Solar & battery
Solar Panel Quantity Calculator
Works module quantity, where string configuration constrains the count.
Result
Order a spare or two on anything sizeable. Glass breaks in transit and on roofs, and a single replacement panel ordered later often costs more in freight and delay than it does in equipment. Matching a discontinued model years later is worse again.
Get a Free Business AuditHow the calculation works
Target array size divided by module rating gives a raw count, which then has to be adjusted to a number that divides into valid strings for the chosen inverter. The practical count is frequently not the raw count, because strings have to be equal and within the voltage window.
What moves the number
- Target array size and module rating.
- Valid string lengths for the inverter.
- Number of inverter inputs available.
- Usable roof area and layout.
Where this one goes wrong
A module count that does not divide into valid equal strings either wastes modules or requires a different inverter configuration, which is why the count is settled with the string design rather than before it. Module-level electronics relax this, allowing more flexible counts at additional cost. The count also has to survive the roof layout, since whole modules have to fit around obstructions and setbacks rather than fitting on average.
Before you quote
- Settle the count together with the string design.
- Check the count divides into valid equal strings.
- Verify the layout fits whole modules around obstructions.
Questions
About this calculator
Why is my module count not what the arithmetic gave?
Because strings have to be equal in length and within the inverter's voltage window, so the raw count has to be adjusted to a number that divides properly. The count is settled with the string design rather than before it.
Do microinverters or optimisers change this?
Yes, they relax the string constraint and allow more flexible module counts, at additional cost per module. Where the roof has multiple orientations or shading, that flexibility is frequently worth paying for on its own merits.
Does the layout affect the count?
It can reduce it, since whole modules have to fit around vents, chimneys and setbacks rather than fitting on an average basis. A count that works on area can fail on layout, which is why the layout is drawn rather than calculated.
The formula
panels = system watts / panel watts, plus spares
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How to use this calculator
- System size (kW DC). Enter your own figure; the page starts at 7.2.
- Panel rating (watts). Enter your own figure; the page starts at 400.
- Spares and breakage (%). Enter your own figure; the page starts at 2%.
- Cost per panel ($). Enter your own figure; the page starts at $210.
The result updates as you type, so you can try a figure, see what it does to panels to order, 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 |
|---|---|
| System size (kW DC) | 7.2 |
| Panel rating (watts) | 400 |
| Spares and breakage | 2% |
| Cost per panel | $210 |
What comes out
| Result | Value |
|---|---|
| Panels to order main result | 19 |
| Before rounding | 18.0 |
| Material cost | $3,990 |
| Cost per kW | $554.17 |
A 7.2 kW system needs 19 panels, about $3,990 in material, or $554.17 per kW.
How sensitive the answer is
Holding everything else steady and changing only cost per panel, this is what happens to panels to order. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per panel ($) | Panels to order |
|---|---|
| $105 | 19 |
| $158 | 19 |
| $210 example above | 19 |
| $315 | 19 |
| $420 | 19 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- System size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
- Panel rating (watts)
- A number, between 0 and 100000, starting at 400.
- Spares and breakage (%)
- A percentage, between 0 and 25, starting at 2%.
- Cost per panel ($)
- A money amount, starting at $210.
What the results tell you
| Result | What it is |
|---|---|
| Panels to order | The headline result. A number. |
| Before rounding | A decimal figure. |
| Material cost | A money figure. |
| Cost per kW | A money 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.
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