Battery Runtime Calculator
Add your electrical devices, enter your battery bank specs, and instantly see how many days of power you have and how many solar watts you need to recharge in a day. Nothing uploaded.
Per-device breakdown
Device Breakdown
Learn more: battery runtime, depth of discharge, and solar sizing
How the days-of-autonomy number is built
The calculator adds up watts times hours per day for every device you list, then divides that total by your inverter efficiency to get what the battery actually has to supply. Usable capacity is battery voltage times amp-hours times your depth of discharge setting.
Days of autonomy is usable capacity divided by that daily draw. The default setup shows the whole chain: lighting at 20 W for 6 hours, a fridge at 45 W for 24 hours, a laptop at 45 W for 4 hours and a phone charger at 10 W for 2 hours come to 1400 Wh. At 90% inverter efficiency the battery has to deliver 1556 Wh, against 960 Wh usable from a 12 V 100 Ah LiFePO4 bank at 80% DoD, so 0.6 days.
The same daily figure drives the solar estimate: 1556 Wh across 5 peak sun hours rounds up to 312 W of panel. Nothing is derated in that division, so it is the minimum that breaks even on a good day, not a shopping spec.
Depth of discharge decides how much of the rated capacity you get
The chemistry dropdown sets DoD for you: 80% for LiFePO4 and lithium-ion, 50% for lead-acid and AGM. Taking lead-acid past the halfway mark on a regular basis costs cycle life, which is why the default sits there.
Between two batteries with the same label the gap is wide. A 12 V 100 Ah lead-acid battery at 50% gives 600 Wh usable, while the same rating in LiFePO4 at 80% gives 960 Wh, 60% more energy from an identically printed number. The slider overrides the chemistry default anywhere from 20% to 100% if you want to model a shallower cycle or a one-off deep drain.
What the number does not account for
The math treats capacity as fixed no matter how fast you pull energy out, and lead-acid does not behave that way. Its amp-hour rating is measured over a slow 20-hour discharge, and heavier loads get less out of the same battery.
BatteryStuff's walkthrough of Peukert's law puts a number on it: a battery rated 100 Ah at the 20-hour rate gives "an effective capacity of 71.9AH at an amperage drain of 15 amps, for 4.79 hours". LiFePO4 is much less sensitive to discharge rate, so a lead-acid result from this calculator is the more optimistic of the two. Cold batteries, wiring losses and an inverter's own standby draw push real runtime down further.
FAQ
Why does lead-acid default to 50% depth of discharge?
Lead-acid and AGM cells lose cycle life quickly when they are routinely taken below half charge, so 50% is the usual working limit rather than a hard cutoff. The slider lets you go deeper, but the extra runtime comes out of the battery's lifespan.
What does the inverter efficiency field change?
It divides your total device consumption before the runtime and solar figures are worked out. At the 90% default, 1000 Wh of appliance use means the battery has to supply 1111 Wh. If your loads run straight off DC with no inverter in the path, set it to 100.
Is the solar figure enough to recharge in a single day?
It is the exact break-even: daily consumption divided by peak sun hours, rounded up to the next watt. Panel temperature, shading, charge controller losses and cloudy days are not in that number, so most builds specify 20-30% above it.