I've been doing some back of envelope (I don't smoke so no fag packets) with regards to power.

I have three sets of Solar Cells, and I've done some measurements and made some calculations.

Using the SAD lamp as a standard light source for each of the tests.

Small Solar Cell

79 mm x 59 mm

Voltage = 1.59 V

Current = 35 mA

Area = 4661 mm2

Power = 55.6 mW

Power per mm2 = 11.29 uW/mm2

Medium Solar Cell

138 mm x 82 mm

Voltage = 4.64 V

Current = 24 mA

Area = 11316 mm2

Power = 111 mW

Power per mm2 = 9.8 uW/mm2

Large Solar Cell

149 mm x 130 mm

Voltage = 5.19 V

Current = 49.3 mA

Area = 19370 mm2

Power per mm2 = 13.165 uW/mm2

What is interesting is that the Medium Solar Cell is not completely 'active' but the if you just use the 'active' area then the Power per mm2 comes up to 15.41 uW/mm2.

Unfortunately you can't remove the inactive area, so putting a Solar Cell array has to use the lower ratio.

Calculations and Assumptions

Initial idea is to use 6 of the large Solar Cells.

Using the numbers from the previous analysis we have this

6 x 130 mm x 149 mm --> 116220 mm2

This gives 1.53 W @ 5.19 V which gives 294 mA.

This assumes that the light level of the SAD Lamp not going to be the light level in the field.
Let it be 50% this brings it down to --> 147 mA.

The Charging of a battery is not going to be 100% efficient.

Let's use 75% which gives 110 mA.

The smallest battery available is 1 AHour.

Using this it would take approx 9 hours to charge the battery.

Short Day

The shortest day is approx 7 hours long.

Having four 1.1 Ah batteries for longevity and power for Torque.

To charge the batteries @ 110mA would take 40 Hours.

This is would be too short even if it's a bright sunny day for the 7 hours on the Shortest Day.

As there are only 7 hours in which to charge the batteries it has to charge 6 times as fast.

Assuming the same technology we have to have

6 x 6 cells --> 36 Solar Cells.

Physically this gives

6 x 130 mm -> 780 mm

6 x 149 mm -> 894 mm

Effectively it is a 1m by 1 m.

This would be physically unstable and also would need a large sheet of Perspex.

Will need to work on the 18 V but this can be solved.

Once I have this I can look at the batteries again.