12 volt Solar Power

Choosing Panels and Regulator

When we began looking at 12volt solar power we found it something of a minefield. It is a complex subject, there was the usual jargon to overcome, and prices of solar panels varied enormously; typically from £350 to less than £100, depending on the source, for what appeared to be essentially the same 80watt panel.

First, we had to decide what we wanted from our solar installation. In the summer, having stopped somewhere for more than 2-3 days (with no electric hook-up), it would be ideal if the output from the solar panels balanced the load on the leisure battery.

With help from the Self Build Motor Caravanners Club (SBMCC) forum and archive we managed to work out what items we would need and where to obtain them. We found one can fit a solar power installation relatively inexpensively by sourcing items on Ebay directly from China and elsewhere. A disadvantage here is if things go wrong in the future - it can be difficult, and costly, to obtain a replacement. So we used items that cost a bit more from reliable UK suppliers (details of suppliers on Sources page).

Although we bought our panels, regulator and sundry items separately, it is possible to buy a solar power kit containing all one would need.

Calculating the panel size (in Watts)

To determine the size of the solar panel(s) required, one needs to calculate the total power consumption of all the 12volt items installed. We had already done this (when we originally worked out the size of leisure battery we'd need). From the table on our Electrics page, the power used is 466watt/hours, or 39amp/hours per day. This is an average - on hot sunny days the figure can increase significantly as our compressor fridge will be running longer; on cold cloudy days with chilly evenings running the space heater for a few hours can also add a noticeable power drain.

One also needs to consider where one is likely to travel in the van, the time of year, and likely weather - this has an important bearing on how much sunlight one can usefully harness (or not!).

A typical 80watt solar panel should output around 4-4.5amps. In theory, it would take 10 hours of constant sunshine to balance our batteries. If we had two 80watt panels, connected in parallel, then we should get 8-9amps and take only five hours. But things are never that simple as there are losses in the panel, wiring and battery, also the sun never shines directly overhead for 5 hours and so forth... To perhaps get a more accurate estimate there are solar panel calculators available on-line one can use (see Sources for one example).

From the data we input into the Motorhomefacts website calculator:
Latitude: 51deg - this is where we live (the figure for any location is shown if you hover over it using Google Earth)
Date: August 2013
160watt panel (2 x 80w)
Panel volts: 17.8v

From this the calculator returned:
Theoretical max output at noon: 7.2amp
Theoretical Daily panel output: 65amp/hrs.

Given we do most of our travelling in summer in southern England and in places such as southern France and Spain (places where our fridge tends to work overtime) this looks as if two 80watt panels will do. This calculator does not take account of the better performance obtained from MPPT regulators, so the output would be higher than that shown.

It is also very worthwhile asking other motorhomers who have solar panels what level of performance they obtain.

However, this is our starting point, we shall have to see how we get on! Update: See section below on our results to date

Schematic Wiring Diagram

solar wiring diagram

The 7.5 amp fuses are housed in a surface mount plastic box in inside the van in the locker cupboard. These fuses are not essential and can be omitted, but they provide a useful test point for fault finding.

The regulator is rated at 30amp - this to allow some future-proofing should we wish to add more panels later.

The remote display is an optional item.


solar panel front face

One of the 80watt Monocrystalline solar panels.

Each panel measures 960x540x22mm and weighs 7kg.

The efficiency of these panels is increasing all time whilst the cost is coming down!

We could, of course, have fitted just one big panel - it probably would have been slightly cheaper. However, in the unlikely event the panel stopped working, we would have no output. By using two smaller panels it means if one failed, we'd still have an output from the other.

solar panel rear view

Rear view of solar panel as delivered.

The 3.5mm sq. wire tails are 700mm long and terminated with MC male and female connectors. We did not use these connectors, preferring to cut them off and terminate the wires directly on a connection block inside the van. This saved having a multitude of connectors on the van roof (see below).

On the long lower edge of the frame are slotted holes for fixings - we didn't use these, but drilled holes in the end frame and epoxied 5mm stainless nuts on the inside to act as captive fixings.

solar panel information label

For those who like statistics: information label on rear of solar panel.

using carboard template on van roof

To decide on the best position on the van roof we made a cardboard template with a length of scrap wire attached to indicate how far the wire tails would reach.

solar panel fixing strips

To fix the panels we used U-section aluminium channel (1in. x 1 1/4in x 1in. by 1/8in. thick) cut to length. Two clearance holes were drilled in the side for the fixing bolts. The channels were fixed using Sikaflex 512 adhesive after the roof area was thoroughly cleaned. These brackets ain't coming off!

One can buy panel fixing brackets that attach at each corner; we were not keen on them as the roof is corrugated and obtaining a good flat surface to mount them could be problematic.

solar panel fixing bolts

End view of one panel showing the two 5mm dia, 20mm long stainless steel set screws with lock washers that secure the panel at each end.

It is important to space the panel away from the roof surface to allow cooling air to flow underneath; the panels can get very hot.

solar panel covered in cardboard to block sun

First panel fitted and covered with cardboard as it is yet to be connected.

solar panels on roof

The finished installation on the roof. Still some space left if we need to fit more!

The front panel is positioned a little way back from the rooflight to avoid any shadows which can seriously affect the solar panel performance.

