A homeowner wants an 8 kW system and you stock 400-watt panels. Convert kilowatts to watts, divide by the panel rating, and round up — you can never install a fraction of a panel.
8 kW is 8,000 watts; at 400 W each that is exactly 20 panels. ROUNDUP guarantees you never come up short.
The example
Three jobs, each with a target system size and a panel wattage.
| A | B | C | D | |
|---|---|---|---|---|
| 1 | Job | System (kW) | Panel (W) | Panels |
| 2 | Home A | 6 | 400 | 15 |
| 3 | Home B | 8 | 400 | 20 |
| 4 | Home C | 10.5 | 425 | 25 |
The formula
The same formula sizes every job:
How it works
Working left to right:
B2*1000converts the system size from kilowatts to watts.- Dividing by
C2, the panel rating in watts, gives the raw panel count. ROUNDUP(…,0)bumps any fraction up to the next whole panel so the array meets or beats the target.
Real arrays add a small derate for losses; multiply the system size by about 1.15 first if you size to AC output.
Try it: interactive demo
Enter a target system size and the panel wattage.
Variations
Roof area limit
Cap the count by available roof area (sq ft) divided by panel area.
Pitfalls & errors
Rounding down leaves the array undersized. Always ROUNDUP so the system hits its target.
Mixing kW and W is the classic slip — remember the *1000 to convert the system size to watts.
Practice workbook
Frequently asked questions
Why round up instead of to nearest?
Do I need to account for losses?
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