How the calculations work, what the numbers mean, and where the model's honest limits are.
For any date and location, the app works out the sun's true position — its altitude above the horizon and its compass bearing — minute by minute. It then compares that position with the direction your panel faces and how far it's tilted, to work out what fraction of direct sunlight is actually landing on the panel's face.
From there it builds an hourly and daily energy picture, and can search across every combination of tilt and facing direction to find the one that produces the most energy over a full year at your location.
Latitude decides how high the sun climbs and how long the day is. Longitude decides exactly when, in clock time, solar noon happens at your specific spot within your time zone — moving roughly four minutes earlier for every degree east of Greenwich, and four minutes later for every degree west.
Without it, every location inside the same time zone would show identical clock times for sunrise, sunset and peak output — which isn't quite true even a short distance apart.
This is your region's standard time difference from UTC — for example 0 for the UK and Ireland, or 1 for mainland Spain. It exists because time zone boundaries are political and historical decisions, not something that can be worked out from a place's coordinates alone. Two locations at nearly identical longitude can legally sit in entirely different time zones.
No, not for UK and Ireland dates — the app works out the last Sunday of March and October each year and automatically applies the one-hour shift for you. The UTC Offset field is your standard time only; you don't need to add or subtract an hour for summer yourself.
Sunrise and sunset describe when the sun is above the horizon at all. "Panel illuminated" describes when your specific, fixed panel — facing one direction, at one tilt — can actually see the sun.
In summer at higher latitudes, the sun rises and sets well to the north of due east and west. A south-facing panel simply can't see it during that early and late stretch, even though the sun is technically up. This is a real, physical property of fixed-tilt panels, not a limitation of the calculation.
It tests every combination of tilt (0–90°) and compass direction, calculates a full year of output for each, and reports the single combination that produces the highest total energy over the year — along with the single best day within that year for the winning combination.
"Tilt equals latitude" is a well-known rule of thumb, but it's really only exact for a single day — the equinox. Because the search is optimising for the whole year's total energy, not one day, the sun's higher summer path and lower winter path pull the ideal angle slightly away from that simple rule. The true optimum is usually close to latitude, but rarely identical to it.
The efficiency figure is the angle between the sun's direction and the panel's face — mathematically, a dot product between the two directions. When the sun sits behind the plane of the panel, that value goes negative and is treated as zero: no light reaches the front face at all. This is exactly what a real, fixed panel experiences.
No. The model calculates light striking the front face of the panel only.
No. Every figure assumes a clear sky, with nothing between the sun and the panel — no cloud, haze, or physical obstruction such as trees, chimneys or nearby buildings. It's a geometric ceiling on what's possible at your location, not a weather forecast.
No. Real panels lose some efficiency as they heat up in strong sun, and any inverter or cabling introduces its own small losses. This app calculates the geometric and solar-intensity potential only — a genuine, real-world system will typically produce somewhat less than the figures shown here.
The model has been tested thoroughly across the UK, Ireland and Spain, including daylight saving transitions, longitude correction and standard time offsets. At genuinely extreme values — latitude within a fraction of a degree of either pole, or longitude far beyond a real location's range — floating-point arithmetic can behave in ways that don't reflect anything physical. These aren't places any real panel will ever be mounted, and results there shouldn't be relied on.
It's used as your panel's rated peak output under ideal, laboratory test conditions — the number usually printed on the panel or in its datasheet. The app multiplies this by the calculated efficiency at each moment to estimate real output, so the accuracy of every energy figure depends on entering a realistic value here.
It's genuinely doing a lot of work — testing every tilt from 0–90° against every compass direction in 10° steps, for every day of the year. That's over a million individual calculations. You'll see a short confirmation before it starts, since it can take from under a minute to a couple of minutes depending on your device, and you can cancel it at any point if needed.
Each field is checked against what's physically possible — latitude between -90° and 90°, tilt and azimuth within their real ranges, and so on. This catches typos and impossible values before they can quietly produce a nonsense result. It can't, however, check whether a value is true for your situation — only whether it's possible at all.
No. Every calculation runs locally on your own device. Nothing you enter is collected, stored remotely, or shared, and the app carries no advertising.