When the sun goes down don’t be stuck in the dark. Forethought about your boat’s daily electrical consumption helps make life afloat infinitely more enjoyable.

A satisfactory 12-volt electrical system on a cruising sailboat should provide all the power you need almost as unobtrusively as the wall outlets in your home. This happens by design, never by accident. If you determine in advance how much power you are likely to need, you can take aboard batteries big enough to supply that power and charging equipment adequate to maintain the batteries.

Always remember that boat electrical systems are demand-side economics. You cannot effectively determine battery or charging requirements until you have in hand an accurate estimate of your daily power consumption. Fortunately load calculations are dead simple, but you must do them and you must write them down. If you don’t you are building your house on sand and you have almost no chance of ending up with a balanced electrical system.

Let’s walk through the process. You are going to need a pen, a sheet of paper—a page in the logbook is the best place for this—and maybe a small calculator. Start by inventorying your boat and listing individually every single 12-volt-powered item aboard. This means cabin lights, running lights, fans, pumps, radios, electronics, refrigerator, inverters, starter motor, cooking gas solenoid—everything with wires connected to it. Add to this list things you plan to take aboard—additional electronics, computer, television, etc.

To the right of your list leave room for three columns. In the first column, write the number of amps each device draws. There are various sources for getting this number. Some electrical devices are labeled with an amp rating. More often they have a watt rating. To convert watts to amps, divide by the supply voltage—in this case 12. A 25-watt light draws 2.1 amps—25 ÷ 12. Note that even if you have 120-volt appliances, if you will be running them away from the dock via an inverter the supply voltage will be 12, so you still divide the wattage by 12, not 120.

If the device isn’t labeled, check the printed materials that came with it (you do have all the manuals on board, don’t you?). Wattage or current draw are usually specified.

If you have a battery monitor installed, selecting the amp setting, then turning on appliances one at a time can be the easiest and maybe the most accurate method of determining current draw. If none of these methods works for you for some appliances, here is a generic list of typical current draw. These should get you close, but there are substantial differences among different devices of the same type, so this list is not intended to substitute for determining the actual draw of the devices you own.

Typical Current Draw:
Device	Amps
Anchor light	0.8
Anchor windlass	150.0
Autopilot	0.7
Bilge blower	6.5
Cabin fan (efficient)	0.2
Cabin fan (oscillating)	1.2
Cabin light (8w fluorescent)	0.7
Cabin light (25w incandescent)	2.1
CD player	0.2
Chart light (10w)	0.8
Compass light	0.1
Computer (laptop)	2.5
Spreader/deck light (20 w)	1.7
Depth sounder	0.2
DVD player	0.3
Gas detector	0.3
Gas solenoid	0.6
GPS	0.5
Ham radio (receive)	2.5
Ham radio (transmit)	30.0
Inverter—standby	0.2
Microwave (700 w)	115.0
Pump—bilge	15.0
Pump—freshwater	3.0
Pump—shower sump	2.0
Pump—washdown	6.0
Radar	4.0
Reading light (10w)	0.8
Refrigerator	5.5
Running lights	2.5
Running lights—Tricolor	0.8
Spotlight	10.0
Television (13-inch)	3.5
Toilet	40.0
Speed log	0.1
SSB (receive)	2.5
SSB (transmit)	30.0
Starter	150.0
Strobe	1.0
VCR	2.0
VHF (receive)	0.5
VHF (transmit)	5.0
Waste treatment	45.0
Watermaker (6 gal)	20.0
Weatherfax	1.0
Wind indicator	0.1

A properly sized electrical system is more than just turning on a light and having it work. It allows the little things in life that can mean quite a lot underway. A happy crewmember receives a weather fax for fair weather.

The amp rating is how much current the device requires to operate measured in amps, but what you need is how much power it consumes, measured in amp-hours—abbreviated Ah. For example, if you have pressurized fresh water, the pump likely draws between 3 and 6 amps. However, if limit your daily water use to, say, 6 gallons per day and your pump delivers a gallon a minute, total pump time will be 6 minutes—1/10 of an hour. So the daily consumption for a 3-amp pump will be just 0.3 Ah.

Let’s do it again for an electric refrigerator. Typical draw is 5.5 amps. In a small, well-insulated box, the refrigerator should not run more than half the time—12 hours in every 24. So we might expect the refrigerator to consume 66 amp-hours daily. Install the same refrigerator in a production boat ice box without improving the insulation and in the tropics the refrigerator will run 75% of the time or more. Eighteen hours of run time translates into 99 Ah of daily consumption. Refrigeration is typically the biggest power hog aboard and it pays to invest in an efficient installation, but be brutally honest in this exercise or you will be a very unhappy cruiser later on.

To get total daily consumption, estimate—liberally—the daily use of every appliance on your list and multiply that use time in hours times the current draw of the appliance in amps. That gives you the daily Ah consumption for each appliance, which you simply add together to arrive at your estimated total daily consumption. Here is an abbreviated example of what your 3-column list should look like.

Appliance	Draw in Amps	Hours Per Day	Daily Ah Consumption
Cabin Light	2.1	4	8.4
Fan	0.2	16	3.2
Freshwater pump	3.0	0.1	0.3
Gas detector	0.2	24	4.8
VHF (receive)	0.5	16	8.0
Refrigerator	5.5	12	66.0
		Total	90.7

Alternative energy sources such as wind generators are an efficient way to keep the batteries topped off in trade wind regions. In more fickle areas, solar panels work better.

This is where I am going to push the pause button. If you are serious about trying to outfit your boat with a hassle-free electrical system, make your comprehensive list of devices, determine the draw of each, estimate daily use, and calculate the daily consumption. The grand total is likely to shock you, but you should believe me when I say the numbers do not lie.

Come back here next month and we will walk through the supply side of the electrical system equation, relating consumption to both battery bank size and charging capacity.