How Battery Runtime is Calculated
Calculating battery runtime requires knowing both the total stored energy and the usable depth of discharge (DoD) of your battery chemistry. Running lead-acid batteries flat destroys their internal lead plates, whereas modern LiFePO4 (lithium iron phosphate) chemistry allows safe daily cycling down to 10% remaining charge.
The Battery Runtime Formula
Usable_Watt_Hours = Total_Ah × Voltage × Safe_DoD_Percent
Effective_Load_Watts = Appliance_Watts / Inverter_Efficiency
Runtime_Hours = Usable_Watt_Hours / Effective_Load_Watts
Effective_Load_Watts = Appliance_Watts / Inverter_Efficiency
Runtime_Hours = Usable_Watt_Hours / Effective_Load_Watts
Comparison of Battery Chemistries
| Chemistry | Recommended DoD | Lifespan (Cycles) | Usable Capacity from 100Ah |
|---|---|---|---|
| Lithium (LiFePO4) | 85% – 90% | 3,000 – 6,000+ | 90 Ah (1,152 Wh) |
| AGM Deep Cycle | 50% | 400 – 600 | 50 Ah (600 Wh) |
| Gel Deep Cycle | 50% | 500 – 700 | 50 Ah (600 Wh) |
| Flooded Wet Lead-Acid | 50% | 300 – 500 | 50 Ah (600 Wh) |
Peukert's Law and High Loads
For lead-acid and AGM batteries, Peukert's Law dictates that the faster you discharge the battery (e.g. running a microwave or induction cooktop on a 2000W inverter), the less actual capacity the battery yields. Lithium LiFePO4 batteries have a Peukert coefficient close to 1.02, meaning they deliver practically their full rated capacity regardless of heavy draw.