Transparent engineering model
Solar calculator methodology and formulas
This page explains how the calculator converts appliance loads into a planning estimate for PV array size, inverter capacity, starting surge, battery storage and roof area. The purpose is to make assumptions visible and auditable rather than hiding them behind a single “exact system size.”
1. Running load
Running load represents the selected appliances operating at the same time. It is used for inverter continuous-power sizing and the balanced PV-sizing check.
2. Daily energy
Operating hours are user inputs. If an appliance cycles on and off, use an equivalent average run-time or improve the appliance record with measured energy data.
3. Starting surge
If an appliance has manufacturer surge watts or a surge multiplier in the catalog, the calculator uses it. If a motor/compressor load lacks surge data, the calculator applies the visible user-adjustable “unknown motor start multiplier.” The default is a planning assumption, not a product specification.
4. PV array from energy
Peak sun hours represent the solar resource for the chosen location and period. The default 14% miscellaneous system-loss input follows the general PVWatts V5 loss allowance for losses such as soiling, mismatch, wiring, connections, degradation and availability, while this calculator models inverter efficiency separately.
5. Balanced PV sizing
Balanced mode checks both daily energy and the selected daytime running load. Energy-offset mode uses daily energy only. Neither mode guarantees full real-time PV output because irradiance, temperature, shading and orientation vary continuously.
6. Panel count
The calculator rounds up to a whole panel and reports the resulting installed DC capacity. String voltage/current design is intentionally not invented; it must be checked against the selected panel and inverter electrical datasheets.
7. Inverter continuous power
The calculator reports continuous kW, estimated kVA using the selected overall power factor, and a separate recommended starting-surge rating. Inverter manufacturers specify different surge durations and overload curves, so the final model must be checked against its datasheet.
8. Battery usable energy
The backed-up load percentage is adjustable. It lets a user model essential loads instead of assuming every selected appliance must remain on during an outage.
9. Battery nominal capacity
The calculator provides planning profiles for LiFePO₄ and lead-acid batteries. Actual battery BMS limits, maximum current, temperature, ageing, warranty conditions and inverter compatibility must be checked before procurement.
10. Roof area
Roof area is withheld if panel dimensions are unavailable. This estimate does not replace structural assessment, setbacks, fire access, row spacing, tilt/shading studies or local code requirements.
External reference used for the default loss assumption
NREL PVWatts documents a 14% default system-loss value for losses not otherwise explicitly modeled in PVWatts V5. The calculator uses 14% as an editable planning default and keeps inverter efficiency as a separate editable input.
NREL PVWattsLimitations
The calculator does not perform hourly irradiance simulation, detailed shading analysis, PV string-voltage calculations, conductor sizing, protection coordination, short-circuit studies, structural checks, utility approval, tariff analysis or code compliance. Use it for planning and comparison, then verify the final system with site-specific engineering.