Start with the right number
A bill is a useful place to begin, but its total is not a measurement of energy. Fixed charges, taxes, adjustments and tariff slabs can distort a bill-divided-by-rate estimate. Copy the billed kWh from each of the last 12 months. Sum them before sizing; a single summer bill can overstate an annual requirement.
If your bill only reports money, use the energy-charge component and an effective variable rate for a first estimate. Mark that input as inferred. Keep the fixed charge separate because it generally remains after installing PV.
| Input | Illustrative value | Where to verify |
|---|---|---|
| Annual electricity use | 3,600 kWh | 12 monthly bills |
| Daily equivalent sun hours | 5.0 h/day | Location and panel-plane irradiation |
| Performance ratio | 0.78 | Loss assumptions |
| Panel nameplate | 550 W | Selected module datasheet |
These are synthetic teaching inputs, not a measured site or a quote. “Five sun hours” means the day’s irradiation expressed at 1 kW/m²; it is not five clock hours of daylight. Panel tilt, orientation and shadows still matter.
Follow the calculation
For 100% annual energy offset, 3,600 ÷ (365 × 5 × 0.78) = 2.529 kWp. Divide by 0.55 kW per panel and round up: five panels, or 2.75 kWp. Recalculate generation from what can actually be installed: 2.75 × 365 × 5 × 0.78 = 3,914.6 kWh/year.
The extra generation comes from panel rounding, not better performance. A smaller offset target may be appropriate when daytime demand is low or exports are poorly compensated. Annual generation alone cannot tell you the best economic size.
Test the assumption that matters most
| Daily sun hours | Same 2.75 kWp array | Annual use covered by generation* |
|---|---|---|
| 4.0 | 3,131.7 kWh | 87% |
| 5.0 | 3,914.6 kWh | 109% |
| 6.0 | 4,697.6 kWh | 130% |
*This is an annual energy ratio, not a bill saving or an hourly self-sufficiency result. A cloudy season can require substantial grid imports even if annual production exceeds annual consumption.
Use a location-specific model to refine the yield. In PVGIS, record coordinates, database, slope, azimuth, horizon and loss settings. Its final kWh/kWp output already includes the selected model losses: do not multiply that output by another performance ratio. The simple calculator instead starts from irradiation expressed as sun hours.
Check the roof and the equipment
The existing solar calculator uses 9.3 m²/kWp as a preliminary roof allowance: this example screens at about 26 m². Actual module dimensions give a separate panel-only footprint. Neither number proves that the roof is suitable.
- Sketch tanks, parapets, trees, maintenance access and shaded strips.
- Ask the installer for a layout and structural/wind-load checks.
- Check inverter input voltage, current, string arrangement and export restrictions.
- For backup, confirm islanding capability, changeover arrangements and a separate critical-load circuit. A standard grid-tied PV array does not establish an outage supply.
Cost and incentive values in the original calculator are planning defaults. Obtain a dated, tax-inclusive quote and verify current programme eligibility independently before using its payback result.
Turn the estimate into a useful brief
Save your annual kWh, resource assumption, candidate panel and roof constraint together. Run a lower-yield scenario before asking for quotes. Then compare the installer’s annual generation estimate against your calculation and ask them to explain any material difference.
Next, read why generated units and saved money differ. You can reproduce the energy example in the planning notebook and retain both your base and lower-yield cases.
Sources and scope
- European Commission PVGIS ↗
Location, irradiation and system-performance inputs; the example above is independently calculated.
- NREL PVWatts Version 5 Manual (2014) ↗
Background on a weather-based PV model. This simplified calculator does not run PVWatts.
Reference links provide technical context; the worked examples and interpretation are specific to this guide. No external source certifies these tools.