Battery Life Calculator

How to use the battery life calculator

Enter the amp-hour capacity, the steady current the battery is supplying, and the usable depth of discharge, and the tool returns the runtime in hours along with the usable amp-hours it worked from. The arithmetic is deliberately short: usable capacity is the rating multiplied by the fraction you are willing to draw, and runtime is that usable capacity divided by the load. There is no electricity rate anywhere on the page, because this answers how long the battery lasts rather than what it costs. The two inputs that move the answer most are the load, which is inversely proportional to runtime, and the depth of discharge, which is the setting most people get wrong by simply leaving it at the maximum.

The idea to internalise is that you never get to use a battery whole nameplate. A lead-acid battery cycled routinely past half empty ages fast, which is why 50 percent is the conventional sizing figure across off-grid design, and why Trojan itself recommends discharging its deep-cycle batteries only 20 to 50 percent of rated capacity even though the battery is capable of going to 80 percent. LiFePO4 is a different animal: Battle Born publishes 3,000 to 5,000 cycles for its LiFePO4 packs at 80 percent depth of discharge, a life no flooded battery approaches at that depth. The practical consequence is that swapping chemistry at the same amp-hour rating changes your runtime by roughly 60 percent without changing a single physical dimension.

Make the load figure realistic, because an optimistic load produces an optimistic runtime and the error compounds through everything downstream. The number the tool wants is the steady current of everything drawing at once, which is rarely the sum of every device you own. Add the amps directly if your equipment is labelled that way, or divide each device wattage by the system voltage and sum the results, which is what the volts-to-watts tool does in reverse. Where a load cycles on and off — a fridge compressor, a water pump, a fan on a thermostat — use the average current over an hour rather than the peak, since the peak only exists for part of the time and sizing to it wastes a lot of battery.

One real-world effect the linear model leaves out is that a battery capacity depends on how fast you empty it. Deep-cycle lead-acid capacity is conventionally quoted at the twenty-hour rate, so a 100 Ah label means 100 amp-hours delivered over 20 hours at 5 A. Empty the same battery in two hours instead and it will yield substantially less, an effect described by Peukert law with an exponent that industry references put around 1.1 to 1.3 for true deep-cycle batteries. Lithium is far less sensitive to this. So read the tool result as a good planning figure at moderate draws and as an optimistic ceiling at heavy ones, and never treat it as a substitute for a licensed electrician or installer sizing a system you intend to wire in.

The formula

Runtime is usable capacity divided by load, and usable capacity is the rating multiplied by the fraction of it you are prepared to draw. Both steps are ordinary division, which is exactly why the depth-of-discharge term carries so much weight: it is a straight multiplier on the answer, so a battery rated at 50 percent usable produces precisely five-eighths the runtime of the same battery rated at 80 percent.

Everything here stays in amp-hours and amps rather than watt-hours and watts, because that is how batteries and DC loads are labelled and because the system voltage cancels out of both sides of the division. If your loads are given in watts, convert them to amps at the system voltage first — a 120 W load on a 12 V system is 10 A — and the arithmetic below then applies unchanged.

usable Ah = capacity × depth of discharge
hours = usable Ah ÷ load
load in amps = load in watts ÷ system voltage
100 Ah × 0.50 = 50 Ah;  50 ÷ 10 A = 5.0 hours

Worked example with the defaults: a 100 Ah battery at 50 percent usable with a 10 A load gives 100 × 0.50 = 50 usable Ah, and 50 ÷ 10 = 5.0 hours. Change one input at a time to see which lever matters. Lift the depth of discharge to 80 percent for LiFePO4 and the same battery under the same load runs 80 ÷ 10 = 8.0 hours, a 60 percent gain. Halve the load to 5 A instead and the lead-acid battery runs 50 ÷ 5 = 10 hours. Double it to 20 A and the runtime collapses to 2.5 hours.

That last case is where the linear model starts to overstate things. A deep-cycle lead-acid capacity is conventionally rated at the twenty-hour rate, which for a 100 Ah battery means 5 A for 20 hours. A 20 A draw is four times that rate, and Peukert law — with an exponent industry references put around 1.1 to 1.3 for true deep-cycle batteries — says the delivered capacity falls as the rate rises. So the 2.5 hours the formula returns at 20 A is a ceiling, not a forecast. LiFePO4 is far less affected, which is another reason lithium banks behave closer to their arithmetic than lead-acid ones do.

Two things this calculation deliberately does not model. It does not track voltage sag, so it will not tell you when an inverter with a low-voltage cutout will shut down before the amp-hours run out, and on lead-acid that cutout often arrives first. And it does not account for temperature, which reduces available capacity in the cold on every chemistry and can be severe on lithium, where charging below freezing is restricted by the battery management system. Use the result to plan and to compare options, and have a licensed electrician or installer size anything that will be permanently wired in.

Frequently asked questions