Solar made simple

Roger McMillan demystifies solar panels with this plain-English look at how they work and factors to consider in choosing a suitable panel for your needs.

When you reach into the boat fridge for a cold drink, it's very satisfying to know that the sun provided all the power to chill your favourite tipple.

Most boats these days have at least one solar panel, even if it's just a little 10-watt “cheapie” to keep the starting batteries charged. Yet there still seems to be some mystique about this essentially basic technology. So let's try to simplify it.

How does solar power work?

The full scientific explanation would be complicated, and the theme of this article is “keep it simple”, so here's the “lite” version. Solar panels are made from thin layers of silicon. Light from the sun hits the panel, which activates the electrons in the silicon and causes them to jump from one atom to another. This creates an electrical chain reaction in the form of a DC (direct-current) charge. Or in even simpler terms, when the sun's light hits your solar panels, it produces electricity in a form that can be fed straight into your batteries.

There's a good reason why you need to know that it's the sun's light, not heat, that makes the electricity. Highest production is at midday in summer, when the sun hits the panels at exactly 90°. In winter when the sun is farther away (and therefore “less bright”), or when the sky is cloudy or when the light is hitting the panels at a more acute angle (eg early morning and late afternoon) your solar panels will produce less power.

I had this clearly demonstrated recently on a trip from Port Geographe to Fremantle. Sailing directly north, the morning breeze was from the east, so my boat was heeled to port. My solar panels were therefore facing west and blanketed from the sun's light which, of course, was shining from the east. In the afternoon, when the sun had moved across the sky to the west, the sea breeze came in, and I was now heeled to starboard, again blanketing the sun from my panels.

Result – no solar power all day. So you need to think about where you position your panels. The easiest place is on a dinghy davit, bimini, radar arch or similar structure. The downside being that these are all flat surfaces, and if you have a monohull you can lose the sun when you're heeled over. A good solution is to hinge the panel off the pushpit or rail, allowing you to tilt the panel towards the sun.

Which panel is best for your boat?

This brings us to the next key question. There are three types of panels, so which one is best for your application? Essentially you can treat the first two types of panels as the same.

Monocrystalline panels (black ones) use a single wafer of silicon cut from a high-grade silicon ingot for each of their 36 cells. Polycrystalline panels (sparkly blue ones) use a wafer of silicon cut from a block of mixed silicon crystals for each cell. While there are some minor differences in cost and output, they share the characteristics that they produce a lot of power for their size but suffer if there's any shading at all. Even the shadow of a halyard lying across one cell will cut the production from all 36 cells.

The third type of panel, amorphous (usually a matt-grey colour), handles shading much better and can be made flexible to wrap around a deck camber. But it has the disadvantage that it requires twice the surface area to produce the same amount of electricity as mono or poly panels. This might not matter on catamarans or powerboats, which usually have a greater area pointing at the sun, but on a monohull it can mean amorphous panels are impractical. Another consideration is that a good-quality mono or poly panel will last 25 to 30 years, while amorphous have a much shorter lifespan. Even top-quality amorphous panels are likely to deteriorate badly after 10-12 years.

How many panels do you need?

This is the million-dollar question, and there are a number of ways of answering it. The right way to size your solar array is to work out what your average daily electricity consumption is, add 20 percent to cover cloudy days, shading, voltage drop and other variables, then do
the sums.

The table shows some typical consumption rates for electrical consumers often found on cruising boats. To get your total amperage, multiply the consumption of each item by the number of hours a day it is in operation. For example, your fridge might consume five amps and run for four hours a day, so its total requirement is 20 amphours. Your anchor winch may draw 100 amps but runs for only two minutes a day – total draw 1.65 amps.

If we need, say, a total of 70 amphours per day to run everything we would add 20 percent to this (our contingency fund) to get 84 amphours. We multiply this by 12 (volts) to get 948 watts. (Amps x volts = watts.) We divide this by the number of hours of sun we expect to hit the panels each day (six hours is a good average, but more if you sail predominantly in the tropics) and this gives us 158 watts of solar panels needed. A couple of 80-watt panels wired in parallel will do the trick.

Another method is to look at the size of your battery bank. Let's say you've got two batteries each rated at 100 amphours. Remembering that you can only discharge your batteries to 50 percent capacity before you risk damaging them, this means you need to be able to put 100 amphours of juice back in every day if you're using them to capacity. To provide 100 amphours of solar power you will need at least 200 watts of panels.

