# Designing an electrical off grid system for winter

I'm currently planning the conversion of a panel van into an RV including heating, water, and electrical. The part which I'm not confident about is the reliable supply of electrical power in the winter months in central Europe (latitude: 48°).

The van will be used starting from September onwards and I'm thinking about skipping the installation of solar panels until spring and using the saved money on extra batteries. The batteries could be charged by the alternator of the van while driving and it would be possible to charge halve the batteries on the grid and switch the batteries once a week.

My electricity consumers would be:

• 1 phone charge `5Ah`
• 2 laptop charges `15Ah`
• lamps run 10h `15Ah`
• water pump `4Ah`
• heater `20Ah`

Which comes to a total of ~60Ah a day, 420Ah a week. I've read that a 100W solar panel will on average produce about 6Ah a day in winter. Considering that the panels have to be mounted flat on the van and have a higher chance of being partially obstructed because of the low height compared to buildings, is it viable to use solar for electricity generation?

Is there some mistake in my thinking or rough calculations or am I right that there is no chance to live off the grid during the winter with my guesstimated power consumption?

My actual consumption is likely to be less because I would be able to charge the phone, power bank, and laptop from grid power most days (work) but ends would still not meet and I'm probably forgetting consumers.

• How many panels can you fit on the roof? If the van will be parked, it should be easy to set them up on a movable rack so you can tilt them toward the sun. Then lower them back to flat for when you're driving.
– LShaver
Jun 18, 2020 at 14:02
• @LShaver Tilting them towards the sun will sadly be not possible. The roof space is about 4m*1,6m. So the maximum I could mount would probably be 2x 360W panels because there also needs to be vents on the roof which take up space. Jun 18, 2020 at 14:20

Reducing your loads is critical. On our sailboat there are always shaded portions of our flat solar panels [2x100 W] yet solar does it all: our 10 LED lights, 4 12 V fans, and 2 phones and laptops charges are negligible, water-pressure and pee-discharge pumps minimal… so our only real load (here at latitude 44°) is our fridge (3.5 Ah but does not run continuously). We have 5 old lead acid batteries totaling 580 Ah and our solar panels and 30A MPPT controller keeps up with our load 98% of the time (3 cloudy days in a row requires that we turn on our Honda 2200i gasoline generator for about an hour to top off the batteries). A \$150 Chinese “breadbox” diesel heater might really make difference in loads for you. So I’d be really surprised if a setup like ours couldn’t meet your needs.

If you live in an area with real winter like southern Finland (I use southern Finland as an example because in northern Finland there is polar night, i.e. absolutely zero sunshine even during the day as sun is always below the horizon in winter), a 100 W solar panel can be expected to produce approximately 2 watt hours in the worst winter days. If you have 12 volt system, then that's 0.17 Ah. Not 6 Ah. 6 Ah is way off.

If you have a battery long enough to "calculate average" of bad and good winter days, then you can expect to get about 17 watt-hours on an average winter day, i.e. about 1.4 Ah. But do note that then your battery needs to be sized for about 30 days of energy usage so that the battery can average both good and bad days.

To produce any reasonable amount of energy in winter, you need panels. Lots of panels. In fact, so many panels it would not be economically viable to do that. You really need gasoline or diesel fuel in the winter.

At latitude 48, it would be slightly better than southern Finland (latitude 60), but not much better. I don't think at that latitude it would be realistic to use entirely solar power, unless you have at least ~2-3 kW of panels (your RV surely doesn't have enough roof area for that) and at least 1500 Ah of 12 V batteries (420 kg if lead-acid). And that assumes perfect orientation of panels; with inoptimal orientation or with some panels shading other panels, that won't work. Flat roof of an RV is not optimal orientation unless you live near the equator.

My estimates assumed MPPT controller. With PWM, add 20% more panels.

• Frankfurt/Germany (ca. 50°N) has about 50 h of sunshine per month in Nov - Jan. That's an average of 1h 40min per day. AFAIK sunshine for these stats is defined as > 120 W/m² perpendicular to sun, which is 12 % of the roughly 1000 W/m² of full direct sun shine. Midday height of the sun is somewhere around 20° during these months. A quick look on actual irradiation data by the German metereological service was about 20 kWh/m² perp. per month Nov - Jan over the last 3 winters. 20 % efficiency times a factor 0.25 for sin sun height since OP cannot tilt the panels => 1 kWh harvest per month Sep 14, 2022 at 18:39
• ... assuming that there is no whatsoever shadowing. I'd call that comparable to your southern Finland data. Sep 14, 2022 at 18:48