What this calculator does
A solar system calculator can answer two different questions: what a given array will produce, or how big an array needs to be in the first place. This one answers the second, more practical question for anyone starting from scratch: given how much energy you need to cover each day, how large a solar power system, and how many panels, does that actually require.
The calculation runs the output formula in reverse. Instead of multiplying array size by sun hours and efficiency to get a yield, it starts from the daily usage target and divides back through those same two factors to get the array size, then converts that into a panel count at whatever wattage the panels being considered are rated for.
The formula
Daily energy usage is divided by the average peak sun hours per day and by the system efficiency (expressed as a decimal) to get the required array size in kilowatts. That array size is then divided by the wattage of a single panel and rounded up to a whole number of panels. If a battery autonomy period is entered, the usage, autonomy and depth of discharge are combined to estimate the battery storage needed to cover that many days without sun.
| Term | Meaning |
|---|---|
| Peak sun hours | The number of hours per day of sunlight at an intensity equivalent to full standard test conditions, not the number of daylight hours. |
| System efficiency | The share of a panel's rated output actually delivered after inverter, wiring, dust and heat losses. |
| Depth of discharge | The share of a battery's capacity that can be used before it is considered fully discharged. |
| Autonomy | The number of days a battery bank is sized to cover with no solar input at all. |
The inputs explained
| Field | What to enter |
|---|---|
| Daily energy usage to cover (kWh) | The daily energy usage you want the system to cover, in kilowatt-hours. |
| Average peak sun hours per day | Average peak sun hours per day for the location, available from local solar irradiance data. |
| System efficiency (inverter, wiring, dust, heat losses) (%) | A typical grid-tied system loses 15 to 25% to inverter, wiring, dust and heat losses; 80% efficiency is a common starting estimate. |
| Panel wattage (W) | The rated wattage of a single panel being considered. |
| Battery autonomy wanted (0 for none) (days) | Set to 0 to size the array only, with no battery. Set to 1 or more to also size a battery for that many days without sun. |
| Battery depth of discharge (%) | How deep the battery can safely be discharged; check the manufacturer figure for the battery chemistry being used. |
When to use it
Sizing an off-grid cabin or shed system
Starting from a measured or estimated daily energy usage is the standard way to size an off-grid system, since there is no grid to fall back on if the array is undersized.
Working out how many panels a roof needs
Once daily usage is known, this converts straight into a panel count, which is the number a quote or roof-space check actually needs.
Sizing battery storage for a set number of days of backup
Adding a target autonomy period turns the same daily usage figure into a battery capacity target, on top of the array size.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the recommended array size changes with daily usage
A fixed sun hour and efficiency figure, across a range of daily energy usage targets.
| Daily usage target | Recommended array size | Panels needed at this wattage |
|---|---|---|
| 5 kWh | 1.39 kW | 4 panels (1.60 kW total) |
| 10 kWh | 2.78 kW | 7 panels (2.80 kW total) |
| 20 kWh | 5.56 kW | 14 panels (5.60 kW total) |
| 30 kWh | 8.33 kW | 21 panels (8.40 kW total) |
| 40 kWh | 11.11 kW | 28 panels (11.20 kW total) |
| 60 kWh | 16.67 kW | 42 panels (16.80 kW total) |
How the recommended array size changes with peak sun hours
A fixed 20 kWh daily usage target, across a range of peak sun hour figures.
| Peak sun hours | Recommended array size | Panels needed at this wattage |
|---|---|---|
| 3 h/day | 8.33 kW | 21 panels (8.40 kW total) |
| 3.5 h/day | 7.14 kW | 18 panels (7.20 kW total) |
| 4 h/day | 6.25 kW | 16 panels (6.40 kW total) |
| 4.5 h/day | 5.56 kW | 14 panels (5.60 kW total) |
| 5 h/day | 5.00 kW | 13 panels (5.20 kW total) |
| 5.5 h/day | 4.55 kW | 12 panels (4.80 kW total) |
| 6 h/day | 4.17 kW | 11 panels (4.40 kW total) |
Questions
How is this different from a solar energy yield calculator?
A yield calculator starts from a chosen array size and works out what it will produce. This one runs the same relationship in reverse, starting from how much energy you need covered and working out the array size and panel count required to produce it.
Where do I find peak sun hours for my location?
Local solar irradiance data, often available from a national weather or energy agency, or from a solar installer's site assessment, gives a location-specific average. It is not the same as hours of daylight, since it accounts for sun intensity, not just whether the sun is up.
Should I size for average usage or peak usage?
That depends on the goal. Sizing for average daily usage gives a system that roughly balances over a year; sizing for peak usage, such as the highest-usage month, gives more margin but a larger and more expensive system.
How accurate is the battery sizing?
It is a straightforward estimate from usage, autonomy and depth of discharge, not a substitute for a proper system design that accounts for battery ageing, temperature effects and inverter sizing. Treat it as a starting figure to take to an installer, not a final specification.
To check what a specific array size would actually produce, see the solar energy yield calculator. For runtime on a given battery and load, see the battery runtime calculator.