Example Data Table
These sample rows illustrate common setups and typical outputs.
| Scenario | Pack | Load | DoD | Reserve | Usable Energy | Estimated Runtime |
|---|---|---|---|---|---|---|
| Router + lights | 12V 100Ah (1×1) | 60W AC @ 90% eff | 80% | 10% | ~690 Wh | ~10.3 hours |
| Office workstation | 24V 100Ah (2×1) | 250W AC @ 92% eff | 85% | 10% | ~1.80 kWh | ~6.6 hours |
| Small fridge | 12V 200Ah (1×2) | 120W avg, 40% duty | 70% | 15% | ~1.00 kWh | ~17.4 hours |
| Backup for TV | 48V 50Ah (4×1) | 180W DC direct | 90% | 10% | ~1.94 kWh | ~10.8 hours |
Formula Used
Runtime (hours) is estimated from usable energy divided by battery draw:
Energy conversions and adjustments:
- Wh_rated from Ah: Ah_total × V_total
- V_total: V_per_battery × series
- Ah_total: Ah_per_battery × parallel
- Battery_Draw_W for AC loads: Avg_Load_W × Duty ÷ Inverter_Eff
- Usable_Wh: Wh_effective × DoD × (1−Reserve) × Aging × Temp
Optional lead-acid Peukert correction (when capacity is entered in Ah):
This uses a common 20-hour rating assumption for lead-acid batteries.
How to Use This Calculator
- Enter battery capacity and nominal voltage per battery.
- Set series and parallel counts to match your wiring.
- Enter average and peak load, plus duty cycle.
- Choose AC via inverter or DC direct load path.
- Adjust DoD, reserve, aging, and temperature settings.
- Press Calculate Runtime to see results above.
For planning, keep a reserve margin and conservative temperature factor.
Capacity to energy conversion
Battery capacity compares best in watt‑hours. A 12 V, 100 Ah battery holds about 1,200 Wh at nominal voltage. A 24 V, 100 Ah series string holds about 2,400 Wh. Series increases voltage; parallel increases amp‑hours. Two 12 V 100 Ah batteries in parallel stay at 12 V but become 200 Ah, doubling energy to about 2,400 Wh.
Load profile and inverter losses
Runtime follows the average draw seen by the battery. If devices average 200 W but run 50% of the time, effective load is 100 W. For AC loads, inverter efficiency matters: at 90% efficiency, a 200 W AC load requires about 222 W from the battery (200 ÷ 0.90). Typical efficiencies are 85%–95%. Use peak load to check surge headroom; a brief 600 W spike can cut runtime to minutes if sustained far faster.
Usable window and planning margin
Real‑world runtime depends on limits you set. Depth‑of‑discharge protects cycle life, and a reserve margin protects against estimation error. If a pack is rated 1,200 Wh, and you set 80% DoD plus 10% reserve, planned usable energy is 864 Wh (1,200 × 0.80 × 0.90). With 90% remaining health, usable energy becomes 778 Wh.
Temperature and high‑current effects
Cold reduces capacity and raises internal resistance. A cautious planning factor might be 0.88 near 0°C and 0.80 near −10°C, varying by chemistry. Lead‑acid also suffers Peukert losses at higher current. With exponent 1.15, doubling current reduces effective capacity by about 11% (2^(1−1.15) ≈ 0.89).
Cost signals for budgeting
Once usable energy is known, finance metrics are direct. Energy value per cycle equals usable kWh multiplied by your tariff. At $0.20/kWh and 1.80 kWh usable, delivered value is $0.36 per cycle. If a $600 pack provides 3,000 cycles, levelized cost is about $0.111/kWh ($600 ÷ (1.80 × 3,000)).
FAQs
1) What is the fastest way to get a realistic runtime?
Start with an average load you measured, then set inverter efficiency, a reserve margin, and a conservative DoD. Enter battery health and temperature for your environment to avoid optimistic estimates.
2) Why does the estimate change when I switch AC vs DC?
AC loads pass through an inverter, so the battery must supply extra power to cover conversion losses. The calculator increases battery draw by dividing load by the efficiency you enter.
3) When should I enter capacity in Ah instead of Wh?
Use Ah when your battery label lists amp‑hours and nominal voltage. Use Wh when energy is already specified. Ah entries are converted to Wh using your system voltage and parallel count.
4) How should I pick the duty cycle?
Set duty cycle to the percentage of time the load is actually running at the entered wattage. For cycling loads like refrigerators, use the long‑term average on‑time, not the startup surge.
5) What DoD limit is reasonable for long life?
For LiFePO4, many users plan 70%–90% DoD. For lead‑acid, 30%–50% is common for longevity. Always follow your manufacturer’s recommended DoD for the cycle rating you expect.
6) How reliable is the Peukert adjustment?
It is a helpful approximation for lead‑acid at higher currents, but real results depend on battery age, temperature, and the reference test rate. Use it for planning, then validate with a short real‑world discharge test.