It is tempting to size a solar system from a single number: annual electricity consumption in kWh. That number is a starting point, but on its own it can lead to a system that is the wrong shape for the property, even when the total is right.
Annual totals hide the shape of demand
Two properties can consume exactly the same amount of electricity in a year and need very different systems, because when they use it differs.
A house that is empty during the day and busy in the evening has a demand curve that runs opposite to a south-facing array's generation curve. A workshop that runs machinery from nine to five has a demand curve that tracks it closely. The annual total says nothing about this difference; the daily and seasonal pattern does.
Interval data is worth asking for
Where it is available, half-hourly or hourly consumption data shows the actual shape of demand across a week or a season, rather than an average. This is the single most useful input to a sizing exercise, and it is worth requesting from your electricity supplier before an assessment.
Where interval data is not available, a written description of the household or business routine is a reasonable substitute: when people are home or on site, what runs continuously, and what happens seasonally.
The roof sets the ceiling, not the target
A common mistake is to size the array to fill the available roof, then decide afterwards whether that generation will be used. It is more useful to work in the other direction: establish how much can realistically be used on site, including anything stored in a battery, and use that figure to decide how much of the roof is actually worth covering.
Filling the whole roof is sometimes still the right answer, particularly when a future EV charger or an extension is already planned. But it should be a decision, not a default.
Storage changes the calculation
If a battery is part of the system, sizing shifts from covering daytime use to covering a defined evening or overnight period. That is a different, and usually smaller, number than the array's total daily output, which is why battery capacity and array size are sized against different questions.
What a proper sizing exercise produces
A sizing decision that can be explained will reference:
- The usable roof area after orientation and shading are accounted for
- The shape of consumption across a representative day or week
- What share of that consumption the system is intended to cover
- Whether storage and future loads such as EV charging are included
Related: Solar Solutions
This article is general, technical background written for the Solara Energy demonstration website. It is not project-specific advice.
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