Vessel Electrical Requirements

The power requirements of a boat dictate the size of certain equipment such as inverters, breakers and wiring, and the energy requirements mostly determine the size of the battery bank, solar panels, and other charging sources. Remember, power is how many watts (or amps at 12V) are being consumed at this moment, and energy is how much power you consumed over an hour or day (amp-hours or watt-hrs). In order to determine the most cost effective sizing of the equipment (e.g. solar panels or battery bank) you must understand not only what the power and energy requirements are, but also how often they will be used. If you only spend a few days a year at anchor, it may not make sense to size your solar system to completely cover your power needs during that short interval.

Power Requirements - The electrical loads (lights, refrigerator, engine starter, etc.) all draw power at some time. Some draw very little, like an LED lamp, and some draw a tremendous amount, like the anchor windlass. Since most boats are equipped with an ammeter for the AC and DC panels, or better yet, a battery monitor, either of these is a good means of finding out how much your individual loads draw. At this point you should make a list of all the individual loads and their corresponding power/amp draws. Some loads, may not run through your panel ammeter and will have to be estimated using the datasheets for the equipment. Other loads like the engine starter and anchor windlass probably don't matter as they are so short in nature and are usually in conjunction with the engine alternator providing any excess power they may require. Your judgement will come into play here.

Energy Requirements - This process is going to be less precise and also more difficult to measure. In fact, much of it will likely have to be estimated as your energy use will be a function of how you use your boat. Develop scenarios for the way the boat is normally used, or is expected to be used, over the course of a year. The (self sufficient) daily energy needs of a boat that spends much of its time at anchor, will be dramatically different those of one that spends most of its time in a marina connected to shore power. Our Electrical Requirements Excel spreadsheet found here should greatly facilitate this analysis. Download Now

Storage Requirements (Batteries) - This topic is also covered in System Design in greater depth, but there are a few things worth mentioning in this section as they pertain to load and charging. While batteries are an energy storage device, they are also a load. Yes, you read that correctly. All batteries self-discharge with no load connected them and that energy must be replenished from somewhere. That is why the batteries are included in the above spreadsheet as a DC load, albeit a small one, with a rough means to estimate it. The other key elements to understand are that 1) All of the energy you put into the batteries does not get stored as some is lost to heat, and 2) The rate at which you discharge a battery will affect how much useful energy that can be extracted from it. These concepts are not independent conceptually as a high rate of charge or discharge generates an exponentially greater amount of internal heat and hence, greater losses. The amp-hr storage capacity of a battery is always rated for a specific discharge rate. If you exceed that rate, the capacity of the battery will be less than rated and vice-versa. Charging efficiencies are affected by rate in basically the same manner.

Summary - Estimating your electrical needs precisely is tough. The reality is that you won't get it exactly right, but you will probably be pretty close. The initial data in the downloadable spreadsheet above should be fairly typical for a 38-40 ft cruising sailboat. This exercise will do a lot more for you than figuring out how many solar panels you might need. It will get you thinking about how much power you consume while aboard and likely get you devising ways to cut that number down. A digital battery monitor, if you don't already have one, is almost an indispensable tool in understanding your electrical use and the condition of your battery bank. It will allow you to actually monitor your amp-hrs in and out of your batteries so you can see exactly what your energy consumption is over the course of a day or longer. With a temporary negative jumper, you can run a test on any piece of equipment that may be variable in nature, like the refrigerator or autopilot to see how much they draw over time as well.