If you're going to spend more than a night away from shore power, you need an energy management plan. Battery capacity and the energy usage of various devices seems complicated at first, but it's really not. The foundation for an energy management plan is the same whether you will be away from power for a day, a week, or months.
Assumptions: You have separate starting and house batteries You have a shorepower connector and charger Your house batteries are in good condition
First, you should understand the math involved in electricity. It's basic, and pretty simple. There's no magic here. Let's talk about electricity in terms of fuel, because the batteries work just like a fuel storage tank. Voltage is like fuel pressure. The more voltage, the more the electricity wants to push through a restriction. Ohms are a measure of restriction, with higher numbers meaning more restriction. The fewer ohms (restriction), or the higher the voltage (pressure), the more you will flow. Pretty simple so far? The measure of flow is amps. One volt of pressure against one ohm of resistance equals one amp of flow. Two volts into one ohm equals two amps. One volt into two ohms equals half an amp of flow. Watts is a measure of total work done. While amps and volts have a variable relationship, the watts stay the same for any given amount of work. Watts are also horsepower, where 1 HP = 745 watts. To calculate watts, you multiply volts times amps. Or reverse the calculation as needed; if you know you need to run a 1/2 HP motor, that's about 372 watts. To run it at 12 volts, you'll need 31 amps (372 divided by 12). Adds up fast, doesn't it? A battery is like a fuel tank with a builtin pump. The pressure (voltage) is known, with most boats using a 12 volt battery bank. Some boats have two or more house batteries. If two batteries are wired in series (positive to negative, with boat power coming from the other positive and negative terminals), then the voltage is doubled. If the batteries are in parallel (positive to positive and negative to negative), then the voltage stays the same while the capacity is doubles (like having dual fuel tanks). A "battery" is literally defined as a group of cells. All flooded leadacid cells produce 2 volts. So a 12v battery has six cells wired in parallel. It's a misnomer to use the word "battery" to describe a household AA, AAA, C, or D cell. Those are single cells that produce 1.5 volts if they are alkalines, or 1.2 volts if they are NiMH/NiCad.
The final calculation is the total capacity, similar to gallons of fuel. This is called "amp hours" or "ampacity." A typical deep cycle battery used in a recreational cruising boat has between 90 and 125 aH, depending on size. Size is stated as a group number, with Group 27, 29, and 31 being most common for boat use. The group number is NOT a reference to ampacity, but is usually correlated because size = capacity, just like in a fuel tank. Amp hours are simple; with a 90 aH battery you can draw 1 amp for 90 hours, or 90 amps for 1 hour. Now, there are other issues that can come into play here at the extremes of this calculation, because batteries don't like to be drained really fast, so you probably wouldn't get a full hour at 90 amps. However you could pull a steady 10 amps for 9 hours from a 90 aH battery. Unless you go to extremes, this calculation is good enough. So now let's talk actual practical usage. I'll use my own boat as an example. It has a single Group 24 starting battery which I won't mention again; it is used ONLY for the engine, and not connected to the house bank in any way. It also has a pair of Group 29 batteries that are rated at 110 aH each, so I have a total of 220 aH capacity. If you want to make your batteries last as long as possible, you should only use about 50% of their capacity, so now we're back to 110 aH of "ideal" capacity. However I regularly run mine down to 10% without immediate issues, and batteries are so cheap ($65 each for these), so I simply don't worry about ideal. If I spend $130 for two batteries every couple years, that's nothing in boat bucks.
Now let's talk about putting that power back into the batteries. All batteries have what is called a "bulk charge" phase, and then a topoff phase. During bulk charge, which is until about 70% full, they accept current at an extremely high rate. For example our 50a charger puts an actual measured 47a into the batteries during this phase. However, the acceptance rate drops off sharply after this phase and the bank will only accept around 6a for the rest of the charge. Why is this critical? Do the amphour math again. If we run the generator we can get to the 70% range very fast, but getting to 100% would take overnight. So for days away from power, you now have to assume you will be starting with a battery that's already 2030% depleted every day, PLUS all your actual usage. This is much worse if you have a smaller charger, which most boats come with. Our OEM charger was only 15a, so it would take all day to put just a little power back into the bank. If you do more than an overnight off shore power, you really must replace your charger with something more powerful. In that case, you should also consider an integrated inverter/charger. Which brings us to the electronics side of this discussion. First off, I cannot state strongly enough how useful our Xantrex Link 1000 battery monitoring panel is. This device is inexpensive ($150) and provides an EXACT measurement of the boat's power usage. It counts amps in and out as well as giving the current power usage and the usage over the last five minutes. With this device I have been able to see exactly how much power each accessory and light uses, and help minimize power usage. It also tells you exactly where your batteries are, just like a fuel gauge tells you about your tank (but it's much more accurate than most fuel gauges). That leads us to inverters, a somewhat controversial topic among boaters. I will try to stick to facts. Fact is, I use one, and wouldn't have a boat without it because it provides convenience. Yes, if misused you can kill a battery bank pretty fast. If you misuse a hammer you can bust up your thumb, too, so the key is to learn how to use it wisely. Remember those calculations from the basic electrical discussion. If a device uses 500 watts at 120 volts, then it will still use 500 watts at 12 volts, but will use 10 times as many amps to get there. 500 divided by 12 is nearly 42 amps, which is a lot. On top of this, the inverter has some efficiency losses which can be in the 715% range depending on the quality of the inverter (don't skimp here!). Further, many things like microwaves have a highly visible output rating, but the input is much higher, because of their own efficiency losses. Our old 600w microwave was actually using over 1000 watts of input power. Most VHF radios have about a 20% efficiency loss on their rated output versus input power. Nothing is 100% efficient. Again, the Link 1000 gives actual usage, but you can make a very close calculation if you assume a 10% loss in the inverter. For appliance losses, simply look at their input ratings on the back/bottom and use that, not the rated output power. Now that you know all the principles, the math is easy. Watts, volts, amps, and time.
