An array is sized from two numbers and constrained by a third: how much electricity you use and when you use it, how much generation the roof can produce, and how much space there is once spacing, edges and obstructions are taken out. Starting from roof area gives you the biggest array; starting from consumption gives you the one that pays.
How many solar panels to run a house: start from the meter, not the roof
Pull twelve months of consumption from your bills or from your smart meter data, and look at when it happens. A household that uses most of its electricity in the evening will export most of what a large array generates, so beyond a certain size each extra panel earns the export rate rather than the import rate, and the payback on those panels is much longer. That inflection point is where an array should usually stop, and it is the calculation most sales sizing skips.
How many solar panels can I fit on my roof? What it can actually hold
Usable area is not roof area. Take off the edge zones the mounting system requires, the space around chimneys, vents, roof windows and hips, and any part of the slope that shades in winter. On a flat roof take off row spacing, which grows with tilt. What is left, divided by panel area, is the honest maximum, and it is routinely a third less than the roof measures. A surveyor produces a panel layout for exactly this reason.
What size solar panels do I need? Output makes fewer panels do the same work
Module ratings have risen substantially, so an array of a given capacity today uses fewer panels than the same capacity did some years ago. That matters when space is tight, and it means comparing quotes by panel count is meaningless: compare kilowatts peak and modelled annual generation. A quote offering more panels is not offering more electricity unless the ratings say so.
Where storage changes the sizing
If you add a battery, the size at which extra panels stop paying moves upward, because generation that would have been exported is stored and used instead. That is why sizing and storage should be decided together rather than in sequence. It also means that a larger array plus a battery and a smaller array without one can have similar economics and quite different resilience, which is a choice rather than a calculation.
Other questions people ask
How many panels does an average house need?
There is no average that survives contact with a real bill. Size it from twelve months of your own consumption and when it occurs, then check what the roof can physically hold. A quote that offers a panel count before seeing your consumption is sizing the roof, not the household.
Can I fill the whole roof?
You can, and beyond the point where you use the generation, the extra panels earn the export rate rather than replacing what you buy. That is a much longer payback, so filling the roof only makes sense with high daytime use, a battery or a good export tariff.
How much roof area does a panel take?
Rather more than the panel itself, once mounting edge zones, obstructions and shading are excluded. A surveyor's layout drawing is the only reliable answer, and it usually fits fewer panels than a rough area calculation suggests.
Does a solar panel calculator UK sites offer size systems correctly?
They estimate. They cannot see shading, obstructions, your consumption pattern or your consumer unit, and they assume a self consumption fraction. Use them to get an order of magnitude and a survey to get a number.