There is a sound that defines a crowded French anchorage on a still August evening, and it is not lapping water. It is the drone of a dozen generators, each one charging a battery bank, running an air-conditioning unit, or topping up a freezer, all of them pumping diesel fumes across an otherwise perfect bay. I used to be one of those boats. Then I started doing the sums on what the genset actually costs, in fuel and in carbon, and the shore-power cable started to look a lot more attractive.
The question of shore power versus generator is usually framed as convenience. The greener angle is more interesting, and in France the answer is unusually clear-cut.
The carbon maths is lopsided
Start with the generator. A marine diesel genset produces somewhere between 0.6 and 0.8 kg of CO2 for every kilowatt-hour of electricity it makes. That is the figure to hold in your head, because the comparison hinges on it.
Now the grid. Shore power's carbon footprint depends entirely on where the electricity comes from, and this is where France is exceptional. French grid electricity ran at about 19.6 grams of CO2 per kilowatt-hour in 2025, one of the lowest figures in the world, because 95.2 percent of mainland generation that year came from low-carbon sources, mostly nuclear. For comparison, the EU average was around 334 grams per kilowatt-hour.
Put those side by side and the gap is enormous. A French shore-power socket delivers electricity at roughly 0.02 kg of CO2 per kilowatt-hour. Your genset delivers it at 0.6 to 0.8 kg. That makes plugging in ashore in France somewhere around thirty times cleaner per unit of electricity than running the diesel. There are very few green choices on a boat where the difference is that stark.
Work it through with a realistic evening. Say you draw 5 kilowatt-hours overnight to run a freezer, charge the house bank and keep the lights on. From the French grid that costs the atmosphere about 0.1 kg of CO2, less than the weight of a lemon. From the generator the same 5 kilowatt-hours costs between 3 and 4 kg. Repeat that every night of a four-week cruise and the genset has put roughly 100 kg of CO2 into the air for a job the shore socket would have done for around 3 kg. The numbers stop being abstract very quickly.
It is worth being clear about why France is the special case. Most countries do not have a grid this clean, so the shore-power argument elsewhere is real but milder. Plugging in to a coal-heavy grid still beats a diesel genset, but only by a little. In France, where 95.2 percent of mainland generation in 2025 was low-carbon, the socket wins by a landslide, and a visiting boater used to dirtier home waters should recalibrate accordingly.
It is not only about carbon
The genset's local effects are the ones your neighbours notice. Diesel exhaust at sea level in a sheltered bay does not disperse the way you imagine; it pools, and it drifts downwind into other cockpits. There is noise, which carries absurdly far over flat water and ruins the very peace people anchored out to find. And there is the small, steady risk of fuel and oil traces in the bilge and overboard.
Shore power has none of that. Plug in and the boat is silent, fumeless and, in a French marina, drawing from one of the cleanest grids on the planet. The contrast is the whole argument.
What it costs you in France
The convenience comparison is real, so be honest about it. Shore power ties you to a marina berth, which costs money and gives up the freedom of the anchorage. French marinas typically include electricity in the nightly berth fee up to a modest daily allowance, then meter heavy users beyond that, or sell power on prepaid cards or tokens at the pontoon. The amounts are small relative to the berth itself. I cover what the pontoon services actually deliver, and how they are charged, in my guide to French marina water, electricity and showers.
The generator's cost is the diesel it burns plus the maintenance it demands, and it is easy to underestimate both. A small marine genset burns in the region of 0.3 to 0.5 litres of diesel per kilowatt-hour at typical part loads, which is why an evening of casual running gets through fuel faster than people expect. With French marina diesel sitting around 1.80 to 2.00 euros a litre in 2025, the "free" electricity at anchor is quietly costing you real money, on top of the oil changes, impellers and filters the engine demands like any other.
One practical wrinkle for visiting boats: French shore power is 230 volts at 50 hertz, the same as the UK and most of Europe, but the pontoon sockets are usually the blue CEE 16-amp or 32-amp connectors, not a domestic plug. If your boat is wired for a different standard, carry the right adaptor lead. A 16-amp supply gives you about 3.6 kilowatts, comfortably more than a cruising boat's charger and domestic loads need, so for most of us the limit is never the issue.
A realistic way to decide
I do not think the answer is to bin the generator. It is to use the cleanest source available wherever you happen to be, and to need less power overall. My rough rules, after several French seasons:
- In a marina, always plug in. In France the grid is so clean that there is no environmental argument for running the genset alongside a working shore socket, and usually no financial one either.
- At anchor, run the engine or genset only as long as it takes to charge the batteries, then shut it down. Continuous running through the evening is the habit to break.
- Invest in solar before you invest in a bigger genset. A decent panel array can carry the daytime house load of most cruising boats and shrink generator hours to almost nothing.
- Lift the house battery bank to lithium if the budget allows. More usable capacity means you charge fast, hard, and rarely, instead of trickling the genset for hours.
- Cut the demand. The single biggest generator-killer at anchor is air conditioning. Anchor for the breeze, rig a windscoop, and you remove the load that makes people run diesel all night.
Sizing the off-grid kit so the genset rarely starts
The genset earns its place as a backup, not a primary source, and a little planning keeps it idle for most of a cruise.
Solar does the heavy lifting in the French summer. The Mediterranean coast averages well over 2,500 hours of sunshine a year, and even the Atlantic coast clears 2,000, so a panel array that would be marginal in northern Europe carries a real daytime load here. As a rough guide, 300 to 400 watts of solar will keep up with the fridge and instruments of a typical cruising boat through a sunny day, and 600 watts and up starts to refill the battery bank as well.
The battery bank is the other half. A lead-acid bank you can safely draw to only about half its rated capacity, whereas lithium gives you most of what is on the label and accepts a fast charge. That combination, generous lithium capacity plus solar, means that on the days the genset does run, it runs hard for a short burst to bulk-charge and then shuts down, rather than trickling for hours at the inefficient part load where it burns the most fuel per unit and makes the most noise.
Done well, a four-week summer cruise on the French coast can pass with the generator started only a handful of times, on the grey days, and otherwise silent.
The simple version
If you are plugged in at a French pontoon, shore power is not a marginal improvement over the generator. It is roughly thirty times cleaner, silent, and usually cheaper once you count diesel and servicing. At anchor the calculation flips, because there is no socket, so the goal becomes running the genset as little as possible and letting solar and a good battery bank do the rest.
There is a quieter dimension to all this that the carbon figures miss. A genset running through the evening at anchor is the single most antisocial thing a boat can do in a shared bay, and in a marine protected area it can stress the very wildlife the zone exists to shelter. If you cruise inside a national park or reserve, the expectation to keep engine and generator noise to the minimum is part of the deal, which I cover in my notes on marine protected areas and your responsibilities.
The generators droning across that August bay are mostly running because it is the habit, not because the boats need them at that moment. Breaking the habit is good seamanship, good manners, and on the French grid, one of the easiest environmental wins a cruiser can claim. It fits naturally with the wider approach I take to a low-impact cruise, which I set out in my notes on leaving no trace at a French anchorage.
