Solar Panel Size Calculator

A solar panel size calculator converts a daily energy target into a preliminary required PV wattage and a whole-number panel count. The result helps establish the starting array capacity for PV layout, inverter review, string planning, DC conductor routing, rooftop space planning, and preliminary production estimates.

Enter daily energy, peak sun hours, system losses, and panel wattage for a transparent preliminary PV capacity estimate.

Updated August 22, 2026

Required PV Wattage

The calculator begins with Daily energy use, expressed in kWh/day. This is the amount of energy the PV system is expected to offset or produce during an average planning day.

Peak sun hours converts available solar resource into an equivalent number of hours per day at full rated irradiance. It is a planning value, not the number of daylight hours.

System loss accounts for stated combined losses, including wiring, temperature, conversion, mismatch, and other assumptions. The calculator converts that percentage into an Applied production factor:

\(\text{Applied production factor} = 1 - \frac{\text{System loss}}{100}\)

Required PV wattage is then calculated as:

\(\text{Required PV wattage} = \frac{\text{Daily energy use} \times 1000} {\text{Peak sun hours} \times \text{Applied production factor}}\)

The result is expressed in watts because PV module ratings and array nameplate capacity are normally stated in W or kW.

A higher system loss reduces the usable output expected from each installed watt of PV. A lower peak-sun-hours assumption also increases the required array size. Both inputs directly increase the required PV wattage when energy demand remains unchanged.

Panel Count and Array Rounding

The calculator divides the required PV wattage by Panel wattage and rounds up to a whole module:

\(\text{Panel count} = \left\lceil \frac{\text{Required PV wattage}} {\text{Panel wattage}} \right\rceil\)

PV arrays cannot be installed as fractional panels. Rounding up means the installed nominal array wattage will commonly exceed the exact required PV wattage.

That installed capacity is:

\(\text{Installed array wattage} = \text{Panel count} \times \text{Panel wattage}\)

The calculator’s Estimated daily production uses the rounded panel count rather than the unrounded required PV wattage:

\(\text{Estimated daily production} = \frac{\text{Panel count} \times \text{Panel wattage} \times \text{Peak sun hours} \times \text{Applied production factor}} {1000}\)

This is why estimated daily production can be higher than the entered daily energy target. The difference is the effect of rounding the array upward to whole modules.

Calculation Example

Using the displayed values:

Input	Value
Daily energy use	10 kWh/day
Peak sun hours	5 h/day
System loss	20%
Panel wattage	400 W

First, calculate the production factor:

\(1 - 0.20 = 0.8\)

Then calculate the required PV wattage:

\(\frac{10 \times 1000}{5 \times 0.8} = \frac{10{,}000}{4} = 2{,}500\text{ W}\)

Next, calculate the panel count:

\(\left\lceil\frac{2{,}500}{400}\right\rceil = \left\lceil6.25\right\rceil = 7\text{ panels}\)

The installed nominal array wattage becomes:

\(7 \times 400 = 2{,}800\text{ W}\)

Estimated daily production from the rounded array is:

\(\frac{2{,}800 \times 5 \times 0.8}{1000} = 11.2\text{ kWh/day}\)

Result: 2,500 W required PV wattage, 7 panels, 11.2 kWh/day estimated daily production, and a 0.8 applied production factor.

PV Circuit and Layout Review

Required PV wattage and panel count establish array capacity, but they do not establish a complete electrical design. The selected module, inverter, and array configuration determine the actual DC electrical characteristics used for field calculations.

After preliminary sizing, the installation design typically needs to verify:

• Series and parallel module arrangement for the equipment’s permitted operating voltage range
• PV source-circuit and output-circuit current based on actual module electrical ratings
• DC conductor AWG or kcmil selection, ampacity, insulation temperature rating, terminal rating, correction factor, and adjustment factor
• Raceway fill, conductor routing, rooftop temperature exposure, and physical protection
• Voltage-drop review for longer DC homeruns, AC branch circuits, and feeders
• Disconnecting means, overcurrent protection, inverter output connection, and available load capacity
• Roof area, module orientation, shading, structural attachment, access paths, and equipment clearances

Panel count alone does not determine string count. For example, seven 400 W modules produce a 2,800 W nominal array, but the acceptable series/parallel arrangement depends on the module voltage and current data and the connected equipment’s limits.

Field Verification

The calculator uses the entered Peak sun hours and System loss as planning assumptions. Actual production varies with seasonal irradiance, array orientation, tilt, shading, soiling, module temperature, equipment operating limits, utility curtailment, and component performance.

Use the required PV wattage as a preliminary energy-sizing result. Final PV circuit design must be based on the selected equipment data, site conditions, installation method, applicable code requirements, utility rules, manufacturer instructions, and the requirements of the AHJ.