Adding fixed solar panels to a Mac

rick.jones's picture

Here are some photos documenting my project to fit solar panels to my Mac 19

In the past I've used a small portable solar panel to keep the batteries from self-discharging when the boat wasn't in use, but it was always a pain to remember to fix it on deck somewhere and plug the cable in etc. I liked the idea of a larger fixed panel, but the problem on a small boat is where to fit one.

Then a while ago I started to see "semi-flexible" panels becoming available, with claims they are robust enough to walk on. Intrigued, I spotted some at the Southampton Boat Show, and they are made by encasing regular solar cells in acrylic sheet - the same stuff the Mac windows are made of. You definitely won't do them any harm trampling all over them!

Location, location

This meant there was no problem screwing one down to a bit of the deck, but where was best? The hatch cover would be a good candidate, but as I have a spray hood the panel wouldn't work very well with the hood up. I therefore decided the upper level of foredeck, just in front of the windows, would be best. I then hunted around online for a panel of a suitable size, and found that Sunstore sold a 20W panel that would allow two to be fitted end-to-end across the deck, giving 40W in total (solar panel wattage figures sound impressive, but in actuality, 40W of panel will only provide 1-2A of charge current on a sunny day in the UK). This arrangement also meant the cabling would be in the middle of the deck, usually the least trodden area.

Small panels of less than 10W output can generally be connected directly to the battery and won't overcharge it, but larger panels need to be used with a charge controller. I bought a Landstar one, rated at 10A, also from Sunstore. It's happens to be waterproof so although it won't be dunked will be safe from degradation in the damp atmosphere of the boat (especially in winter). It's also no dumb charger, it is a proper multi-stage unit, and includes a temperature sensor so it can adjust charge voltage according to ambient temperature.

Solar panel cabling

The majority of solar panels available today are fitted with industry-standard cables. There are two separate cables, one positive, one negative, terminated in MC4 connectors. There's nothing magic about MC4s in relation to solar power, the connectors have been adopted because they are weatherproof, and can be quickly clipped together by non-electrically-qualified fitters when installing large solar arrays. You don't have to use them, you can just cut them off if you want. If I'd used the MC4s, and hence the full cable lengths, I'd have ended up with a mass of wire floating around the deck - not good!

The cable is also very heavy duty. It's stranded 4mm2 tinned copper, double-sheathed (see picture). The tough sheath is good news, but the thickness makes it quite stiff so you need to plan how to route it. 4mm2 is complete overkill for the 1A or so that each panel will generate, but very necessary for the huge panels used in large arrays, and it's just standard fitting.

Cable routing

Unlike the 26, there is no clearance between the ceiling liner and the deck to route cables in a 19. I was going to have to drop the cable through into the cabin and run it along the ceiling. I therefore got some white flat 2-core "5 amp" cable, and white self-adhesive cable clips, to be as unobtrusive as possible. The cable is .75mm2 section, which although a lot thinner than the cable on the panels, is quite adequate for the 2A max current that the panels will generate.

To hook it all together I found a black waterproof junction box, made by Wiska, that I could screw to the deck. It also has the advantage of membrane cable entries, meaning the cables can simply be pushed through and are watertight without the need for bulky glands. The box is positioned on a black area of deck, so is quite unobtrusive.


The pictures show the installation sequence. I started by fitting the junction box, then drilled through the box into the cabin and fed the cable through, fixing and sealing it with Sikaflex.

While that was setting, I positioned the panels and drilled the deck for self-taps, one in each corner of each panel. I put a drop of Sikaflex in each hole to both seal the GRP and help prevent the screws working loose.

Next, I routed the cables into the junction box, cut them to length and joined them into the box's connector block, together with the wires into the cabin. To feed the cables through the membranes it's best to start a small hole with a sharp point (e.g. an awl), then gently push the cable through so as to expand the hole without tearing it. The panel leads are marked + and -, it's important to join + to + and - to - (the panels are in parallel).

It just remained to clip the cover onto the junction box to complete the external work.

Internally, it was simply a matter of attaching self-adhesive cable clips to the ceiling in order to route the white cable to the corner by the starboard seat, where I attached the charge controller in the spot where the original cabin light had been (I find that battery LED lights fixed in other places work better for cabin lighting, so this light was no longer used).

The charge controller has three pairs of flying leads, which I hooked up using a screw connector strip. One pair connects to the solar panels, one to the battery, and the third pair can optionally be used for all the equipment running off the battery. Use of this pair is optional, you can just leave the rest of your electrics as normal, and the controller will take care of charging the battery.

Battery protection

However, I also wanted to add a low-voltage cut-off relay, a device from China I found on Ali Express, and now also on eBay. This is the white box beside the controller. The cover flips off to reveal a digital display and a couple of buttons you can use to program the disconnect and reconnect voltages as well as a time delay. The device needs to be wired so that everything running off the battery goes through it, and if the voltage drops too low it will disconnect, saving the battery from over-discharging. 12V lead-acid batteries should not be allowed to fall below 12.2V if you want them to last. Letting the voltage fall below this will drastically shorten their life each time it happens. Leaving something like a small fridge running overnight without shore power can easily kill a battery if there is nothing like this to protect it.

Installing the device thus entailed re-configuring the wiring from the battery to the main switch panel, and it worked out simplest to wire it to the output connections of the charge controller. The relay in the cut-off device is rated at 20A, which is well above the total amount of equipment I use.


All in all I'm happy with the result. The panels don't look out of place on the deck, and being all-black look almost like two extra windows (many plastic solar panels use clear plastic, so very obviously look like what they are).

The 85Ah battery wasn't completely charged when I started, but after only a day in September sun, the light on the charge controller was flashing indicating full charge, so they definitely work!

I am a little concerned that although they are robust enough to walk on, the glossy panels might present a bit of a slip hazard when wet. That said I don't actually step on that part of the deck much anyway, and almost never need to go to the foredeck when underway, so I shall just have to take care.

If you want to know more, feel free to post a comment below.