Plan backup hours before outages strike today. Adjust batteries, wiring, and inverter losses easily here. See runtime, usable energy, and safety notes in seconds.
The calculator converts the battery bank into usable watt-hours, then divides by your load:
Optional Peukert adjustment approximates how effective capacity changes with discharge current.
| Scenario | Load (W) | Bank (V × Ah) | DoD | Eff. | Estimated runtime |
|---|---|---|---|---|---|
| Small essentials | 150 | 24V × 100Ah | 80% | 90% | ~11.0 hours |
| Home office | 300 | 24V × 100Ah | 80% | 90% | ~5.5 hours |
| Heavier backup | 800 | 48V × 200Ah | 80% | 92% | ~8.8 hours |
Examples are illustrative. Real performance depends on battery chemistry, discharge rate, and load cycling.
Backup runtime planning links storage to operating cost, downtime risk, and equipment protection. A 300 W critical-load panel draws 7.2 kWh per day, so a modest bank can be exhausted during long outages. Knowing expected hours helps you budget capacity, inverter size, and resilience. For businesses, even one hour of outage can exceed battery costs quickly.
Battery labels show nominal energy: voltage times amp-hours. A 24 V, 100 Ah bank is 2,400 Wh nominal. Usable energy is lower because you limit depth of discharge, lose conversion power in the inverter, and face wiring or standby losses. The usable factor combines these effects.
At higher currents, many chemistries deliver less effective capacity. Lead-acid is most sensitive: a bank rated at a 20-hour test may provide fewer amp-hours at a 1–3 hour discharge. The optional Peukert adjustment reduces effective capacity as load current rises, improving realism for heavy loads.
Cold conditions reduce available capacity, while older batteries show degradation. Temperature and age factors let you derate estimates, useful for garages or harsh climates. If you store energy for several days, self-discharge matters; reserve days apply a small reduction to reflect storage loss.
Use the graph to test “what if” cases. Lowering average load from 400 W to 250 W increases runtime by about 60% because hours scale inversely with watts. Adding a parallel string increases amp-hours linearly, often improving runtime more than raising voltage. Plan conservatively and keep buffer for surges. Export CSV or PDF to document assumptions and share scenarios with teams internally.
1) What inputs matter most for accurate runtime?
Load watts, depth of discharge, and inverter efficiency drive the biggest differences. Next are battery configuration (series/parallel) and battery health. Use realistic averages rather than peak surges.
2) Should I use the Peukert option?
Enable it for lead-acid banks or when runtime is only a few hours. High discharge rates reduce effective capacity. For many lithium systems at moderate loads, the impact is smaller.
3) Why is my real runtime lower than the estimate?
Loads cycle, inverters consume standby power, and batteries deliver less energy under high current or cold temperatures. Wiring losses and aging also reduce capacity. Use conservative inputs and keep safety margin.
4) How do I model mixed loads like fans and a fridge?
Estimate an average watt draw over an hour, including duty cycles. Add a separate check for starting surges to confirm the inverter can handle them. Then run the calculator using the average watts.
5) What depth of discharge should I choose?
Follow your battery manufacturer guidance. Many lithium banks allow 80–90% for planning, while lead-acid is often kept closer to 50% to extend life. Choose a conservative value for critical systems.
6) How can I improve backup time without replacing everything?
Reduce average load, add a parallel battery string, improve inverter efficiency, and minimize standby losses. Better wiring and cooler operating temperatures can also help. Small reductions in watts often yield large runtime gains.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.