Inverter Size Calculator: Continuous Watts, Surge Duration, Battery Current, and Energy Limits
Last reviewed August 10, 2026. Verify current product documents, official rules, and local requirements before acting.
Direct answer
Estimate inverter output, battery current, surge duration, and energy limits without treating wattage as the only compatibility check.
Inputs
Results
Enter or confirm the inputs to see the result.
Interpretation
The calculator distinguishes inverter wattage, battery-bank current, surge duration, AC configuration, and stored energy. A larger inverter does not fix a battery or BMS current limit, a short-duration peak rating used for a longer event, or an incompatible phase arrangement.
Pass, warning, and fail conditions
- Continuous-current PASS, peak-current PASS, and duration PASS mean the modeled value is below the entered limit. WARNING means it is within 10% of a limit; FAIL means the limit is exceeded or the duration/configuration is inconsistent.
- The bank energy check is separate from the power/current check. A bank can pass current and still be energy-limited for the requested runtime, while a current or wattage failure is power-limited.
- Read the named result row to identify which input caused a warning or failure.
Formula or method
continuous AC watts = (running watts × (1 + margin)) ÷ power factor + idle watts
surge AC watts = (running watts + startup watts) ÷ power factor + idle watts
DC current = AC watts ÷ (battery voltage × efficiency)
usable bank energy = usable kWh per battery × battery quantity
Assumptions and limitations
- Battery quantity is total installed modules; batteries in parallel controls the current-limit aggregation. Confirm the manufacturer’s permitted series/parallel topology rather than assuming modules can be combined.
- The power-factor input is an optional planning approximation; motor and electronic-load behavior may require measured or manufacturer data.
- The calculator does not verify listing, transfer equipment, grounding/bonding, conductor ampacity, fault current, generator interaction, or utility interconnection.
Worked example
With 3,000 W running load, 2,500 W startup load, 48 V, 92% efficiency, 20% margin, two 5 kWh batteries, and a four-hour request, the output should show separate continuous and peak current decisions plus whether the 10 kWh bank can cover the modeled runtime. The exact inverter and battery manuals still control.