- MozGlobal Engineering
- Energy
- 7 min read
Sizing from a roof area or a budget produces either wasted investment or a system that falls short. It starts from the consumption profile, and there is no shortcut.
Most undersized photovoltaic installations were not sized at all. They were quoted from a roof area, or from a budget, or from what the supplier had in stock. The result is predictable: either capital spent on generation that is never consumed, or a system that covers a fraction of the load and disappoints everyone who approved it.
Sizing properly is not complicated, but it does require information that takes effort to gather.
The consumption profile, not the monthly total
A monthly electricity bill tells you how much energy was consumed. It does not tell you when. For an industrial installation that distinction decides the whole design, because solar generation follows the sun and your load follows the production schedule. Two facilities with identical monthly consumption can need very different systems.
What you need is the shape of the load across the day and across the week:
- Operating hours, including whether there are night or weekend shifts
- The base load that runs continuously regardless of production
- Peak demand and when it occurs
- Seasonal variation, if the operation has one
Where metering data is available, this is straightforward. Where it is not, it can be reconstructed from equipment ratings and operating schedules with reasonable accuracy — it is more work, but it is far better than guessing.
Match generation to the hours you actually consume
A system sized to cover total consumption without regard to timing will generate a surplus at midday and nothing at night. Whether that surplus is worth anything depends entirely on what you can do with it: feed it back, store it, or shift load into the daylight hours.
Storage changes the economics substantially, and not always in the direction people expect. Batteries are a significant part of the cost and have a service life measured in cycles. Sizing storage to carry an entire night load, for an operation that only runs during the day, is a common and expensive mistake.
The right question is not "how much can we generate?" but "how much of what we generate will we actually use?"
What the site imposes
Available area is the hard constraint, and it is rarely the roof area someone measured on a drawing. Shading from adjacent structures, roof orientation and pitch, structural capacity to carry the additional load, and access for cleaning and maintenance all reduce what is genuinely usable.
Ground-mounted arrays remove the structural question but introduce land use, security and cabling distance. Neither option is universally better; the site decides.
Before you generate, look at what you waste
It is frequently cheaper to reduce consumption than to generate the equivalent. Compressed air leaks, motors running unloaded, oversized pumps throttled at the valve, lighting left on in unoccupied areas — these are unglamorous and they are where the quick returns usually are.
An energy assessment before a generation project is not a delaying tactic. It changes the size of the system you need to buy, which changes what the project costs.
What a proper proposal should tell you
A proposal worth accepting states the assumed consumption profile, the expected generation across the year, what proportion of your load it covers, and what it does not cover. If it gives you a peak power figure and a price without any of that, it has not been sized. It has been quoted.
