Solar Panel Calculator: method, checked example and practical limits
Estimate array size, panel count and annual production from energy use and solar-resource assumptions. The array estimate balances an average energy budget rather than guaranteeing instantaneous supply or annual production.
Inputs used by this calculator
Solar Panel Calculator uses the fields below. The starting values reproduce the page example; replace them with values from the same units and time period before using the result.
| Input | Starting example | Accepted value | How it is used |
|---|---|---|---|
| Average daily energy use (kWh) | 30 | 0 to 1000000000 | Supplies the average daily energy use (kwh) quantity used by the published equation or result breakdown. |
| Peak sun hours per day | 5 | 0.01 to 24 | Supplies the peak sun hours per day quantity used by the published equation or result breakdown. |
| System derate efficiency (%) | 80 | 1 to 100 | Supplies the system derate efficiency (%) quantity used by the published equation or result breakdown. |
| Panel rating (W) | 400 | 1 to 10000 | Supplies the panel rating (w) quantity used by the published equation or result breakdown. |
| Installed price per watt | 3 | 0 to 1000000 | Supplies the installed price per watt quantity used by the published equation or result breakdown. |
Inputs and measurement conventions
The quantities used on this page are Average daily energy use (kWh), Peak sun hours per day, System derate efficiency (%), Panel rating (W), Installed price per watt. Start with values for the same situation and read the units attached to each field. The field reference below shows supported choices and numerical limits.
Peak-sun hours are an irradiation measure, not daylight duration. Shade, tilt, weather and storage affect usable generation.
Method and equation
Divide daily energy demand by equivalent peak-sun hours and system efficiency to size nameplate kilowatts, then round panel count upward.
Solar Panel Calculator uses the following stated relationship: array kW = daily kWh ÷ peak-sun hours ÷ system efficiency. Its assumptions determine which question the answer can resolve.
array kW = daily kWh ÷ peak-sun hours ÷ system efficiencyWorked calculation with the starting values
Average daily energy use (kWh): 30; Peak sun hours per day: 5; System derate efficiency (%): 80; Panel rating (W): 400; Installed price per watt: 3. These starting values give 19 solar panels. The calculation breakdown is Required array (kW): 7.5; Installed nameplate (kW): 7.6; Annual target energy (kWh): 10,950; Estimated installed cost: 22,800.
30 kWh/day, five sun hours and 80% system efficiency needs about 7.5 kW, or 19 whole 400 W panels.
Reading the answer
The array estimate balances an average energy budget rather than guaranteeing instantaneous supply or annual production.
Required array (kW): 7.5; Installed nameplate (kW): 7.6; Annual target energy (kWh): 10,950; Estimated installed cost: 22,800. These intermediate quantities make the stated Solar Panel Calculator model inspectable; they should be read together with the main answer rather than treated as independent guarantees.
Common errors and model boundaries
Peak-sun hours are an irradiation measure, not daylight duration. Shade, tilt, weather and storage affect usable generation.
Use a site-specific solar assessment for shade, orientation, weather, utility rules, roof loads and production guarantees.
Independent verification and next decision
No. Storage capacity, charge control, autonomy and peak loads require their own time-dependent design.
For the Solar Panel Calculator method, compare array kW = daily kWh ÷ peak-sun hours ÷ system efficiency with the starting inputs and the calculation table. The linked NREL PVWatts Calculator and U.S. Department of Energy: homeowner's guide to going solar references identify the relevant definitions, conventions or domain limitations. Retain intermediate precision when comparing a manually worked answer.
Solar Panel Calculator worked-example ledger
| Quantity | Example value | Role |
|---|---|---|
| Average daily energy use (kWh) | 30 | Entered assumption |
| Peak sun hours per day | 5 | Entered assumption |
| System derate efficiency (%) | 80 | Entered assumption |
| Panel rating (W) | 400 | Entered assumption |
| Installed price per watt | 3 | Entered assumption |
| Main answer | 19 solar panels | Calculated from the starting inputs |
| Required array (kW) | 7.5 | Intermediate or contextual output |
| Installed nameplate (kW) | 7.6 | Intermediate or contextual output |
| Annual target energy (kWh) | 10,950 | Intermediate or contextual output |
| Estimated installed cost | 22,800 | Intermediate or contextual output |
Frequently asked questions
Does this also size a battery?
No. Storage capacity, charge control, autonomy and peak loads require their own time-dependent design.
Which relationship does the Solar Panel Calculator use?
Divide daily energy demand by equivalent peak-sun hours and system efficiency to size nameplate kilowatts, then round panel count upward. The displayed relationship is array kW = daily kWh ÷ peak-sun hours ÷ system efficiency.
What do the starting values produce in the Solar Panel Calculator?
Average daily energy use (kWh): 30; Peak sun hours per day: 5; System derate efficiency (%): 80; Panel rating (W): 400; Installed price per watt: 3. These starting values give 19 solar panels. The calculation breakdown is Required array (kW): 7.5; Installed nameplate (kW): 7.6; Annual target energy (kWh): 10,950; Estimated installed cost: 22,800.
How should I interpret this Solar Panel Calculator answer?
The array estimate balances an average energy budget rather than guaranteeing instantaneous supply or annual production.
Which input mistake most affects the Solar Panel Calculator?
Peak-sun hours are an irradiation measure, not daylight duration. Shade, tilt, weather and storage affect usable generation.
What is outside this Solar Panel Calculator model?
Use a site-specific solar assessment for shade, orientation, weather, utility rules, roof loads and production guarantees.
What to keep in mind
Use a site-specific solar assessment for shade, orientation, weather, utility rules, roof loads and production guarantees.
Results are rounded for display in US format, with money in dollars and cents; CSV exports keep full calculation precision. This is a calculation summary, not an official certificate.