The formulas
Declination δ = 23.44° × sin(360° ÷ 365 × (284 + day of year))
Day length = 2 ÷ 15 × arccos(−tan(latitude) × tan(δ)) hours
Noon sun height = 90° − |latitude − δ|
Day length here is from sunrise to sunset of the sun's centre. Atmospheric refraction adds a few minutes, so published sunrise and sunset times show slightly longer days.
Why it matters for solar
At UK latitudes the midwinter sun only climbs about 13–17° above the horizon, and the days are under 8 hours long. Flat panels on a van roof catch very little of that low sun, which is why winter solar output can be a fifth or less of summer. Tilting panels towards the sun in winter makes a big difference.
Frequently asked questions
How many hours of daylight in the UK in winter?
Around 7–8 hours in southern England and 6–7 hours in northern Scotland at the winter solstice.
What is the best solar panel angle in December?
Roughly your latitude plus 23°, so about 75° in the UK. Use the result above for your exact location and date.