We spent the whole of last July at anchor in the Morbihan and the Glenans without once plugging into shore power. Two years earlier that would have meant running the engine an hour a day to top the batteries and listening to a diesel grumble over a sunset that deserved silence. The difference was a winter spent fitting solar and swapping out the old lead-acid bank for lithium. If you are bringing a boat to France for a season of anchoring rather than marina-hopping, this is the upgrade that changes how the holiday feels.
I am not an electrician and I will not pretend the maths is glamorous. But the principle is simple, and once you have done your own power budget the rest is shopping.
Start with what you actually burn
Before you buy a single panel, work out your daily consumption in amp-hours at 12 volts. Most cruising boats running off the batteries alone land somewhere between 60 and 200 amp-hours a day, and the figure roughly doubles on passage when the autopilot and instruments run round the clock.
The big drinkers are predictable. A compressor fridge pulls 5 to 6 amps while the compressor runs, cycling on and off, which over a hot French summer day adds up fast and can climb toward 100 amp-hours on its own if the box is poorly insulated. Navigation electronics, a chartplotter and autopilot together, draw roughly 8 to 12 amps when in use. Add lights, a water pump, charging phones and a laptop, and a typical week at anchor in the Med might run 90 to 120 amp-hours a day.
Write your own list. Multiply each device by the hours it runs. The total is the number every other decision hangs on. If you want the offshore version of this calculation, where the loads run continuously, I worked through it in detail for power management on a French coastal passage, and the anchored numbers here are the gentler cousin of those.
Why lithium changes the sums
Lead-acid batteries hate being run flat. You can safely use about half a lead bank before you start shortening its life, so 200 amp-hours of lead gives you maybe 100 usable. Lithium iron phosphate, the LiFePO4 chemistry that has taken over the leisure market, lets you use roughly 80 to 90 per cent without complaint. A 100 amp-hour LiFePO4 cell therefore does the work of nearly two equivalent lead batteries, in a quarter of the weight, and it accepts charge far faster so your solar fills it in the hours that count.
The proven rule of thumb is to carry three to four times your daily consumption in battery capacity. If you burn 120 amp-hours a day, you want 360 to 480 amp-hours of bank. In lead that is a back-breaking, locker-filling installation. In lithium it is three or four 100 amp-hour blocks you can lift one-handed. Prices have fallen hard: a basic 100 amp-hour LiFePO4 unit was selling around 189 dollars in mid-2025, and even quality marine-grade cells with Bluetooth monitoring sit well below what lead cost a few years ago.
One caution. Lithium needs a charger and an alternator regulator that know what they are doing, and a battery management system to protect the cells. Do not bolt lithium onto an old charging setup and hope. Budget for the supporting kit, not just the batteries.
Sizing the solar
Solar is the half that makes anchoring sustainable. A 100-watt panel in five hours of useful French summer sun produces around 500 watt-hours, which at 12 volts is roughly 40 amp-hours. That is a real, measured figure, not a brochure one, and it already accounts for the panel never quite hitting its rated output.
So if your fridge and your evenings cost you 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 a margin for cloudy days. The Atlantic coast of Brittany sees more grey than the Cote d'Azur, so a northern-coast cruiser should oversize where a Riviera cruiser can run lean. Flexible panels bond onto a bimini or sprayhood, rigid ones go on a stern arch where they catch more sun and run cooler, which matters because panels lose efficiency as they heat.
Pair the panels with a proper MPPT controller rather than the cheaper PWM type. MPPT squeezes noticeably more out of the same panels, especially in the partial shade that a mast and boom throw across a deck all day. It is a small box and a worthwhile one.
A word on shading, because it bites harder than people expect. A single shadow from a boom, a furled headsail or a backstay can knock a panel down to almost nothing, since the shaded cells choke the whole string. On a sailing boat you cannot avoid this entirely, which is the real argument for the stern arch: it lifts the panels above most of the rigging clutter and gives you a few clean hours either side of noon when the sun is high. If you can only deck-mount, wire your panels so that one shaded panel does not drag the others down, and accept that your harvest figures will be on the optimistic side of what you actually see.
What it buys you in France
The honest answer is freedom from the marina. French marina costs in high season are not trivial, and 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 can sit 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 rather than a lifeline.
Solar also quietly removes the engine-charging ritual that annoys your neighbours and yourself. No more running a cold diesel at half load every morning, which is bad for the engine anyway. The panels work while you sleep in, swim, or row ashore for bread.
And it frees you to add the modern kit that makes a long cruise liveable without dreading the power cost. A Starlink dish, a laptop for working remotely, a decent anchor-watch setup left running all night: all of these are easy on a lithium-and-solar boat and a constant worry on a tired lead bank. If staying connected matters to you, the panels are what make staying online in the French canals and at anchor a non-issue rather than a daily rationing exercise.
My short version
If I were specifying for a 35-foot cruising boat heading to France for a summer of anchoring, I would fit 300 watts of solar on a stern arch, an MPPT controller, and 300 to 400 amp-hours of LiFePO4 with a matched charger and BMS. That covers a fridge, instruments, lights, devices and a connectivity dish for a family, with enough margin to ride out a couple of grey Atlantic days without touching the engine.
Do the power budget first. Buy the batteries to match your real consumption, not the biggest bank you can fit. Size the solar to refill that bank in the sun you will actually get on your coast. Get those three numbers right and the rest is bolts and cable. The reward is a season of quiet anchorages where the only sound at sunset is the halyard tapping the mast, not your engine.