The gap between the two panels is to give access to the fixings should it ever be necessary to remove a panel.

solar regulator in sofa box

The 30amp MPPT (Multi Power Point Tracking) solar regulator fitted in sofa bed base near to the leisure battery. Not the ideal location as there is little air movement to keep the unit cool. It had to be fitted close to the battery as the unit has a sensor (with quite a short lead) that is attached to the battery casing to monitor its temperature to avoid overcharging. Although more expensive, this type of regulator can produce around 20% more power than the standard regulator.

To limit voltage drop between the solar panels, regulator and leisure battery, 10mm sq. cable was used throughout with soldered ring terminals at the battery.

To prevent damaging (possibly terminally!) the solar panels it is important to connect the cables in the correct order.

The regulator must be connected to the battery before connecting the solar panels to the regulator. Similarly, if it is sunny, and necessary to disconnect the solar panel cables from the regulator, the panel in-line fuse must be removed (if you have one), or the panel covered over to reduce the output to a low figure.

Surprisingly, whilst doing some testing, we found even with thick corrugated cardboard covering the panel, 5 volts was registering on the voltmeter.

solar remote display on main control panel

An option is to fit a solar regulator Remote Display. Although our regulator has LEDs to give an indication when the unit is (or isn't) charging the leisure battery, LEDs aren't that helpful when one wants to know actual voltage or current (and being hidden away in the sofa bed base the LEDs cannot be seen anyway).

Pic shows the reading after we fitted one panel; output from panel: 16.2volts, 4.5amp charging current. Whilst below...

solar remote display on main control panel

....output after connecting both panels: 16.6volts and charging current of 8.4amps - not bad for a mid-August afternoon in southern England (the leisure batteries were almost fully charged at this point).

The remote unit can also show the leisure battery voltage and current being drawn. A slot in the base accepts an SD memory card if one should ever want (unlikely) to look at the performance of the unit and solar panels over time.

solar cable roof entry box

One method of cable entry through the roof is to use a streamlined box fitted with two gland nuts as shown. A hole would be drilled in the van roof, the cables fed through the glands and roof hole, and the box then stuck in place with Sikaflex. This would leave one pair of MC connectors on the roof for every panel fitted.

In the pic, we did attempt to dismantle one of the MC connectors to see if it was possible to feed its cable through the gland with the terminal part attached (the idea being not to use the connector on the outside of van roof, but inside) - but we could not remove the terminal from the body.

If using more than one panel one has a choice whether to tee all the cables together using a special connector (see below) to leave just one pair of cables to enter the box, or use multiple boxes - one for each panel.

We didn't use this method.

solar cable roof entry box

We used a waterproof box fixed down with Sikaflex with a hole drilled through the box base and roof. We fitted glands in the box sides and grommet in roof hole. Note, the cables supplied with the panels have tough insulation which makes them difficult to bend in a tight radius.

The box screw-down lid allows inspection and cable changes, if desired.

solar panel cable roof entry box

We came across this method of joining the cables of two solar panels (in parallel) using tee-connectors. The connectors on the left (arrowed) are from one panel - these plug into the tee. A second set of connectors from a second panel (not shown) would also plug into the tee on the left. The connectors on the right are for the two cables going away and subsequently through the gland box and roof to the regulator. We assume all this lot would be left on the roof open to the elements.

Some observations.

Update 3 - November 2015.

Vilanova Park campsite, near Barcelona

Having now had the solar installation working for two full years, there's no doubt the system is well worth installing.

In fact it is easy to forget, particulary when one is relaxing in sunny Spain in late summer, to suddenly find the leisure batteries are only half charged. It's then one realises the nice shady spot (as we had here at Vilanova Park near Barcelona) means the solar panels are getting very little sun! We had to connect up our mains charger to the leisure batteries until we moved to a less-shady pitch a few days later.

Update 2 - November 2014.

MCC rally at Weyhill Fair, near Andover

In the latter part of September 2014, we spent five nights at a super rally organised by the MCC East Wessex group at the Weyhill Fair, near Andover in Hampshire.

The weather was changeable, often cloudy for long periods and chilly some evenings. With no electric hook-up we wondered if the leisure batteries and solar panels would cope. They did, but only just, the battery voltage dropping to about 50% capacity at the end of the rally.

We believe the extra drain on the battery from using the space heater (it takes 1.7 amps on gas setting) in the evenings, and the long cloudy days not generating enough power to balance the load, were the cause. Also, it would have helped if we had turned down the fridge which, we discovered later, was set inadvertantly on an unnecessarily high setting for warmer more southerly climes! It would appear even with sufficient leisure batteries and solar panels one still has to monitor the effects of the weather, the location and time of year, and adjust the power demands accordingly.

Update 1 - October 2013.

Now we've had a few months use from the solar panel installation we are happy to say that, so far, it is working out very well. We should perhaps add that in 2013 most of Britain and Europe had a wonderful sunny, warm summer, so that may tilt the results somewhat!

Since we've had the panels fitted, and on our recent trip down through France, at no time have we seen less than 12.6 volts reading on the leisure batteries. This also in temperatures of 32deg C. when our compressor fridge was working flat out and, at times, pulling around 4.5 amps load on the batteries for some hours. Even on cloudy days we were getting more than sufficient charge from the panels. We suspect the MPPT solar regulator is making a significant difference to the performance - justifying its greater cost (manufacturers claim that MPPT type regulators will produce around 20% greater output than a standard PWM regulator). We shall have to see how we get on in an English winter!