Volts, Watts, Amps, and Ohms: 12-Volt Terms for New Boaters

If you’re new to boats, or even if you’ve been around them for a while but have been avoiding the electrical bits, sooner or later you’re going to need to become more familiar with DC electricity than you are if all you know how to do is jump-start your car. To help get you going, here are some very basic definitions you’ll need to understand.

DC means Direct Current. It’s the kind of electrical current produced by batteries. Batteries found in cars, trucks, RVs, and boats are almost always 12-volt DC.  Flashlight batteries are DC, too. In fact you can create 12-volt potential by linking together just eight 1.5-volt flashlight batteries – AA, C, or D (1.5 x 8 = 12).

Fuses are rated for a certain maximum amperage flow, above which they will 'blow,' interrupting the circuit. Amp ratings are shown on the fuses above. Circuit breakers work the same way but 'trip' when they're overloaded.
Fuses are rated for a certain maximum amperage flow, above which they will ‘blow,’ interrupting the circuit. Amp ratings are shown on the fuses above. Circuit breakers work the same way but ‘trip’ when they’re overloaded.

Basic 12-Volt Definitions

  • Voltage/Volts = the amount of potential energy available to push electrical current. Since electricity is invisible, it’s convenient to picture voltage as the potential pressure in a water system. For a battery-powered system, think of a water tower with a big tank on top. It’s drained by gravity, and the way water flows out of it depends partly on the volume and weight of the water in the tank at any given time (the “voltage” equivalent), and partly on the characteristics of the drain pipe. In an electrical system, wires and other conductors are the equivalent of water pipes.
  • Amperage/Amperes/Amps = the flow of electrical current through conductors like wires. Think of it as the amount of water flowing past a single point in a pipe at a given time.
  • Wattage/Watts = the amount of energy expended, or used. Think of it as the water needed to fill a glass (a few watts) or a swimming pool (lots of watts).
  • That’s as far as the plumbing analogy goes, though, because unlike water systems that run from source to drain, electrical systems run in “circuits,” in other words in a circle, from power source to usage (“load”) and back to the power source, with switches and fuses or circuit breakers in between to interrupt the flow of electricity as necessary.
  • Ampere-hours (Ah) = the current in amperes multiplied by the amount of time it flows. Batteries have ampere-hour capacity ratings that give a general idea of how many amps can be drawn from the battery for how long. In a perfect world, a battery rated for 90 amp-hours would be able to give you 90 amps for one hour, 45 amps for two hours, one amp for 90 hours, and so on. In reality, you can and would use only a portion of those amp-hours before the battery should be charged again.
  • Ohm = a measure of resistance in a wire or other conductor. Resistance is determined both by the wire’s length and its thickness, or gauge. The thicker the wire, the more easily current will flow through it. Resistance always creates heat, and the greater the resistance, the more heat. Try to put too much current through too small a wire, and you can create enough resistance to start melting things and causing fires. This can happen even in a simple 12-volt system, so always use common sense and generous wire gauges.

In most cases it’s unrealistic for a boatowner to measure for ohms, simply because boat gear manufacturers don’t usually offer a baseline of resistance to measure against.  Instead we measure for an abnormal drop in voltage in a circuit, which would indicate corrosion, too small a wire size, or a poor connection.

Now, here are some of the easiest equations you’ll ever have to use:

Volts x Amps = Watts (example: 12 volts x 5 amps = 60 watts)

Watts / Volts = Amps (example: 60 watts / 12 volts = 5 amps)

Amps x Time = Ah (example: 3 amps x 5 hours = 15 Ah)

If you read the owner’s manual for any piece of electrical gear, or the stamped information on the gear itself, you can usually discover what it needs for energy input and how much energy it uses. Most marine gear, whether a chartplotter, a bilge pump, a windlass, or an electric windshield wiper, will tell you its current draw in amperes. Then it’s a matter of arithmetic to find out if your 12-volt system can handle the task, and for how long.  (Note: there’s also marine gear made for 24-volt systems, but those are usually on larger or specialized boats; the great majority of small pleasure-boats use 12-volt systems, and in any case the DC principles are the same.)

12-Volt Safety Issues

Working with 12-volt systems is easy, and relatively safe, compared to 120-volt AC. But 12-volt systems are far from benign, for several reasons.

First, a 12-volt battery can deliver a whopping big discharge of current all at once. That’s how a 12-volt battery can run a starter motor to get a car or a boat running. That discharge of current can be violent, as you’ve noticed if you’ve managed to bridge the the terminals with a wrench or a wire. If you have a metal watch strap or a ring on, and that metal becomes part of the short circuit, you can get hurt. If you’ve used jumper cables, you’ve probably seen a good-sized spark as a clamp comes in contact with a terminal. So be alert to the danger of short circuits and sparking.

Working with marine 12-volt batteries is relatively safe, but there are precautions and habits to pay attention to.
Working with marine 12-volt batteries is relatively safe, but there are precautions and habits to pay attention to.

Second, when you produce a spark in the wrong environment, you can cause an explosion. Batteries produce hydrogen and oxygen when they’re charging  — no problem if the battery is well-ventilated, but a potentially explosive mixture if the battery is in an enclosed, poorly ventilated space or container.

Third, inadequate wiring, unfused wires, poor connections, and corrosion can cause electrical fires, which in turn can catch other flammable material like cloth or paper on fire. If you get involved in a 12-volt installation on your boat, follow the instructions, use big enough wire with short enough wiring runs; use fuses where necessary, and never run wiring under or through any flammable material.

Fourth, the liquid electrolyte in flooded-cell batteries is mostly sulfuric acid, which will eat your clothes and burn your skin, and can blind you if you get it in your eyes. When inspecting and testing the flooded cells in your battery it’s a good idea to wear glasses or other eye protection, and rubber gloves. When you pry off the cell covers with a flat-bladed screwdriver, do it gently and carefully, and make sure the battery is on a stable surface. If you get electrolyte on you, rinse thoroughly with fresh water.

These warnings aren’t meant to scare you off — far from it. Working with 12-volt systems is relatively safe, but you must read the directions, use common sense, and be alert. That’s a pretty good combination of habits for any project.

Good References

Electrical Resistance on Boats: Keep That Voltage Drop in Check

The 12 Volt Bible for Boats by Miner Brotherton and Ed Sherman

Powerboater’s Guide to Electrical Systems, Second Edition, by Ed Sherman

Boatowner’s Illustrated Electrical Handbook by Charlie Wing

This article originally appeared on Boat Trader in January 2016.


Written by: Doug Logan

Doug Logan has been a senior editor of YachtWorld.com since 2010. He's a former editor-in-chief of Practical Sailor, managing editor and technical editor of Sailing World, webmaster for Sailing World and Cruising World, contributing editor to Powerboat Reports, and the editor of dozens of books about boats, boat gear, and the sea.