium will give you 80% or more, which is most of the reason it wins despite the price.
Inverter efficiency is next. A good pure sine inverter is 90% or so, meaning a 150 W load pulls about 167 W from the battery. Cheap modified sine units are worse, and most inverters also draw a watt or two just sitting idle.
Peukert's law, which almost every other calculator leaves out
A lead-acid battery rated 100 Ah is rated at a gentle discharge, normally over 20 hours. Pull it harder and you do not merely empty it faster, you get less out of it altogether. At 50 A that 100 Ah battery behaves like a 56 Ah one.
The exponent n measures how badly. Lithium sits near 1.0 and is effectively immune. Flooded lead-acid is around 1.25, AGM about 1.15. The results above show you what the answer would have been without this correction, because the gap is often the difference between a bank that works and one that dies at three in the morning.
This is why swapping lead-acid for lithium of the same nameplate can nearly triple your usable runtime under a heavy load: 80% depth instead of 50% is only a 1.6× gain, and Peukert supplies the rest.
Amp hours, watt hours and kilowatt hours
Amp hours mean nothing without a voltage. A 100 Ah bank at 12 V and a 25 Ah bank at 48 V hold the same 1.2 kWh. Watt hours are the honest unit for comparing banks, which is why the results show all three.
Running the same bank at a higher voltage is usually worth it. Four times the voltage is a quarter of the current for the same power, which means thinner cable, smaller fuses and far less heat. Above roughly 100 A, 12 V systems start needing cable thick enough to be awkward and expensive.
Questions
How long will a 100Ah battery last?
It depends entirely on the load and the chemistry. A 100 Ah lithium bank at 12 V runs a 150 W fridge for about 5 hours; the same nameplate in flooded lead-acid manages roughly 2, because you can only use half of it and Peukert takes a further bite. At a 10 W load both last for days.
What is Peukert's law?
It says a lead-acid battery gives you less total capacity the faster you discharge it. A 100 Ah battery rated over 20 hours behaves like about 56 Ah when pulled at 50 A. The effect is governed by an exponent, roughly 1.25 for flooded lead-acid and near 1.0 for lithium, which is why lithium is barely affected.
How deep can I discharge my battery?
About 50% for flooded, AGM and gel lead-acid if you want reasonable cycle life, and 80% or more for LiFePO4. Going deeper works once but costs you cycles, so a lead-acid bank has to be roughly twice the size of a lithium one for the same usable energy.
How do I convert amp hours to watt hours?
Multiply by the system voltage: 100 Ah at 12 V is 1,200 Wh, or 1.2 kWh. Go the other way by dividing. Amp hours alone are meaningless for comparing banks at different voltages, which is why watt hours are the better unit.
Should I build a 12V, 24V or 48V bank?
Higher voltage for anything substantial. The same power at 48 V draws a quarter of the current of 12 V, so cable, fuses and busbars are smaller and cheaper, and losses drop. 12 V is fine for a van or a small cabin; past about 2 kW of load it becomes awkward.
Why does my battery die faster than the calculator said?
Usually one of four things: the stated load is lower than the real one, the inverter's idle draw was ignored, the battery is cold, or it is simply old. Capacity falls with age and temperature, and a lead-acid bank a few winters in can be well under its nameplate.
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