Solar System Size Calculator

How to calculate solar system size

The calculator first converts your electricity use to an average daily figure. It then applies your desired solar offset and divides the target daily generation by the usable solar energy available after system losses.

The basic sizing formula is: solar system size (kW DC) = target daily generation ÷ [peak sun hours × (1 − system losses)].

This provides a preliminary DC array size based on energy use and average solar resource. It does not model hourly production or the detailed electrical design of a photovoltaic system.

What are peak sun hours?

Peak sun hours are a solar-energy measure, not simply the number of daylight hours. One peak sun hour represents the same solar energy as one hour of sunlight at an irradiance of 1,000 watts per square metre.

For example, 4.5 peak sun hours means that the total solar energy received during the day is equivalent to 4.5 hours at 1,000 W/m². The actual sunlight may be spread across a much longer period at varying levels of intensity.

Solar resource varies with location, season, weather and the orientation of the receiving surface. For preliminary sizing, use an annual-average solar-resource value appropriate to the proposed installation location.

Why include system losses?

Solar panels do not normally deliver their rated output as usable energy throughout the year. Real photovoltaic systems experience losses from several sources.

These can include soiling, shading, module mismatch, wiring and connection losses, degradation and periods when the system is not available. Temperature and inverter performance can also affect the energy ultimately delivered by a complete PV system.

This calculator uses 14% as its initial system-loss assumption. A 14% system-loss value is also used in an example in the current PVWatts documentation. The appropriate value for an individual installation may be higher or lower.

For more information about photovoltaic energy-production modelling, see the PVWatts V8 documentation .

How the solar panel count is estimated

Once the required DC system capacity has been calculated, the system wattage is divided by the rated wattage of one solar panel. The result is rounded up to the next whole panel.

For example, a calculated requirement of 5.8 kW is 5,800 watts. Using 450 W panels:

5,800 W ÷ 450 W = 12.89 panels

Because a fraction of a panel cannot normally be installed, this would be rounded up to 13 panels, giving an installed DC panel capacity of:

13 × 450 W = 5,850 W, or 5.85 kW DC.

Actual array design must also consider available roof or ground area, module dimensions, mounting layout, inverter requirements and electrical design constraints.

What does solar offset mean?

Solar offset is the percentage of your electricity consumption that you want the solar system to generate on an energy basis. A 100% offset targets annual solar generation equal to the entered electricity consumption, while an 80% offset targets four-fifths of it.

A 100% annual energy offset does not mean that solar will supply 100% of your electricity at every moment. Solar generation varies throughout the day and year, while electricity consumption follows its own pattern.

A grid-connected system may therefore export surplus electricity at some times and import electricity at others. Battery storage can change when solar energy is available for use, but battery sizing is outside the scope of this calculator.

Why location, tilt and shading matter

Two solar systems with the same installed panel capacity can produce different amounts of electricity because solar energy reaching the panels depends on the installation site.

Geographic location affects the available solar resource, while panel orientation and tilt influence how effectively the array receives that energy. Nearby trees, buildings, roof features and other obstructions can create shading and reduce production.

The U.S. Department of Energy notes that photovoltaic energy yield is affected by factors including heat, dirt and shade, and that system configuration has an important effect on annual energy production.

See the U.S. Department of Energy guidance on photovoltaic system design and energy yield for further information.

Worked solar system sizing example

Suppose a property uses 900 kWh per month and the goal is to offset 100% of that consumption. Annual electricity use is estimated as:

900 × 12 = 10,800 kWh/year

The average daily generation target is therefore approximately:

10,800 ÷ 365 = 29.59 kWh/day

If the location averages 4.5 peak sun hours per day and system losses are estimated at 14%, the effective solar-hours value is:

4.5 × (1 − 0.14) = 3.87 hours/day

The estimated required DC system size is then:

29.59 ÷ 3.87 = approximately 7.65 kW DC

With 450 W panels, the preliminary panel requirement would be approximately 17 panels, giving an installed panel capacity of 7.65 kW DC.

Important limitations

This calculator provides a preliminary energy-based sizing estimate. It is not a detailed photovoltaic production model and does not guarantee how much electricity a particular installation will generate.

Actual production depends on site-specific solar resource, weather, panel orientation and tilt, shading, module temperature, equipment performance, inverter configuration, system availability and other factors.

Final system design should also consider available installation area, structural requirements, electrical protection, inverter and string configuration, local grid-connection requirements and applicable electrical and building standards.

Frequently asked questions

Can I use monthly or annual electricity consumption?

Yes. Choose Daily, Monthly or Annual to match the electricity-use figure you enter. Monthly use is annualised using 12 months and annual use is converted to a daily average using 365 days.

How do I find my electricity consumption?

Electricity bills normally show energy consumption in kilowatt-hours (kWh). Using consumption from several bills, or preferably a full year, gives a better picture of annual electricity use than relying on a single month.

Does this calculator size a battery?

No. Battery-storage sizing is a separate calculation involving factors such as overnight load, desired backup duration, usable battery capacity, maximum discharge level and charging efficiency.

Does a larger solar system always mean a higher solar offset?

Increasing installed capacity can increase annual solar generation, but actual output still depends on the available solar resource and installation conditions. Grid-export limits, available installation area and electricity-use patterns can also affect the practical system size.

Is the calculated size the exact system I should install?

No. It is a preliminary estimate. Final solar design should use site-specific solar-resource data and account for orientation, tilt, shading, equipment performance, electrical constraints and available installation area.