Electrical self sufficiency on the hook starts with being realistic about how much energy you’ll be consuming and preparing the boat for it.

Last month in Part 1 we walked through the process of accurately estimating daily electrical consumption for a planned cruise. This single number becomes the cornerstone of your electrical system. If you are both thorough and honest in arriving at this number, hassle-free electrical power is simply a matter of taking aboard an adequate battery bank and a satisfactory means of keeping it charged. Fail to accurately predict your daily consumption and not only is your electrical system likely to become a source of constant annoyance, but this unrelenting irritation can jeopardize your entire cruise.

For convenience we are going to work with a daily consumption calculated at 100 amp-hours (Ah). This is a not unrealistic estimate for a cruiser of moderate size outfitted with a well-insulated refrigerator and a normal compliment of 12-volt appliances, but I choose this number mainly to make the calculations easier to follow. The first question we need to answer is how big our house battery bank needs to be to supply 100 Ah of power daily.

Allow me a quick aside here. Once upon a time, the typical battery configuration was dual banks of more or less equal capacity rotated on a daily basis. This is not the best configuration. You should have dual banks alright, but one bank should be the engine starting battery. I like this to be connected through an automatic combiner or echo charger so that it is never discharged for house use and always charged when the charging system is operational. The second battery bank is the house bank, and the most efficient configuration is for that to be a single bank since charge acceptance is directly related to the size of the bank. Maximum charge rate is typically around 25% of capacity, so if you recharge two 200 Ah battery banks on alternate days, your regulator is going to start at around 50 amps and decline from there, whereas a discharged 400 Ah bank will accept an initial charge rate of 100 amps if you have an alternator or other source capable of that level of output. This is one of those rare instances when it is better to put all your eggs in one basket.

Back to the question of what size the house bank should be, the answer depends on your daily consumption number. If you have adequate solar and wind-powered charging on board, these can replenish power as you use it, making the size of the battery bank seemingly moot. But this overlooks overcast, windless days. For such eventualities, unless you are willing to run your engine more than once a day, you need a battery bank capable of sustaining your usual consumption for at least 24 hours.

So for my chosen example, we need a battery bank capable of providing at least 100 Ah of power between charges. Because deep-cycle batteries are rated in Ah, this is where the neophyte gets into trouble. A 100 Ah battery is most emphatically NOT capable of supply 100 Ah of power, at least not on a recurring basis. What we are interested in is useable capacity and as a rule we can estimate useable capacity at approximately 40% of rated capacity. Why? Because the life of lead-acid batteries is severely shortened by discharging them more than 50%, so we only have half of the rated capacity available. Additionally, batteries get increasingly reluctant to accept charging current the closer to full charge they get, so replacing that last 10% is impractical with engine-driven charging unless the boat is underway. This means that house batteries typically operate between 50% charged and 90% charged. Momentarily we will consider alternative power sources, which have the advantage of charging for more hours. This has the effect of increasing useable capacity to 50% when alternative energy is available, but bank sizing should be based on 40% of rated capacity. A simpler way of stating the same relationship is that that battery capacity in amp-hours should be at least 2 ½ times daily consumption.

So back to our example, 100 Ah of daily demand will necessitate a battery bank with at least a 250 Ah rated capacity. This is the minimum bank capacity. A larger bank, if you have both the budget and the space for it, will allow longer times between charges, providing added charging flexibility. Greater capacity also allows for a higher rate of charge, which in turn reduces charging times and engine hours.

This takes us to charging capacity. The big gun is typically the engine-driven alternator, and appropriate alternator size is based on the size of the battery bank, not the daily discharge rate. This is because, as I already mentioned, lead-acid batteries cannot be charged at a rate exceeding about 25% of capacity. For a 250 Ah house bank, alternator capacity beyond around 60 amps is a waste. Gel cell and AGM batteries will accept a somewhat higher rate of charge without damage, but rapidly rising voltage very quickly reduces the amount of current even these types of batteries will accept. As a practical matter, there is little to be gained by fitting an alternator rated at much more than 25% of your house bank capacity. An alternator that is too large for your battery configuration doesn’t really hurt anything other than your bank account, but it will not recharge your batteries any quicker. Here is the harsh reality: discharge your batteries to 50% and you can expect that no alternator/regulator configuration will take the charge level back up to the 90% level in less than 3 hours.

This interconnectedness between charge level and bank size makes it possible to also work backwards from alternator size. If one of the options on your new engine is, say, a 110-amp alternator, this will provide maximum-rate charging for a bank up to 440 Ah. It will fully charge an even larger bank, but not in the minimum possible time. So if you already have this alternator aboard, it makes sense to size your house bank to around 440 Ah rather than the 1-day capacity of 250 amps.

As a by-product of propulsion, charging the batteries with an engine driven alternator is essentially free power. However, running the main engine at anchor just to spin the alternator is a costly and inefficient method of keeping the batteries charged. A cruising boat should have alternative energy sources--either solar panels or a wind generator or both.

While there will be cloudy and windless days in which alternative energy sources like wind generators and solar panels will sit dormant, over the long run these systems are indispensable on any cruising boat.

Sizing alternative energy sources follows directly from daily consumption. The recharging process consumes some energy, so you should plan to replace about 20% more power than you consume. A daily consumption of 100 Ah could be fully offset with around 120 Ah of solar power, but how much panel capacity would it take to put 120 Ah into the battery bank? Panels are rated for the sun directly overhead, but since the sun actual scribes an arc over the boat, you should anticipate no more than the equivalent of 5 hours of rated output daily. Working backwards, we can divide our target output of 120 Ah by 5 hours to get the amp output required to keep up with the daily discharge—in this example, 24 amps. But panels are typically rated in watts. You convert amps to watts by multiplying by the rated panel voltage, typically around 16 volts. Doing the math yields 384 watts. In sunny conditions, a panel configuration of four 100-watt panels should handle 100 Ah of daily consumption.

If your alternative charging source will be a wind generator, your equilibrium target remains 120 Ah of daily output. Wind generators are typically rated in amps according to wind speed, and it is hard to resist basing expected output on an optimistic wind speed estimate. After all, in a trade wind area where the winds average 25 knots, what could possibly be wrong with using 15 knots as the average wind speed in your calculations? Everything. Consider that most anchorages are shielded from the prevailing winds—the reason they have become anchorages—so at-anchor wind speeds will be much lower than those outside the anchorage. Anchorages are also near land, which is likely to have its own wind patterns that can and often do cancel the trade winds at night. In my experience, the 10-knot rating for a wind generator will come the closest to reality. For example, if a generator is rated at 1.5 amps at 10 knots, expect not more than 36 Ah from this generator daily.

Working backwards from 120 Ah, if we divide by 24 hours, we find we need a wind generator capable of a 5-amp output in 10 knots of wind to keep up with the load. The output of a typical small blade (3 feet) wind generator is about 20% of this target; output of a large-blade (5 feet) generator output will be around 80%. You will need to supplement both, either with multiple wind generators, solar panels, or engine hours.

It is impossible to convey the sense of satisfaction that comes from sitting quietly at anchor with all the electrical power you require available all the time, without consuming fuel and with no bill to pay. Balance your electrical system before you leave your home dock and it will enhance your cruise far beyond the up-front dollars you spend.