Solar Panels for your Camper The future's so bright, we've got wear gas masks!

Although we're not planning on going totally back-woods Bear Grylls / Ray Mears with the camper, we are planning around being able to be fully self-sufficient if we need to for short periods. Hence the toilet and shower (work in progress at time of writing) and also hence the need for a source of power other than the engine or mains.

As it stands we don't actually have any mains provision in the ambulance, no mains charger and nothing that requires mains. We did get grumbled at on a Swiss campsite for running the engine to charge the battery after the Mk1 fridge proved way jucier than anticipated.

The Short Version

TL;DR - We used:

  • 100W Panel eventually 2x 100W panels
  • Cheap eBay MPPT charger (eventually x2 of them)
  • 100Ah battery
  • Plus split-charge form the alternator when running
  • See Mk2 Electrics Setup for full info.

Realistic Solar Panel Performance

The short version

Over an average day, average weather in Europe, you'll get 10% of whatever the panel claims

So a 100W panel will put 10W per hour over 24 hours = 240Whr back into your system.

It's worth noting that XKCD has a pretty good take on the whole solar-panel thing…

The Long And Tedious Version

Finding realistic solar output

This has proved an incredible pain in the arse to find accurate information on. There are hundreds of solar calculators online but they are all on websites that sell solar panels, hence are as trustworthy as a Labrador at an archaeological dig.

Wikipedia has information on solar irradiation that looks like it was measured by real adults doing science, that indicates in the UK our average irradiation is about 1000W/m2. That's often touted about as if that's the amount of electricity you could actually get from 1m2 of solar panels, whereas it's actually the amount of power hitting a 1m2 square of earth on an average day, which the solar panel must then convert to electricity. The current world record for that conversion efficiency is about 22%, and according to some physics article I saw online, the theoretical limit for traditional solar panels is about 32%.

So, for your 1kW of sunlight falling on your 1m2 panel, you can expect about 200W of electricity out at most.

After distilling a lot of reading I came to the conclusion that the (very well concealed) reality was likely to be somewhere around 10-15% of the rating of the panel, so a 100W panel might average 10-15W of output, averaged over 24 hours per day and 365 days.

Best Answer Yet

Whilst discussing this with a friend, he said another friend might know something about solar… so I got in touch and it turns out that friend has a friend who worked in “remote power”, basically providing electricity by whatever means to things like remote cellphone towers, research stations, weather monitoring stations, etc. way beyond the reaches of the electrical grid. This is excellent news, because their job is not to sell solar panels, it's to provide reliable power in the most effective way possible. That means they can't rely on marketing bullshit and have to really know what a system is going to really do in the real world - be it solar, wind, battery, generator, whatever.

He offered to run our requirements through their software which takes vast amounts of data about the weather, solar radiation, etc. etc. as well as some pretty damn accurate info on real-world behaviour of solar panels and batteries, and then tells you how well your system will perform.

Long story short the answer is about 10% of the rating of the solar panel. For an average load of 1kWh/day he calculated 500W of panels plus 200Ah of battery would be required for mostly reliable operation in northern Europe for most of the year.

By a happy coincidence the BBC Radio4 Inside Science program on 14/4/16 had a piece on solar panels:

Many listeners wrote in after a recent piece on solar panels. We had queries about how to store the electricity, and whether PV panels are worth the energetic cost of producing them and what units to use.

We put all these questions to Jenny Nelson, Professor of Physics at Imperial College and author of 'The Physics of Solar Cells.'.

In this program, Prof nelson gave a ballpark estimate that for a given “peak power” rating (the headline rating by which solar panels are sold) the average output of a solar panel over a year is about 1/10th of that figure.

So, pretty much what cynical old me had suspected!

Solar Chargers

There is a vast range of solar chargers out there, and people are adding bells and whistles all the time. I'll summarise a few basics here


There are two main types of charger: What's known as PWM is the basic version, MPPT is a bit smarter and supposedly up to 25% more efficient.

PWM is a bit of a bad name because PWM is just a method of controlling //anything//, it might be better to call PWM controllers “non-MPPT” or “dumb” or something like that. Basically they just regulate the charge from the panel to protect the panel & battery, avoid over-charging and all that sort of thing.

MPPT is a scheme to try and make sure you're getting the best performance possible from your solar panel. MPPT stands for Maximum Power Point Tracking, and in basic terms means the controller varies the load on the panel to keep it operating at its “sweet spot” for the current conditions. Think of it a bit like an automatic gearbox in your car - cruising down the road you could be in 3rd, 4th or 5th gear for exactly the same speed, but one of those will be the “best” one for the current conditions, the best balance of engine revs & load. An MPPT charger is basically doing that balancing act with the solar panel volts & amps. The “dumb” ones can't do this matching, basically it's like they never “change gear” and you get what you get, which is obviously a bit less efficient.

