Using the Battery & UPS Runtime Calculator
Choose watt-hours directly or amp-hours multiplied by nominal voltage. Then account for the fraction of capacity you intend to use and delivery losses. Enter the equipment power rating so a load beyond that rating is flagged before a runtime is shown.
Worked example
Use the example inputs below to reproduce this result. These are the form's starting values, not recommended targets. Changing your inputs updates your result above; this worked example stays fixed for comparison. Results are rounded for display.
Example inputs
- Capacity format
- Amp-hours × voltage
- Rated capacity (Ah)
- 100
- Nominal voltage (V)
- 12
- Usable capacity (%)
- 80
- Delivery / inverter efficiency (%)
- 85
- Continuous load (W)
- 100
- Equipment continuous power rating (W)
- 1000
Example result
- Nominal stored energy (Wh)
- 1,200
- Estimated delivered energy (Wh)
- 816
- Approximate runtime (hours)
- 8.16
- Approximate runtime (minutes)
- 489.6
- Power headroom (W)
- 900
Method and assumptions
Nominal Wh = Ah × voltage, or supplied Wh. Delivered Wh = nominal Wh × usable fraction × efficiency; hours = delivered Wh / continuous watts. Usable fraction includes your reserve/aging assumptions. Rejects loads above the supplied continuous rating. Ignores surge power, Peukert behavior, temperature and changing loads; use manufacturer discharge curves for real equipment. Not a critical-load reliability or electrical-design guarantee.
Reference: victronenergy.com — method and further reading
Distinguish a model from a circuit specification
Electrical calculations describe the assumptions entered in the form. Real components, cables and supplies also have ratings, tolerances and operating conditions. Confirm whether the model is for DC resistance, energy over time or another specific quantity. A mathematically consistent result is not by itself a complete design or installation check.
Common question
Can this estimate size backup for critical medical or safety equipment?
No. The model assumes a steady load and user-supplied capacity and efficiency. It does not verify surge compatibility, battery discharge curves or system reliability. Critical backup requirements need manufacturer data, appropriate professional design and validated operating tests.