What size solar system do you need?
Bigger is not better. Here is why an oversized system pays back slower.
The short version
- • One kilowatt of panels makes about four units a day.
- • A 550W panel needs about 26 sq ft. Flat roofs need a third more.
- • Extra units you sell back are worth far less than the ones you use.
Too small and you keep paying a bill you did not expect. Too big and you paid for panels you do not need.
Three things decide the answer.
1. What your bill needs
One kilowatt of panels makes roughly four units a day across the year.
Take your daily units and divide by four.
Using 900 units a month? That is 30 a day. Thirty divided by four is about 7.5kW.
2. What your roof allows
A 550W panel needs about 26 sq ft. A flat roof needs about a third more, because the rows have to be spaced so they do not shade each other.
| System | Panels | Sloped roof | Flat roof |
|---|---|---|---|
| 5kW | 10 | 260 sq ft | 351 sq ft |
| 7.5kW | 14 | 364 sq ft | 491 sq ft |
| 10kW | 19 | 494 sq ft | 667 sq ft |
Usable roof is smaller than it looks. Stairs, water tanks and the parapet shadow all take space.
If your roof cannot fit the size your bill wants, the roof wins. A system covering 70% of your units still pays for itself.
3. Shade
Panels are wired in strings, so a shadow on one panel drags down its neighbours.
A tree or a neighbour's wall can cost most of a tenth of your output. Tall buildings cost much more.
South facing is best. East and west work with a small loss.
Why bigger stops paying
The units you use yourself save you the full rate you would have paid. The extra units you send to the grid are bought from you at about a third of that. So a system much larger than your usage takes longer to pay back, not less. If someone says you will "earn from the extra", ask what your DISCO actually pays per unit.
It uses sunlight figures for your exact area, and your plot size in marla.
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