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Rooftop Solar System Sizing Calculator

This calculator estimates the rooftop solar array capacity needed to meet a chosen share of annual electricity consumption. It uses annual load, target solar offset, daily peak sun hours, and total system losses, then converts the required capacity into an approximate panel count.

Use it for an initial project scope before detailed roof layout and electrical design. The output is an energy-based size: available roof area, interconnection limits, shading, structural constraints, and export rules can reduce the practical system size.

Calculator inputs

kWh/year
%
hours
%
W
Result
—
Estimated required solar capacity
Target annual solar energy—
Approximate panel count—
Output from rounded panel count—

1. Enter annual electricity use
Use a recent 12-month total from utility bills or a forecast for the property.

2. Choose the target offset
Enter the percentage of annual consumption you want the solar array to produce.

3. Enter solar resource and losses
Use site-average peak sun hours and a combined system-loss assumption.

4. Enter panel wattage
Use the planned module nameplate rating to estimate whole-panel count.

5. Review capacity and panel count
The main result is continuous required capacity; the panel count rounds upward to a buildable whole number.

Target solar energy = Annual electricity use × Target offset %

Annual yield per kW = Peak sun hours × 365 × (1 − System loss %)

Required array capacity = Target solar energy ÷ Annual yield per kW

Panel count = ceil(Required capacity × 1,000 ÷ Panel watts)

What the result means

The main result is the DC array capacity estimated to produce the selected annual energy offset under the entered assumptions.

A target above 100% is mathematically allowed for planned electrification or export, but local design and interconnection rules may limit it.

Given: 12,000 kWh annual use, 80% target offset, 4.5 peak sun hours, 14% losses, and 420 W panels.

Calculation: Target energy = 12,000 × 0.80 = 9,600 kWh. Annual yield per kW = 4.5 × 365 × 0.86 = 1,412.55 kWh. Required capacity = 9,600 ÷ 1,412.55 = 6.80 kW. Panel count = ceil(6,800 ÷ 420) = 17.

Result: About 6.80 kW DC, represented by 17 panels or 7.14 kW of rounded nameplate capacity.

Why can the rounded panel system be larger than the required capacity?

Panels are installed in whole units. Rounding upward avoids undersizing relative to the energy target.

Should I use electricity consumption before or after efficiency upgrades?

Use the load expected after planned upgrades so the system is sized for future rather than historical consumption.

Can I target more than 100% offset?

The math allows it, but excess annual production may be constrained by roof area, export compensation, and interconnection rules.

Does battery storage reduce the array size required?

Storage shifts energy in time but does not create energy. It may improve self-consumption, while the annual production requirement remains driven by load and losses.

How is this different from the annual output calculator?

This tool works backward from desired annual energy to required capacity; the output calculator works forward from an existing capacity to annual generation.