Power budget:

Note: the 2GB version of the Pi5 has a tested current of less than 1A when running with lite OS, though possibly with a peak current higher than this on startup.

Assume on-time of 20 mins, energy used by Pi:

5V * 1A * 0.33h  = 1.7 watt hours (Wh)

Assuming 0.05 Watt for standby during rest of day
0.05W * 24h = 1.2 Wh.  (ignoring 20 mins less)

Total energy required for 24hr = 3.0 Wh

Waveshare power management unit, has 3x18650 Li-ion, and can give 3A output - so the possible higher startup current of the Pi is covered.

Batteries are rated 3.7V, 2500mAh

Max battery energy capacity 3 x 3.7V x 2.5Ah = 27.75 Wh

So fully charged batteries have plenty of capacity.

Assuming 6V 2.0W Solar panel available from PiHut.

Assume UK weather only gets 1W, then after four hours of daylight:

  1.0(W) x 4(h) = 4.0 Wh

So should get the 3Wh energy required per 24hr.

However after further consideration: UK winter sun may produce less than 1W. This solar panel is advertised as 6V. If panel output drops below 6V, then the management unit may fail to work as it is advertised for an input of 6V and above. So decided to get two 6V solar panels and wire them in series to get a nominal 12V, together with the increased wattage output.