How runtime works
Without corrections, runtime is just energy divided by power. A 12V, 100Ah battery holds 1,200 watt-hours of energy. Run a 120-watt load and it lasts 10 hours.
The real world chips away at that number from several directions. Inverters waste 5–15% as heat. You can’t cycle most batteries to zero - usable depth of discharge is 50% for traditional lead-acid, 80–95% for modern lithium. And lead-acid specifically delivers less total capacity when pulled hard.
runtime = (capacity × V × usable × efficiency) ÷ load_watts
Frequently asked
Should I use the Peukert correction?
Only for lead-acid, AGM, and gel batteries. Lithium chemistries (LiFePO₄ especially) deliver near-rated capacity even at high discharge currents - their Peukert exponent is essentially 1.0, meaning no correction needed.
Where do I find my battery’s Peukert exponent?
Some datasheets publish it directly. Many don’t. If you have C/20 and C/5 (or C/1) capacity ratings, you can derive it. Otherwise use the typical defaults: 1.10 for newer AGM, 1.15 for typical flooded lead-acid, higher for older or undersized banks.
Why is my actual runtime shorter than the calculator says?
The usual suspects: cold temperature (lithium loses 15% at freezing, lead-acid more), aged battery (loses 10–30% over years), inverter idle draw (10–40W just sitting there), surge loads (motor startup pulls 3–5x running watts for seconds), and voltage cutoff (BMS protects below ~10.5V on a 12V system, before the battery is truly empty).