Solar & battery
Off-Grid Solar Calculator
Sizes an off-grid system, where the worst month rather than the average governs.
Result
Off grid is sized on the worst case rather than the average, which is why it costs several times a grid-tied system of the same daily output. Almost every off-grid installation includes a generator, because sizing for a two-week December overcast is far more expensive than running one for a few days. Reducing the load is nearly always cheaper than generating more.
Get a Free Business AuditHow the calculation works
Daily energy consumption, divided by the peak sun hours of the worst month rather than the annual average, with battery storage for the intended days of autonomy and a charge source for recovery. Off-grid has no fallback, so the design has to work in December rather than on average.
What moves the number
- Daily consumption, measured rather than estimated.
- Worst-month peak sun hours at the site.
- Days of autonomy required.
- Whether a generator provides backup charging.
Where this one goes wrong
Sizing on annual average insolation produces a system that works most of the year and fails in winter, which off-grid cannot tolerate, so the worst month is the design condition. Load reduction is also far cheaper than generation: every watt removed from the load reduces array, battery, controller and wiring together, which is why serious off-grid design starts with the loads rather than the panels. A generator for the worst few weeks is frequently cheaper than sizing solar for them.
Before you quote
- Size on worst-month insolation, not annual average.
- Reduce loads before sizing generation.
- Consider a generator for the worst weeks rather than oversizing solar.
Questions
About this calculator
Why size off-grid on the worst month?
Because there is no grid to fall back on. A system sized to the annual average works most of the year and fails in winter, which off-grid cannot tolerate. The worst month is the design condition rather than a conservative extra.
What is the cheapest way to improve an off-grid design?
Reducing the load. Every watt removed reduces array, battery, charge controller and wiring together, so efficiency work compounds through the whole system. Serious off-grid design starts with the loads rather than the panels.
Is a generator an admission of failure?
It is usually sound economics. Sizing solar and storage for the worst few weeks of the year costs far more than a generator used occasionally, and most well-designed off-grid systems include one for exactly that reason.
The formula
array kW = daily kWh / (worst-month sun hours x derate); battery = daily kWh x autonomy / depth of discharge
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How to use this calculator
- Daily energy use (kWh). Enter your own figure; the page starts at 8.
- Peak sun hours, worst month. Enter your own figure; the page starts at Pacific Northwest (3.5).
- Days of autonomy. Enter your own figure; the page starts at 3.
- System derate (%). Enter your own figure; the page starts at 75%.
- Usable battery depth (%). Enter your own figure; the page starts at 80%.
- Battery cost per kWh ($). Enter your own figure; the page starts at $900.
The result updates as you type, so you can try a figure, see what it does to array size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Daily energy use (kWh) | 8 |
| Peak sun hours, worst month | Pacific Northwest (3.5) |
| Days of autonomy | 3 |
| System derate | 75% |
| Usable battery depth | 80% |
| Battery cost per kWh | $900 |
| Array cost per watt | $2.2 |
What comes out
| Result | Value |
|---|---|
| Array size main result | 3.0 |
| Battery bank | 30.0 |
| Equipment cost | $33,705 |
| Annual production | 2,920 |
| Generator hours in a bad week | 4.6 |
You need about 3.0 kW of array and 30.0 kWh of battery, roughly $33,705 in equipment. Size the array on the worst month, not the annual average, or winter will be spent running a generator.
How sensitive the answer is
Holding everything else steady and changing only battery cost per kwh, this is what happens to array size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Battery cost per kWh ($) | Array size |
|---|---|
| $450 | 3.0 |
| $675 | 3.0 |
| $900 example above | 3.0 |
| $1,350 | 3.0 |
| $1,800 | 3.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Daily energy use (kWh)
- A number, between 0 and 500, starting at 8.
- Peak sun hours, worst month
- Choose from Pacific Northwest (3.5), Northeast, Midwest (4.0), Mid-Atlantic (4.5), Southeast, coastal California (5.0), Texas, inland California (5.5), Arizona, Nevada (6.0).
- Days of autonomy
- A number, between 0 and 14, starting at 3.
- System derate (%)
- A percentage, between 40 and 100, starting at 75%.
- Usable battery depth (%)
- A percentage, between 20 and 95, starting at 80%.
- Battery cost per kWh ($)
- A money amount, starting at $900.
- Array cost per watt ($)
- A money amount, starting at $2.2.
What the results tell you
| Result | What it is |
|---|---|
| Array size | The headline result. A decimal figure. |
| Battery bank | A decimal figure. |
| Equipment cost | A money figure. |
| Annual production | A number. |
| Generator hours in a bad week | A decimal figure. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
Keep going
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