Inverter Capacity Calculator
Estimate inverter continuous watt/VA capacity and surge requirement from simultaneous loads, headroom and power-factor assumptions.
Estimate inverter capacity from connected loads
Calculate continuous load, recommended inverter headroom and surge requirement from user-entered loads.
How the Inverter Capacity Calculator works
For backup/load-side sizing, the first requirement is simultaneous continuous real power. The calculator adds a configurable headroom percentage to that running load and divides by the entered power factor to provide a rough VA requirement. Startup surge is reported separately as running load plus additional surge watts.
How to use this inverter capacity calculator
List or measure loads that may run at the same time and enter their combined watts. Add surge above running watts for motors, pumps, compressors or other high-inrush equipment. Choose conservative headroom and power factor based on equipment/inverter documentation rather than treating the defaults as universal engineering standards.
How to interpret the result
An inverter must satisfy both continuous and surge requirements. A unit with adequate continuous watts can still trip if its surge duration/rating is insufficient. Conversely, oversizing solely for a brief inrush may not be necessary if the inverter has a documented short-duration surge capability that matches the load.
Assumptions and limitations
Real sizing must consider DC battery voltage/current, wiring, ambient temperature derating, inverter efficiency, waveform compatibility, neutral/grounding, code requirements and load diversity. For grid-tied PV, array-to-inverter DC/AC ratio is a separate design topic; NREL PVWatts exposes DC/AC ratio and inverter efficiency as system parameters.
Practical example and workflow
With 2,500 W of simultaneous running load and 25% headroom, the continuous planning target is about 3,125 W. At power factor 0.9 that corresponds to roughly 3,472 VA. If startup adds another 1,500 W, the inverter must also tolerate about 4,000 W for the required surge duration.