I will give you the punchline first, because it took me three seasons to learn it and it would have saved me a lot of money. A 100-watt panel does not make 100 watts. On a good French summer day, mounted on a boat, with the sun crossing the sky and a mast throwing shade, that panel gives you somewhere near 40 amp-hours into a 12-volt bank. Plan around that real figure, not the brochure, and your solar will never disappoint you.
Everything else in this guide is detail hung off that one honest number.
What a panel really produces
The maths is simple once you stop believing the marketing. A 100-watt panel in five useful hours of sun makes roughly 500 watt-hours. At 12 volts that is about 40 amp-hours. The five-hour figure is a fair southern-French summer average. Sources that measure real-world output put a 100-watt panel at around 389 watt-hours a day in a cloudier northern climate and 560-plus in a sunny one, so 500 is a sensible middle for a French season that mixes Brittany grey with Mediterranean glare.
Note the spread. The Atlantic coast of Brittany sees far more cloud than the Cote d'Azur, so a northern-coast cruiser should oversize where a Riviera cruiser can run lean. If you cruise both in one summer, size for the gloomier end.
Start with your daily burn
Solar sizing is pointless until you know what you spend. Work out your daily consumption in amp-hours at 12 volts before you buy anything. Most boats living off the batteries at anchor land between 60 and 200 amp-hours a day. A compressor fridge is usually the villain, drawing 5 to 6 amps while it runs and easily climbing toward 100 amp-hours over a hot day if the box is poorly insulated. Add instruments, lights, water pump and charging your devices, and a typical week at anchor in the Med might run 90 to 120 amp-hours a day.
Write the list. Multiply each device by its hours. That total is the number your whole system hangs on. I worked the continuous, round-the-clock version of this for an offshore leg in my notes on power management on a French coastal passage, and the anchored numbers here are the gentler cousin.
Doing the simple sum
So put the two halves together. If you burn 100 amp-hours a day, two 100-watt panels giving you 80 amp-hours of harvest get you most of the way, and three panels comfortably cover it with margin for cloud. That is genuinely all the arithmetic there is. Daily burn divided by 40 gives you the rough number of 100-watt panels you want, then add one for the grey days.
For a 35-foot boat running a fridge, instruments and a family's devices for a French summer of anchoring, I would fit 300 watts. It is enough to ride out a couple of dull Atlantic days without touching the engine.
MPPT, not PWM
There are two kinds of charge controller and the choice is not close. A PWM controller is cheap and crude. An MPPT controller squeezes noticeably more out of the same panels. On a 100-watt panel, MPPT typically delivers around 95 watts to the battery against roughly 78 watts for PWM, and under passing cloud or light rigging shade the MPPT advantage can be 14 to 22 per cent more watt-hours over a day.
That is free energy for the price of a slightly better box. Buy the MPPT. It matters most in exactly the conditions a sailing boat lives in: a mast and boom dragging shadows across the deck all afternoon.
The shading problem nobody warns you about
This is the one that catches everyone. A single shadow from a boom, a furled headsail or a backstay can knock a panel down to almost nothing, because the shaded cells choke the whole string. On a sailing boat you cannot avoid this entirely. It is the real argument for a stern arch, which lifts the panels above most of the rigging clutter and gives you clean hours either side of noon when the sun is high.
If you can only deck-mount, wire the panels so one shaded panel does not drag the others down, and accept that your real harvest will sit on the optimistic side of the calculation.
Flexible or rigid
Flexible panels bond onto a bimini, sprayhood or coachroof, sit flush and survive being walked on. They run hotter, which costs efficiency, and they tend to die younger. Rigid panels on a stern arch catch more sun, run cooler because air passes behind them, and last longer, but they are a structure you have to build and they catch the wind. For a season or two in France I would take flexible on the bimini if the boat has no arch; for a long-term boat I would build the arch and fit rigid.
Mounting in the real world
Where you put the panels decides how much of that theoretical harvest you actually see, and most boats compromise here. A stern arch is the gold standard, holding two or three rigid panels clear of the boom's shadow and angled toward the sky, but it is a fabrication job that costs real money and adds windage aft. Guardrail-mounted panels swing to face the sun and clear the deck, though they are vulnerable when you come alongside and they foul lines. Bimini-bonded flexible panels are the tidy, cheap answer for a boat without an arch, accepting the efficiency loss from heat and shade as the price of simplicity.
A detail people forget: cable run matters. Long thin wires from a deck panel to a battery deep in the bilge waste power as voltage drop, and on a 12-volt system that loss is not trivial. Use generously sized cable, keep the MPPT controller close to the battery rather than the panel, and you keep the watt-hours you worked to make.
What it buys you in France
The honest reward is freedom from the marina. The joy of a French summer is the anchorages: the Glenans, the lee of Belle-Ile, the calanques near Cassis, the bays off Porquerolles. A boat that makes its own power sits in those for days. When you do want a pontoon for laundry and a long shower, you arrive with full batteries and treat the water, electricity and showers in French ports as a top-up, not a rescue.
Solar also kills the morning engine-charging ritual that annoys your neighbours and your engine. The panels work while you sleep in, swim, or row ashore for bread.
Pair it with the right battery
Solar and lithium belong together. Lead-acid and AGM accept charge slowly as they fill, so the last fifth crawls in just as your panels are making their best power at midday. LiFePO4 takes a high charge rate almost all the way up, so it banks the sun instead of wasting it. I made the full case in lithium vs agm boat batteries for a French summer, and the combined sizing exercise lives in solar and lithium for a French summer cruise.
If you want to run the genuinely thirsty kit, a watermaker for French cruising leans hardest on a generous solar-and-lithium setup, because making water is the one luxury that turns the sun directly into staying out longer.
The short version
Size your panels off your real daily burn, divide by 40 amp-hours per 100-watt panel, add one panel for the clouds, fit an MPPT controller, and get the panels clear of the rigging if you possibly can. Do that and you will spend a French summer where the only sound at sunset is the halyard tapping the mast, not your engine charging a battery you could have filled for free.
