Solar Panels (winter)

If you have not read the original article I suggest you follow the link here in order to familiarise yourself with the project so far.

To refer back to the Autumn results follow the link here .

The winter solstice was a very dreary business and the days around it. Christmas day was however sunny. As all of the midday measurements have been in bright sunlight this is the one I have used.

A surprising 3.3 amps was being produced from the 260 watt array. That is the equivalent of 2.5 amps on the original 200 watt set-up.

So to tabulate the midday sun results, they are.

July (original fitting of the panels)12.015.6
Autumnal equinox 9.011.7
Midwinter 2.5 3.3

Which look very good, however they are solar panels, and these are maximums. The sun shines a lot less in the winter (both in time and intensity). There is also less penetration of the cloud cover by sunlight. So the results do not tell the whole story. Subjective results are in this case more important than objective measurement.

My overall assessment is that from about the end of March to September the 260 watt array will provide us with the majority of our electricity and that our normal cruising, about 2 hours a day, will keep the batteries in a, more or less, fully charged condition.

During the winter months any power produced should simply be counted as a bonus. The engine will be required for the majority of battery charging. In reality, even on a beautiful mid-winter day the total power production from the panels will only be in the order of 10-12 amps over the whole day, Our engine driven charging system can produce that in less than 10 minutes. On a deeply overcast mid winter day we are lucky to see 1 or 2 amps in total over the day.


The system was very much a worthwhile investment. I would expect to break even in 3-4 years with the current diesel price. The total investment was £700 and a few pence. My diesel/engine costs have reduced by about £200 a year.

The losses in efficiency caused by mounting panels flat and permanently are no real barrier to the effective use of solar panels. They are, of course, less efficient mounted this way but still offer very useful levels of power on our narrowboat.

The charging rate is very low (the main battery bank is 825 amps), this is good for lead/acid batteries. It is certainly kinder than charging them at 80 amps. The panels could well add an extra year to the life of the battery bank. If that is the case (and we shall never know) then their payback will be very much faster as a years worth of battery bank is in the order of £400.

Some of the problems envisaged have turned out to be very real, Particularly the reduction of roof space and having to be particularly careful with stacking coal and wood. A further problem turned up, and that is having to clear the panels of snow in the worst of winter.

However some were figments of my imagination, the controllers do not fight with each other, the main charging system voltage is high enough to override the panel charging system controller.

I believe the MPPT controller has been a major factor in the results by optimising the output of differing panel voltages, as well as matching the panel output to the battery bank state of charge. While MPPT controllers are expensive items they do seem to produce very good results.

What would I have done differently if I had known what I know now.

The answer is surprisingly little.

If I had known that I could mix panels with the impunity that I have experienced I would have planned in a further 60 watts. (Please note all of the panels were from the same supplier, I doubt that panels from different suppliers or different types can be indiscriminately mixed even with a MPPT controller).

I would have installed a 30 amp controller at the outset. Having installed a 20 amp controller I cannot add further panels and safely stay within the current controller capacity.

I would have installed a second set of wiring and a small panel for the starter battery. This would not be to charge the starter battery, but to keep it active (and therefore warmer) to assist engine starting in very low temperatures.

I hope that this helps somebody