SpecCalc Hub · Planning estimate
Inverter Size Calculator
Estimate continuous and surge inverter size from load, surge, margin, and power factor.
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Formula
recommended_continuous_w = continuous_load_w * (1 + margin) recommended_continuous_va = recommended_continuous_w / power_factor recommended_surge_w = max(largest_surge_load_w, recommended_continuous_w)
Assumptions
- Inputs are user-provided.
- Results are preliminary estimates.
- User provides realistic continuous and surge load.
Limitations
- Does not guarantee motor startup compatibility or validate wiring, fuses, thermal limits, or waveform requirements.
Sources / methodology
Calculates active power in W with the selected margin, then apparent power in VA using the load power factor. Both manufacturer limits must be satisfied.
- Formula version
- 1.1.0
- Formula review date
- 2026-09-15
Source links
Quick answer: Inverter Size Calculator estimates recommended continuous from transparent input parameters and a visible formula. Results are informational estimates only and should be checked against the stated assumptions and limitations.
Input parameters
- Continuous load (W): 1,000 W
- Peak total load, including other running devices (W): 2,000 W
- Planning margin (%): 25 %
- Combined load power factor (PF): 1 PF
Output values
- Recommended continuous (W)
- Recommended continuous (kW)
- Required continuous apparent power (VA)
- Recommended surge (W)
Step-by-step example
Calculation for the example inputs shown above:
- Pₙ = 1,000 × (1 + 25/100) = 1,250 W
- Sₙ = 1,250 ÷ 1 = 1,250 VA
- Pₚ = max(2,000; 1,250) = 2,000 W
W and VA are separate limits. Confirm both ratings and the permitted peak duration in the inverter datasheet.
Worked examples
Worked example with the form defaults
Example inputs: Continuous load: 1,000 W; Peak total load, including other running devices: 2,000 W; Planning margin: 25 %; Combined load power factor: 1 PF. Pₙ = 1,000 × (1 + 25/100) = 1,250 W. Sₙ = 1,250 ÷ 1 = 1,250 VA. Pₚ = max(2,000; 1,250) = 2,000 W. W and VA are separate limits. Confirm both ratings and the permitted peak duration in the inverter datasheet.
Alternative calculation scenario
Example inputs: Continuous load: 1,000 W; Peak total load, including other running devices: 2,000 W; Planning margin: 25 %; Combined load power factor: 0.8 PF. Pₙ = 1,000 × (1 + 25/100) = 1,250 W. Sₙ = 1,250 ÷ 0.8 = 1,563 VA. Pₚ = max(2,000; 1,250) = 2,000 W. W and VA are separate limits. Confirm both ratings and the permitted peak duration in the inverter datasheet.
What the result does not establish
Enter the simultaneous total peak, including devices already running. A peak-watt number alone does not establish motor starting: the inverter’s peak duration, output waveform and battery current limits also matter.
Common mistakes
- Keep all units consistent with the field labels.
- Check whether the input values are measured, nameplate values or planning assumptions.
- Review the stated assumptions and limitations before using the estimate.
What to check next
Check the inputs, limitations, sources and related calculators before using the estimate in a real decision.
FAQ
What does Inverter Size Calculator calculate?
Inverter Size Calculator estimates recommended continuous from transparent input parameters and a visible formula. Results are informational estimates only and should be checked against the stated assumptions and limitations.
What formula does this calculator use?
The visible formula starts with: recommended_continuous_w = continuous_load_w * (1 + margin). The full formula, assumptions and limitations are shown on the page.
Are the results stored?
No account is required and SpecCalc Hub does not save personal calculation history.
Related calculators
This calculator provides an estimate for informational purposes only. It is not a certified engineering design, electrical safety approval, or professional installation recommendation. Always verify final decisions with a qualified professional and applicable local codes.