Battery Bank Runtime Calculator
How long a bank actually runs a load, after depth of discharge, inverter losses, and the derate lead-acid suffers under load.
The total for the whole bank, not one battery.
The continuous draw you want to sustain.
Sets the depth of discharge and how much capacity is lost at higher discharge rates.
Uncheck for a DC load wired straight to the bank, which pays no conversion loss.
Runtime
11h 16m
Includes the inverter conversion loss, which a naive calculation misses.
- Nameplate energy
- 2400 Wh
- Actually usable
- 1920 Wh
- Current drawn from bank
- 14.2 A
Why this is shorter than the obvious calculation
The intuitive answer is capacity times voltage divided by watts. A 200 Ah 12 V bank holds 2,400 Wh, so a 150 W load should run 16 hours. In practice it will not, and three deductions explain the gap. All of them make the real number smaller, which is the direction a calculator ought to be honest about.
Depth of discharge
You cannot use all of a battery. Lead-acid chemistries are conventionally limited to about half their rated capacity, because discharging deeper shortens their life dramatically. LiFePO4 tolerates 80% or more. The same nameplate capacity therefore delivers very different usable energy depending on chemistry.
Inverter losses
Converting stored DC into household AC costs roughly 10 to 15% as heat, so a 150 W AC appliance draws closer to 170 W from the battery. A DC load wired straight to the bank avoids this entirely, which is why many off-grid cabins run lighting and pumps on DC.
Discharge rate
Lead-acid delivers less than its rated capacity when discharged quickly. Ratings are quoted at a gentle 20-hour rate; pull the same battery hard and the usable capacity shrinks. This is the Peukert effect, and the calculator applies it.
LiFePO4 is close to immune, holding near its rated capacity across normal discharge rates. On a system with large intermittent loads such as a well pump or power tools that difference is substantial, and it is much of the practical case for lithium.
What this does not account for
- Temperature. Batteries deliver less when cold, and lead-acid can lose 20 to 30% near freezing. A bank in an unheated shed in January will underperform this figure. See cold climates.
- Age. A battery at the end of its warranted life is typically down to 80% of its original capacity.
- Cycling loads. A fridge runs only part of the time, so a bank will carry one far longer than it would carry a constant load of the same rated wattage.
For choosing a chemistry, see LiFePO4 versus AGM and flooded lead-acid. For sizing a bank from scratch rather than testing one, see how to size an off-grid battery bank.