| Appliance | Power (W) | Hours/day | Daily Energy (kWh) |
|---|---|---|---|
| Ceiling Fans (4) | 280 | 10 | 2.80 |
| LED Lights | 180 | 6 | 1.08 |
| Refrigerator | 150 | 12 | 1.80 |
| Water Pump | 750 | 1 | 0.75 |
| Air Conditioner (1 ton) | 1200 | 5 | 6.00 |
| Total | 12.43 |
derate = (1 − losses%) × inverter_eff% × temperature_factor
adjusted_daily_kWh = daily_kWh × (1 + growth%)
system_kW = adjusted_daily_kWh ÷ (PSH × derate)
panels = ceil((system_kW × 1000) ÷ panel_W)
Note: This is an energy-based estimate; exact designs require site survey and shading analysis.
- Select daily or monthly usage mode.
- Enter your energy usage and a conservative PSH.
- Set losses, inverter efficiency, and temperature factor for realism.
- Add growth headroom if your demand may increase.
- Press Calculate to see panels and system size.
- Use Download CSV/PDF for sharing and quotes.
- If shading is common, increase losses and consider panel-level optimizers.
- For winter sizing, use lower PSH and a lower temperature factor.
- Always check roof orientation, tilt, and structural limits.
Translate Consumption Into a Daily Target
Your starting point is energy, not power. If your bill shows 360 kWh per month, the calculator converts it to about 11.83 kWh/day using 30.44 days. Add growth headroom to avoid undersizing; a 10% margin turns 12.00 kWh/day into 13.20 kWh/day. This adjusted target is what the array must produce on an average day, before net‑metering choices are considered. It helps align expectations between homeowners, designers, and installers early and accurately.
Peak Sun Hours Drive System Size
Peak sun hours (PSH) represent equivalent full‑sun hours per day. A location averaging 5.0 PSH delivers more energy from the same array than a 4.0 PSH site. In the sizing relationship, system kW equals daily kWh divided by PSH and derate. Changing PSH from 5.0 to 4.5 increases required capacity by about 11% for the same target energy.
Derate Factors Reflect Real Output
Real systems lose energy to heat, wiring, soiling, mismatch, and conversion. The calculator combines these with inverter efficiency and temperature factor into one derate multiplier. For example, 14% losses (0.86), 96% inverter efficiency (0.96), and a 0.90 temperature factor yield a derate near 0.743. If shading is frequent, raise losses rather than inflating PSH for conservative quotes.
Panel Rating Converts kW Into Count
Once required DC size is known, panel wattage turns kW into an integer count. With a 13.20 kWh/day target, 5.0 PSH, and 0.743 derate, required size is roughly 3.55 kW. Using 550 W modules gives 6.46 panels, rounded up to 7. The installed DC size becomes 3.85 kW, typically producing about 14.3 kWh/day under the same assumptions.
Roof Area and Budget Checks
Roof planning prevents late surprises. If each panel occupies 2.6 m², seven panels need about 18.2 m², plus spacing and access paths. Use the optional cost per watt to create a quick budget baseline; multiplying installed watts by your chosen rate enables comparisons. Export the results to CSV or PDF, then verify tilt, orientation, structural loading, and interconnection rules before purchase.
1) Should I use daily or monthly usage?
Use daily when you have monitoring data. Use monthly when you only have utility bills; the calculator converts monthly kWh into daily kWh using an average month length.
2) What PSH value should I enter?
Enter a conservative annual average for your area. If you are unsure, choose a lower value to avoid undersizing, then validate later with local solar resource data.
3) Why does the derate reduce output so much?
Derate accounts for unavoidable losses: soiling, wiring, mismatch, inverter conversion, and heat. These effects compound, so multiplying factors gives a more realistic production estimate.
4) How do I handle shading or dusty conditions?
Increase the losses percentage to reflect shading, dirt, and downtime. This is better than lowering panel wattage, because losses directly reduce energy production.
5) Does this include batteries?
No. This tool sizes the solar array from energy demand. Battery sizing depends on backup hours, depth of discharge, and charging constraints, which should be calculated separately.
6) Why is my recommended panel count rounded up?
Panels are discrete units. The calculator uses ceiling rounding so the installed system meets or exceeds the energy target after losses, rather than falling slightly short.