Solar Panels Needed Calculator

Demand and solar resource drive system size

Sizing starts with average daily electricity demand. Convert monthly bills to kWh per day, then divide by peak sun hours and a performance ratio. A realistic ratio combines wiring and soiling losses, inverter efficiency, temperature derate, and roof orientation. For many homes, PR falls between 0.75 and 0.88. If you use 30 kWh/day, 5 sun hours, and PR 0.82, the required array is about 7.3 kW DC.

Module wattage determines panel count

Panel count is array size divided by module power. Higher-watt modules reduce panel quantity, but roof geometry still matters. A 550 W panel needs fewer units than a 450 W panel for the same kW. The calculator rounds up because partial panels are impossible, which increases the DC size. After rounding, it estimates an inverter rating using a DC/AC ratio near 1.2, balancing clipping and cost.

Roof area and utilization constrain layout

Roof area is the hidden constraint. Multiply panel length by width to get one-module area, then multiply by panel count. Divide by utilization to account for walkways, setbacks, vents, and spacing. Utilization commonly ranges from 65% to 85% on roofs. For example, fourteen 550 W panels at 2.30 m by 1.13 m occupy about 36.4 m² of module area; at 80% utilization you may need about 45.5 m² of roof.

Production links sizing to savings

Energy output is estimated from DC size, sun hours, and PR. Daily output times 365 produces an annual figure you can compare with annual consumption. Seasonal variation can shift monthly production by 20% or more, so annual planning is safer. If annual production is 10,500 kWh and your rate is 0.20 per kWh, gross bill offset is about 2,100 per year. Net savings depend on export compensation and fixed charges.

Cost and payback are decision metrics

Cost and payback are planning indicators, not bids. The estimate combines module cost, inverter allowance, and an installed cost per DC watt that includes racking, wiring, labor, and permits. Small systems often have higher cost per watt due to fixed overhead. Payback equals total cost divided by annual savings; many markets fall between 4 and 10 years. Use the buffer setting to accommodate load growth, electric vehicles, or future heat pumps.

1) What are peak sun hours?

Peak sun hours represent daily solar energy as equivalent hours at 1,000 W/m². They summarize irradiance, so higher values mean more expected production from the same system size.

2) Why does the calculator use a performance ratio?

Real systems lose energy to heat, dirt, wiring, inverter conversion, and suboptimal orientation. PR compresses these effects into one factor, helping convert panel nameplate power into realistic energy output.

3) Should I size for 100% of my bill?

Not always. Net-metering rules, export rates, and fixed charges can make 60–100% offset optimal. If you expect load growth, use the buffer option rather than oversizing without policy context.

4) What DC/AC ratio should I choose?

Many rooftop systems use 1.1–1.3. Higher ratios lower inverter cost per kW but may clip midday power. Lower ratios reduce clipping but increase inverter size and cost.

5) How accurate is the roof area estimate?

It is a planning estimate using module dimensions and an assumed utilization factor. Obstructions, setbacks, roof pitch, and racking layout can change usable area, so confirm with a site survey.

6) Why do my results differ from an installer proposal?

Installers may use hourly weather data, shading models, specific equipment curves, and local permitting costs. Use this tool to sanity-check assumptions, then refine inputs to match your project details.