An MPPT charger basically constantly adjusts itself to find the sweet spot, the thin blue line on this graph of solar panel characteristics Vs sunlight:


I have discovered that many solar chargers are common-positive, which creates a serious risk of fire if wired into a regular negative-earth vehicle. At the very least makes it very easy to completely bypass the charger by accident if you common the grounds.

Beware Shadows!

Something not commonly mentioned is that ANY shadows (or dirt, leaves, etc.) on solar panels pretty much kill their output, for example this picture (from HandyBobSolar shows a small shadow reducing a panel to -50% output:

Indeed, we've seen output drop massively even on very sunny days because we were parked near a tree. You can imagine that a single large wet leaf falling on your panel & sticking (or a patch of bird poo) is going to seriously dent your output.

This happens because the panel is made up of cells and if one cell is shaded it acts much like a dead battery - not only does it generate no power but it has a high resistance, so prevents current from the other cells in the chain from flowing through it.

End Result

The upshot of all this faffing about can be seen in our Mk2 electrical system write-up.

The answer for “What size solar panel do I need?” is basically “About 10x the advertised rating”, and MPPT controllers are worth the few extra pennies

Further reading

Unfinished Draft Rubbish...

The problem I have with most charging systems / solar controllers is they don't really tell you what they're doing, even the really expensive ones, so I ended up with a cheap 10A MPPT eBay controller (but which at least has a display to show you what it's doing), a £30 intelligent split-charge relay from Furneaux Ridall, and called it good enough.

Unless you're drawing loads of power you'll be fine with a 100W panel and basic controller. Also, if you're driving it around with any regularity (EG not parked up for a week) you're topping the battery off daily anyway at a far faster rate than any solar panel is going to.

We ended up with 2x 100W panels and 2x MPPT controllers because we're restricted on battery size (due to not wanting to make major mods to the vehicle) and the fridge works hard for 5 days with zero engine running while we're parked up at LeMans - a 2nd panel and controller was cheaper than any of the alternatives and we already had the roof rack to sling it up there.

We also spent “extra” to get the most efficient fridge (Waeco CRX50) which means you need less battery and less solar. TBH it's been worth every penny, it will often freeze stuff unexpectedly even on the medium setting and it holds way more than you'd think as it's deeper than a 3-way fridge despite being physically smaller. Again, we didn't want to hack holes in the side needed to run a 3-way on gas 9the only efficient way to run one!) so the compromise was a flash 12v-only fridge.

You can't put two panels into one controller - well, you *could* but they might fight each other, solar panels are weird… I ended up running two sets of wires back to the battery terminal to prevent the controllers fighting each other.

As I said, the problem with all this solar stuff is no-one tells you what any of it really does (and most of them don't know themselves) - the power claims for panels are total marketing optimism, the intelligence of controllers is vastly exaggerated and most of them are re-packaged so many times no-one remembers what the thing will actually do in any given situation. Ironically CTEK, the really expensive ones, tell you almost nothing useful either in the manual or on their display (usually the minimum number of LED's they can get away with).

Given more time and enthusiasm I'd have made my own controller, and I might yet one day…

For your purposes though, a single 100W panel + basic controller will be more than fine - adding battery capacity is far more efficient especially if the truck will be driven fairly regularly.

The key to it all is working out how much power you need over an average 24h period, how much power the solar panel will be able to put back, and how much battery you need to sustain this. There's no point having a big solar panel if your battery is “full” after an hour of sunshine, as you're then throwing power away and not extending your “range”.

For example - our average load over a day is maybe 2A/h (24W) - 1.2A/h for the fridge, 0.8A/h for everything else (lights, water pump, heater). We have 200W of solar panels, which averages ~20W back into the system over 24h So the battery has to cope with ~4W * 24h = 0.3A/h * 24 = 7.2AH of capacity lost per 24h

Now, the elephant in the room is that the solar panels don't put out a nice even 20W of power 24 hours a day, they put out maybe 120W at midday and 0W overnight, and if the battery and fridge can't usefully absorb 120W, some gets wasted - which means that the battery has more to do when the sun's down and might average 10AH or more of lost capacity per day which the solar cannot fully replenish when the sun's up.

alfie/solar.txt · Last modified: 2019/05/15 23:34 (external edit)
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