The third (and possibly simplest) method is to work out how much suitable space you have on the boat and ask your solar supplier to fill it. If there's any shortfall between what you can generate from solar and what you use each day, you'll just have to run the diesel.

More bang for your buck

On boats that have a limited amount of space available for solar panels a good way to boost the output is to use a maximum power point tracker. In simple terms, this works by processing the power coming from the panels and converting it into the most efficient form to charge the batteries. The voltage of a panel can vary slightly depending on the intensity of the light. Even though your panel is rated at 12 volts, it might produce up to 18 volts in perfect conditions.

However, your batteries can only absorb about 13.8 volts, so the extra production is wasted. The MPPT captures this excess energy, converts it into the ideal voltage and feeds more amps into the battery bank. Using a good-quality piece of gear like the Blue Sky Solar Boost I have on my boat, you can expect up to 40 percent more power from your panels.

A few more things you need to know

• Solar panels are heat-sensitive. Their ideal operating temperature is 25°C and when the mercury climbs to over 30° their output can start to decline quite quickly. The best way around this is to make sure that you mount the panels with plenty of space underneath them – they're much better on davits or a rail than they are flush-mounted on the deck.

• You need to control the charge. Batteries are sensitive beasts, and they hate being overcharged. So you should fit a charge controller to all but the smallest panels (anything 15 watts or under). These aren't expensive and they will certainly extend the life of your batteries. Solar panels can also draw power out of the batteries at night, so you need a blocking diode to ensure the power can run only one way. Most charge controllers incorporate this, and all maximum power point trackers will too.

• Solar panels need virtually no maintenance. Apart from washing off salt spray and bird poo with a non-abrasive cloth now and again (remember, any shading reduces output) they should give many years of service without input from you.

• Use quality cable. The cables running from the panels to the batteries are exposed to the elements, so use only marine-grade, tinned cable. And make sure you use the right thickness to avoid voltage drop. Tell your panel supplier the distance of your cable run, and they'll advise the minimum diameter required.

• Make sure your panel voltage matches your batteries. If you have a 12-volt battery bank you must use 12-volt panels. Similarly, a 24 volt battery bank requires 24 volt panels. A maximum power point tracker or a DC/DC converter can be used to reduce voltage if necessary.

Future trends

Solar panels have improved in efficiency over the years and the current trend to renewable energy means scientists all over the world are working to make them even better. We hear rumours of ultra-thin panels, panels that will be much more shade tolerant, smaller but more efficient modules and even “spray on” panels where you spray your boat with a silicon compound and the whole damn thing becomes a massive generator. My advice would be “don't hold your breath”.

Yes, some of the new technologies will get up, but in the meantime what we've got is pretty good. Once you get over the initial cost, your maintenance-free solar array will produce steady, efficient DC electricity for many years to come, ensuring you can keep your beer cold, run all your nav gear, pump out tunes from your iPod and even fire up your onboard computer without having to start that noisy, smelly, expensive diesel. I'll drink to that!


AM/FM radio 1 amp

Anchor winch 100 amps

Autopilot 3-5 amps

Bilge pump 2 amps

Cabin lights 1 amp each

Cabin lights (LED) 0.3 amps each

Computer 4 amps

Electric toilet 16 amps

Electric winches 150-200 amps

Fridge 5 amps

Instruments 1-2 amps

Nav lights 2 amps

Nav lights (LED) 0.3 amps

Radar 3-4 amps

VHF 1 amp

Water pump 4.5 amps

To get a really accurate picture of your total consumption, you will need to get the handbooks for all your electrical consumers and find out their watts or amps rating. Then work out how many hours you will use them per day. Add up the total amphours per day and add 20-percent safety margin.

Obviously, consumption while sailing is different to consumption at anchor or in the marina.


Top-of-the-range 215-watt, 24-volt SunPower module $1,995.00

Schott 170-watt, 24-volt module $1,575.00

GE Energy 85-watt, 12-volt module $745.00

Polycrystalline 65-watt, 12-volt module (Bias Boating) $649.00

Polycrystalline 10-watt, 12-volt module (Bias Boating) $169.00

Tristar 12-volt, 60-amp charge controller $305.00

Sunsaver 12-volt, 25-amp dual-battery charge controller $230.00

Panel Regulator 1-20-amp (Bias Boating) $79.90

Blue Sky Solar Boost 3048 Maximum Power Point Tracker $889.00

SunSaver Maximum Power Point Tracker $370.00

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