Whether you sail a small and simple boat with few systems, or a larger one with complex systems and significant power drains, optimising the electrical system can be a challenge with expensive consequences if you get it wrong. So how can you save power and ensure you use what you have in the most effective way?

Wind power generator on a yacht

Using wind power can be a useful way to supplement your battery power. For many solar power is a cheaper, more effective and quieter choice.

The problem is that even the best batteries don't like to be discharged below 50 per cent of their rated capacity – any more than this and the lifespan of the batteries declines. This is compounded by the fact that it becomes progressively more difficult to pack the last 20 per cent of charge into batteries, so in many cases the effective working capacity is only around 30 per cent of the battery’s rated capacity.

A simple solution to an apparent lack of battery capacity would be to add more batteries. While there are definitely boats that will benefit from this approach, it's not automatically the best first move and it adds weight and complexity, as well as a recurring additional cost to replace the extra batteries when they inevitably reach the end of their life.


How much consumption?

It's worth starting out by figuring out your total daily consumption of electrical power. The most accurate way of doing this is with a sophisticated battery monitor that measures all the current that leaves the battery bank. In the absence of such a monitor you can work out an approximate figure for your boat’s daily power consumption by adding up the power rating of each device in Amps and multiply that figure by the number of hours it’s used for.

Checking battery power with a meter

Using a digital voltmeter to check battery state.


Ideally total maximum daily consumption will not exceed 30 per cent of the size of the service battery bank. If it does, before adding an additional battery, it's worth investigating ways to reduce consumption. For instance, changing to LED interior lighting and navigation lights has the potential to save 10 amp hours per day on a small boat, and two or three times that on a larger one. Similarly switching MFD chart plotter displays to stand by when they're not being used can save 1-2 amps per display, which can easily add up to a further 10 amp-hours saved per day.

Many fridges are also a source of unnecessary power drain - a well insulated fridge will use less than an 24Ah per day, even in Mediterranean summer weather, but many use double that. A minimum of four inches of well fitting insulation all-round will save a considerable amount of power. It also helps if items are already cold when they go in the fridge - at the start of the trip if you carry items in the car an electric cool box in the boot will help keep them cool. But don't be tempted to use the portalble cool box on-board – they tend to be very power hungry, using around 4Ah.

Adding this savings up, on many boats it’s possible to save up to 50 amp hours per day, even on a vessel with relatively modest power requirements. This equates to a saving of around 160Ah in terms of the size of the battery bank required. As well as potentially avoiding the need to buy an additional battery, this approach also reduces the charging inputs that will be required.


How much charge is left?

If properly calibrated, the battery monitor will give a precise indication of how much charge there is left in the batteries, and when they will next need charging. It does this by measuring the total charge delivered to the batteries as well as the charged used by the boat’s systems. However in the absence of a montior, the open circuit (ie no load) voltage of the battery bank, measured with a cheap digital voltmeter, will give an approximate indication:

12.9V full charge.
12.5V 75 per cent charge.
12.2V 50 per cent charge.
12.0V 20 per cent charge.
11.8V battery flat.


Charging inputs

For most boats the engine is the prime means of charging. Ideally the alternator should be rated at around 30 to 50 per cent of the total size of the battery bank, in order for there to be a chance of replacing the daily consumption with around one hour of engine running.

However, you can't expect to get all the theoretical output from a standard alternator - the charging rate typically drops off very rapidly after the first 10 to 15 minutes of engine running. That's where three or four stage smart regulators come in – they will maintain the initial rate of charge until the batteries reach approximately 80 per cent of their fully charged capacity. The regulator will then continue to charge them faster than a standard alternator, although this is still significantly slower than the bulk charge phase between 50-80 per cent of battery capacity. This is the reason for sizing battery banks at around three times larger than your daily electrical consumption.


Shorepower chargers

For boats kept in marinas, and those that regularly stay in a marina overnight while cruising, a 240V battery charger makes a lot of sense. A three or four stage type will ensure that the batteries are 100 per cent charged at the start of every trip and, during overnight stops will power the fridge, lighting, laptops and more, significantly reducing daily electrical consumption.

When sizing a shorepower charger, remember that it will be relatively quick to charge the batteries to 80 per cent, but then progressively slower. This means a small, and therefore inexpensive, model may be ample to replenish the bulk of the charge (to 80 per cent of battery capacity) overnight, but a much larger and more expensive model may be needed to reach 100 per cent charge.


Self discharge

12V batteries lose around 1-3 per cent of their charge per month, which in the past could be a considerable problem for infrequently used boats that were kept on moorings without access to mains electricity. For example, a boat that returned to her moorings with 60 per cent charge would see this figure fall to less than 50 per cent after around 12 weeks.

However, the considerable reduction in price of solar panels over the past few years means this no longer needs to be a worry. The rule of thumb is that the panel rated at less than 8 to 10 per cent of the battery’s capacity can be connected without a regulator – panels of this size can be bought for £10-20 and will maintain the battery’s charge, even in winter.

A slightly larger panel, connected via a regulator, will also replenish the batteries while the boat is not being used, such that each time you arrive at the boat they are already fully charged. This is a particularly good, yet simple, solution for small outboard powered boats that don't have a diesel engine’s powerful alternator to charge the batteries.


Solar and wind power

Boats that cruise further afield, whether it's long-term voyaging or simply a fortnight's holiday on which you need to be more self sufficient, may well benefit from additional solar or wind power. This in conjunction with ensuring you save power where you can, means you are able to cruise long distances in comfort. In the Mediterranean summer, it is now feasible to provide the boat's entire electrical needs – including the fridge – via a couple of large solar panels.

Even in northern Europe, the long summer days mean that large solar panels can provide a very significant power input. This is also helped by the fact that solar panels are more efficient at lower temperatures. In the Caribbean the situation is arguably slightly different, especially as the peak season is in winter, where daylight hours are significantly shorter. In many cruising areas wind generators can produce a disappointing output - most harbours are, by definition, sheltered from the wind. However the Caribbean’s reliable tradewinds make this a viable source of electrical power.

See How can I save battery power with my refrigeration system? for more helpful tips.


Written by: Rupert Holmes
Rupert Holmes has more than 70,000 miles of offshore cruising and racing experience, in waters ranging from the North Sea to the Southern Ocean and Cape Horn. He writes about all aspects of boat ownership and marine travel, including destinations, seamanship and maintenance, as well as undertaking regular new boat and gear tests. He currently sails around 5,000 miles per year and in the past couple of seasons has cruised from the UK to the Azores, as well as winning his class in the 2014 two-handed Round Britain and Ireland Race. He also owns two yachts, one based in the Mediterranean and the other in the UK.