Off grid use

The 12v leisure battery is intended to provide low current power for water pump, lights, control circuits of fridges and control circuits of heating. The refrigerator should not be cooled by 12v except when the engine is running and alternator output is generated. The vehicle should be wired up with a relay to prevent the leisure battery from being used for the refrigerator. When you are pitched and off grid, you should use gas for the fridge, cooking and heating. A 100wat solar panel will not power a refrigerator, but may support the daily power requirement for the low current circuits, if the sun is good enough.

I googled whether this was ok and it looks as though a bigger solar panel would be needed, 150w rather than 100w.

Modern solar panels work well even without full sun JVB.

Realistically for sustained off-grid use you’re not going to get reliably far with a 12v fridge unless you use gas. A compressor fridge will use less power than absorption, but in hot conditions assuming 4 amps/hour for 50% of the time a 100AH battery will be flat in about 24hrs (assuming 50% discharge) maybe 48 hours in cooler conditions? You might get 60AH from a 100w solar panel in absolutely ideal circumstances, but for the most part it will a fraction of that in the UK (maybe 5AH on a bad day in winter) so no guarantee it will keep the fridge (or other electrical stuff) going. 

To run a compressor fridge off grid, you need a serious amount of solar plus, for preference a decent capacity lithium battery. The good news about lithium is that they are about 35 - 40% of the weight of a normal leisure battery. You need to do your sums but somewhere around 3-400W of solar and 200Ah lithium if you want to be off grid for more than a day or two, I would think. I've got 300W of solar and 240Ah of lithium but my fridge is absorption and runs off a refillable gas cylinder when off grid, which is most of the time.

A further consideration - location. You don’t say what you’re intentions are for off-grid use, but you’ll get a lot more solar for your bucks if you’re travelling in southern Europe, apparently up to 3x better than typical UK. Even in the UK position can make a big difference. I understand there can be 30% difference in solar gain between North of Scotland and where I live on South Coast of Cornwall same day/same conditions.

I think that if you want to use your fridge off grid for more than a few hours, then a 3way absorption fridge is a must, running on gas. However, absorption fridges rarely provide as much cooling as compressors, which can lead to marginal efficiency in hot weather. Ours struggles to go below 8deg C, and it really needs to cool to 5deg C. You’ll notice the considerable increase in gas consumption too. Even with extra solar power, fridges running on 12V are really only meant to do so for holding an already cold fridge’s temperature. That’s more of an absorption fridge problem than it is for compressors.

Another problem which applies to batteries is the gap between the rated power and reality. In many instances their capacity can be as little as half the advertised figure. A leisure battery supplying a low demand will deliver more energy before it needs recharging, than one which is the subject of higher loads. Worse still is a memory effect which reduces capacity after being run down to less than half. Smart chargers can restore performance, but a solar panel doesn’t have that capability.

We do use our caravan off grid with 110W of solar power which can’t keep up in dull weather, so we have a backup battery to swap in when required. We’ve also tapped into the car’s split charge relay output so that the backup battery can recharge when driving without the caravan. Substantial gas consumption is catered for with refillable Safefill bottles because Autogas is less than half the price of Calor. As you have a Motorhome, simply driving or running the engine will recharge your batteries.

Theres as much art as science in going off grid.