The arithmetic takes thirty seconds. The judgement takes an engineer. Both are below.
The thirty-second version
Take your annual electricity consumption in kilowatt-hours. It is printed on your utility bill, usually as a 12-month total or a monthly average you can multiply by twelve. Divide it by the annual production per kilowatt of solar in your area. In Phoenix that is roughly 1,700 kWh per kW per year; in Colorado Springs about 1,550; in Fresno about 1,600.
That gives you system size in kilowatts. Divide by your panel wattage (call it 0.42 kW for a modern 420 W module) and you have a panel count.
A home using 14,000 kWh a year in Phoenix: 14,000 ÷ 1,700 = 8.2 kW. 8.2 ÷ 0.42 = about 20 panels.
That number is the starting point for a conversation, not an answer. Here is why.
Why the biggest array that fits is usually wrong
The instinct is to fill the roof. Sales compensation encourages it. But every additional panel produces electricity you either use or export, and in most of our service territory exported power is worth substantially less than the power you consume directly.
Under full retail net metering, a kilowatt-hour exported at noon offsets a kilowatt-hour imported at midnight one-for-one. Under a reduced export rate, Arizona's export credit, or California's NEM 3.0. That same exported kilowatt-hour might be worth a quarter of what you would have paid to buy it.
So the last few panels on an oversized array can have a payback period of eighteen years while the first fifteen pay back in nine. Averaged together the system still looks fine on paper. You are still buying something that does not pay for itself.
The five things that actually move the number
1. Your consumption, not your house size
Two identical 2,400 square foot homes on the same street routinely differ by a factor of two in consumption. Occupancy, thermostat behaviour, pool pumps, and whether anyone works from home matter more than square footage. We use twelve months of your actual meter data, pulled from your utility.
2. Roof orientation and pitch
In the northern hemisphere, south-facing is best, west-facing is often better than east if your utility has an afternoon peak, and north-facing is usually not worth using at all. A panel on a north-facing plane in Phoenix produces roughly 30% less than the same panel on the south face six feet away.
3. Shading
The single most underestimated factor. A mature tree, a neighbouring two-storey addition or a chimney can cost 20–35% of annual production, and shade that looks trivial at 2pm in June can be severe at 9am in December. This is why we model hour-by-hour across a full year rather than eyeballing a satellite image.
4. What your utility pays for exports
Covered above, and it is worth repeating: this variable can change the optimal system size by 30% either direction on otherwise identical homes in different utility territories.
5. What your consumption will be, not what it is
An electric vehicle adds roughly 3,000–5,000 kWh a year. A heat pump replacing gas heat adds more. A pool adds 2,500–5,000. If any of those are plausible within five years, size for them now, adding panels later means a second permit, a second mobilisation and an interconnection amendment that together often cost as much as the panels.
What we actually recommend
For most homes in our markets we design to an 80–95% offset of current consumption, adjusted upward if there is a known load coming and downward if the export rate is poor. We almost never recommend 100%+ offset unless you are on full retail net metering, which is increasingly rare.
If your proposal from any company shows a 110% offset and a poor export rate in the same document, ask them to explain the last fifteen panels. It is a reasonable question and the answer tells you a great deal.