What will these panels actually make?

The angle your panels sit at, measured up from flat.

Flat panels collect less over a year than tilted ones at most latitudes, because the sun is rarely overhead. The best annual angle is roughly your latitude minus fifteen degrees. Most pitched roofs already sit between 30 and 45 degrees, so in practice the roof decides this rather than you.

A steeper angle trades a little annual output for noticeably more in winter. If you are sizing around dark-month performance rather than the yearly total, steeper usually wins.

The compass direction your panels face.

In the northern hemisphere a south-facing array collects the most over a year; in the southern hemisphere it is north. Turning away from that costs output, but far less than most people assume, and a split east-west roof produces a flatter, longer generation day.

East and west facing roofs lose less than people expect — typically ten to twenty percent, depending on latitude and tilt; the probe above gives the figure for this roof. They also spread generation across the morning and evening, which can be worth more than the total suggests if you are home at those times and your exports pay poorly.

The total area of the panels themselves, not of your roof.

A typical modern panel is about 1.9 square metres and around 400 watts. Generation scales directly with total area — double the panels, double the output.

Output scales almost exactly with area, so this is the one input with no diminishing return in the physics. The limits are roof space, budget, and what your inverter and your export terms will absorb.

How much of the light landing on a panel becomes electricity, at test conditions.

Budget crystalline panels run around 18 to 20 percent; premium ones reach 21 to 23. The difference decides how much power fits in a given area, not how good the electricity is.

Efficiency only matters when roof space is the binding constraint. If you have room to spare, more cheap panels beat fewer expensive ones for the same money.

How much of the sky your panels can actually see.

This estimate assumes nothing shades your roof. That is almost never exactly true. A shaded panel still generates — diffuse light reaches it from across the whole sky — but direct sun is what carries most of the output, so losing morning or afternoon sun to an obstruction matters more than the shaded fraction alone suggests.

Shading is the single largest source of error in a rooftop estimate, and the only input here we cannot infer for you. A chimney or a neighbouring tree can cost more than choosing the wrong panels ever would.