Daylight hours and solar panel calculators both deal with the sun, but one measures geometry and the other measures scenarios. The daylight hours calculator estimates how long the sun is above the horizon at a latitude and date, including polar day and polar night. The solar panel calculator turns usable panel area and user-entered specific-yield values into low and high annual energy scenarios. Day length says how much sun is available in principle; the solar calculator says how much energy a modeled array might produce, given yields its source has already adjusted for real-world conditions.
What each calculator does
The daylight hours calculator takes latitude and a date, converts the date to day of year, estimates solar declination with a cosine approximation, combines it with latitude into an hour-angle argument, and converts the hour angle to daylight duration L = 2ω/15. Sunrise and sunset are placed symmetrically around local solar noon at 12:00, so the displayed times are solar times, not civil clock times — longitude, time zone, elevation, and refraction are omitted. When the geometry indicates continuous daylight it returns 24 hours labeled polar day; when it indicates no sunrise it returns zero hours labeled polar night. Its worked example: latitude 51.5074°N on June 21 gives 16 hours 24 minutes of daylight, sunrise 03:48, sunset 20:12.
The solar panel calculator takes usable panel area, area per panel, panel nameplate rating, and two annual specific-yield values (kWhac/kWdc/year) that you identify with source, version, geography, and configuration details. Panel count is the floor of usable area divided by area per panel, DC nameplate capacity is that count times the rating, and each annual-energy scenario is the unrounded capacity multiplied by its entered yield. It adds no weather, orientation, shading, thermal, or inverter effects — the entered yields are expected to already embody them — and it applies no performance ratio. Its worked example: 40 m² of area, 2 m² panels, 0.4 kWdc rating, yields of 1,200 and 1,600, gives 20 panels, 8.00 kWdc, and a 9,600–12,800 kWhac/year range.
Side-by-side
| Daylight hours calculator | Solar panel calculator | |
|---|---|---|
| Inputs | Latitude and date | Usable area, panel area, panel rating, low and high annual specific yield, plus source provenance |
| Primary output | Daylight duration in hours and minutes, with solar sunrise, sunset, and noon | Low and high annual energy scenarios in kWhac/year |
| Formula basis | Declination and hour-angle geometry, L = 2ω/15 | Panel count = floor(area ÷ panel area); capacity × entered specific yield |
| What the number means | How long the sun is geometrically above the horizon | Modelled annual energy for the entered capacity and yield scenarios |
| Weather and system effects | Omitted by design | Not added — yields must already embody them from their source |
| Typical uses | Gardening, outdoor work, seasonal planning | Solar energy scenarios from a named report or assessment |
When to use which
Use the daylight hours calculator when your question is about day length itself: how long the sun is above the horizon at a location through the year, when local solar sunrise and sunset fall, or whether a high latitude approaches polar day or polar night. It is a geometric baseline for gardening photoperiod, outdoor work, and seasonal planning — not a light-intensity or energy measure.
Use the solar panel calculator when your question is about energy scenarios: how much annual AC energy a specific panel configuration could produce given identifiable specific-yield values. The page is explicit that this is a screening maximum, not a physical layout or site-suitability determination, and that PVWatts can supply input values without the calculator fetching them itself.
Limits and disclaimer
Both pages are educational only. Daylight times are approximate local solar times and must not be used for navigation, legal deadlines, or safety-critical fieldwork, and the polar-season checks can oversimplify southern-hemisphere cases. The solar panel calculator’s range is user-defined scenarios, not a forecast or guarantee: the yield values you enter determine the result, and provenance fields describe rather than validate the source. Day length is one factor in solar production, but clouds, low sun angle, and site conditions can dominate it.