The math behind the result
Solar panel calculator methodology
The calculator converts an annual electricity target into a whole-panel estimate using panel wattage, peak sun hours and a transparent performance factor.
Last reviewed: July 23, 2026
1. Estimate monthly electricity use
If you do not enter actual monthly kWh, Watt My Roof starts with one of four planning baselines. These are internal estimating assumptions—not claims about a national average—and are intended to represent general household load before the selectable major loads are added.
| Home-size starting point | Monthly baseline |
|---|---|
| 1 bedroom | 310 kWh |
| 2 bedroom | 420 kWh |
| 3 bedroom | 560 kWh |
| 4+ bedroom | 740 kWh |
2. Add selected appliance loads
The following monthly allowances are added to the home baseline. Actual use can be substantially higher or lower based on equipment efficiency, climate, fuel type, occupancy and operating habits.
| Selectable load | Planning allowance |
|---|---|
| Refrigerator | 55 kWh/month |
| Washer | 12 kWh/month |
| Electric dryer | 75 kWh/month |
| Central HVAC | 240 kWh/month |
| Dishwasher | 25 kWh/month |
| Electric water heater | 320 kWh/month |
| Electric vehicle | 260 kWh/month |
| Pool pump | 300 kWh/month |
When you enter actual monthly kWh from your electricity bill, that number replaces the home-size and appliance estimate. For the best starting result, average 12 months of electricity usage.
3. Set the electricity offset
Monthly kWh is multiplied by 12 and then by the selected offset: 75%, 90%, 100% or 110%. An offset above 100% may be useful for planned future loads, but utilities may restrict system size or compensate exported electricity differently.
4. Estimate annual production per panel
The 0.80 performance factor reserves 20% for real-world losses such as temperature, inverter conversion, wiring, mismatch, soiling and other operating conditions. It is a general planning factor, not a site-specific production model.
Example: a 400-watt panel at five peak sun hours produces an estimated 584 kWh per year after the 20% allowance.
5. Divide, round up and report system size
The annual electricity target is divided by annual production per panel. Watt My Roof rounds the result up to the next whole panel. The displayed DC system size is the panel count multiplied by panel wattage.
What the estimate does not model
- Roof direction, pitch, usable area, shade, setbacks or structural condition
- Hourly demand, appliance startup surge, inverter sizing or battery capacity
- Monthly weather patterns, snow cover, degradation or equipment-specific performance
- Utility interconnection limits, net-metering terms, rates, incentives or permits
- Project cost, financing, tax treatment, payback or guaranteed savings
For a professional production model, compare the estimate with a site-specific tool and a qualified installer. The companion calculation guide includes a worked example.