| Scenario | Ah | V | Batteries | DoD | Eff | Load (W) | Usable (kWh) | Runtime (h) |
|---|---|---|---|---|---|---|---|---|
| Small essentials | 100 | 12 | 2 | 80% | 90% | 200 | 1.73 | 8.64 |
| Home office | 200 | 12 | 4 | 80% | 90% | 500 | 6.91 | 13.82 |
| Higher load backup | 280 | 24 | 2 | 70% | 88% | 1500 | 8.27 | 5.51 |
- Nominal Wh = Ah × Voltage × Number of batteries
- Usable Wh = Nominal Wh × (DoD% ÷ 100) × (Efficiency% ÷ 100)
- Usable kWh = Usable Wh ÷ 1000
- Runtime (hours) = Usable Wh ÷ Load (W)
- Required nominal Wh = (Load × Desired hours) ÷ (DoD × Efficiency)
- Required Ah each = Required nominal Wh ÷ (Voltage × Batteries)
- Lifetime kWh = Usable kWh per cycle × Cycle life
- Levelized cost ≈ (System cost + Maintenance) ÷ Lifetime kWh
- Enter your battery capacity, voltage, and battery count.
- Set usable depth of discharge based on your chemistry and warranty limits.
- Enter inverter efficiency from the label or datasheet.
- Estimate your average load in watts across the backup window.
- Add a desired runtime to see whether your bank meets the goal.
- Optionally add system cost, cycle life, and maintenance for a cost-per-kWh estimate.
- Press Calculate; download CSV or PDF for records or planning.
Backup capacity in kWh
Battery banks are labeled in amp‑hours, but planning is clearer in kilowatt‑hours. Nominal energy equals amp‑hours × voltage × battery count ÷ 1,000. Usable energy is lower because depth of discharge and inverter efficiency reduce what reaches AC loads. Example: four 12‑volt 200 Ah batteries are 9.6 kWh nominal; at 80% depth and 90% efficiency they deliver about 6.9 kWh.
Runtime drivers and realistic loads
Runtime is mostly controlled by average watts, not brief peaks. With 6.9 kWh usable, a 500 W average load runs about 13.8 hours, while 1,500 W runs about 4.6 hours. Temperature and discharge rate can reduce capacity, so base your load on measured usage or a duty‑cycle estimate for each device.
Sizing for a target outage window
To meet a planned outage, convert the goal to load energy: watts × hours. Then divide by usable factors (DoD × efficiency) to find required nominal capacity. If you need 2.0 kWh delivered and usable factors equal 0.72, the bank should be about 2.78 kWh nominal. The calculator also converts this into required amp‑hours per battery for your voltage and battery count.
Cost per delivered kWh
Levelized cost helps compare systems with different cycle ratings. Delivered lifetime energy equals usable kWh per cycle × expected cycles. A 6.9 kWh usable bank rated for 3,000 cycles can deliver about 20,700 kWh. If total cost is 2,900 including maintenance, the modeled cost is near 0.14 per delivered kWh, excluding financing, taxes, and replacements.
Interpreting results for purchasing
Use shortfall to decide whether to add capacity or cut load. If shortfall is positive, increase batteries, improve efficiency, or prioritize essential circuits to lower average watts. Keep margin for surge loads and aging; many planners add 10–20%. Export results to document assumptions and compare scenarios consistently. For critical medical or telecom loads, verify inverter surge rating and battery temperature limits before installation.
Nominal kWh is the battery bank’s rated energy (Ah × V × count). Usable kWh applies your chosen depth of discharge and inverter efficiency, which represent practical limits and conversion losses.
Add the watts of devices that will run, then multiply each by its expected duty cycle. A fridge might average far less than its label because it cycles on and off. A plug‑in power meter improves accuracy.
Most backup loads are AC. The inverter converts DC battery power to AC and loses some energy as heat. Even a 90% efficient inverter reduces delivered energy by 10%, directly shortening runtime.
Choose a conservative value based on your battery chemistry and warranty guidance. Many lead‑acid setups use 50%. Many lithium systems use 80–90%. Higher DoD increases usable energy but may reduce long‑term life.
Use average watts for runtime, but confirm the inverter can handle startup surges from pumps, compressors, or fans. Consider a buffer in load input or add a separate surge allowance when selecting inverter and wiring.
No. It is a simple levelized estimate for the battery system only, based on cycle life and optional maintenance. It does not include charging energy cost, financing, taxes, downtime risk, or replacement components.