Why real battery backup is lower than theory
See which visible assumptions make measured battery runtime lower than a simple volts-times-Ah calculation.
Not all nominal energy is used
A planning calculation applies a usable depth of discharge instead of assuming the full label capacity is delivered.
Conversion has losses
The inverter, cables and battery discharge process reduce the energy reaching appliances. Use a manufacturer value when one is available.
Age and discharge rate matter
Battery condition, temperature, charge level and high load can change runtime. Use the calculator as a range and test your actual system safely.
Start with the label-only theory
A 12 V, 150 Ah battery contains 12 × 150 = 1,800 Wh of nominal label energy. Dividing directly by a 300 W load gives six hours, but that assumes every nominal watt-hour reaches the appliance. It ignores discharge limits, inverter loss, battery condition and rate effects, so it is a useful ceiling calculation rather than a dependable runtime.
Apply visible reductions one at a time
At 50% usable depth of discharge, the example retains 900 Wh. At 85% inverter efficiency, about 765 Wh is delivered. Dividing by 300 W gives 2.55 hours before further condition and discharge-rate effects. Keeping the two percentages separate lets the user replace each one with compatible manufacturer evidence.
| Stage | Calculation | Example result |
|---|---|---|
| Nominal | 12 V × 150 Ah | 1,800 Wh |
| Usable | 1,800 × 0.50 | 900 Wh |
| Delivered | 900 × 0.85 | 765 Wh |
| Runtime | 765 ÷ 300 W | 2.55 h |
Load is not always constant
Fans change with speed, displays and processors change modes, and fridges or pumps cycle and surge. Measure or estimate the simultaneous load that actually remains on. A high short surge tests inverter capacity, while the average watts over time drive energy use.
Condition changes the available capacity
Age, temperature, charge level, maintenance and discharge rate can reduce delivered energy. A label value is not a guarantee for an aged battery. Do not change protective cut-offs or exceed manufacturer limits to make an observed test match a calculation.
Use observation to refine, not overpromise
Record the connected load, starting charge indication, runtime and cut-off during a safe test. Treat the observation as evidence for that battery and condition. Maintain reserve for important needs and use purpose-designed professional planning for emergency or safety-critical backup.
Diagnose a shorter observed runtime
Check whether the connected load changed, the battery began fully charged, the cut-off setting is appropriate, and the observation used the same voltage, battery bank and conditions as the calculation. Then review age, temperature, maintenance and manufacturer test information. Do not compensate for a discrepancy by silently increasing efficiency or usable discharge. Record it as evidence and seek battery or installer assessment when capacity appears abnormal.
- Keep the theoretical ceiling and delivered estimate separate.
- Do not bypass safety or discharge cut-offs.
- Replace assumptions only with compatible evidence.