There is an intuitive logic to filling a roof: the space is there, panels are cheaper than they have ever been, so why not use it. The logic breaks the moment you look at what happens to the generation you cannot consume.
The units you do not use
A solar array produces most of its output in a narrow band around the middle of the day. If your consumption during that band is smaller than what the array makes, the surplus has to go somewhere — exported, stored, or simply not generated because the inverter throttles back. Depending on your connection agreement, export may earn little or nothing. Storage costs money. Throttling is pure waste. In all three cases, the marginal panel you added to fill the roof earns far less than the first panel did.
What a profile shows that a total hides
A monthly total is a single number pretending to be information. Two sites with identical annual consumption can need systems that differ by a factor of two, because one runs flat through daylight hours and the other spikes at seven in the evening. Interval data — readings every fifteen or thirty minutes across a representative period — shows you the shape, and the shape is what you size against.
A seasonal operation makes this vivid: a coffee washing station sized on annual-average consumption gets an array far too small for the only months that matter.
Match the asset to the job
Once you have the profile, the design almost writes itself. Size the array to the daytime base load — the floor your consumption rarely drops below. Size any battery to the peak, or to the evening block you want to shift. These are two different assets doing two different jobs, and conflating them is how sites end up with an oversized array charging an undersized battery to serve a load nobody measured.
When filling the roof is right
Occasionally it is. If your daytime load genuinely absorbs everything the roof can make, fill it — that is a site with an excellent solar case, and the constraint really is the roof. The point is not that big arrays are wrong; it is that the roof should be the answer to the question, not the question itself.
- Size the array to the daytime base load, not the roof area
- Size storage to the peak or the evening block, separately
- Get interval data — a monthly total cannot show you shape
- For seasonal operations, size to the season, not the average
- If the load absorbs everything the roof makes, then fill the roof
Written by Engineering team. This article is general guidance, not a substitute for a site-specific assessment.