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Converting Amps, Watts and Volts: Boating Electrical Basics

Ian Fortey by Ian Fortey Updated on November 6, 2022. In nauticalknowhow

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If you’re going to be operating a boat you need to have a basic understanding of how the electrical system works. The problem with that for many newcomers is that not everything electrical is explained in plain English. The terminology can get confusing, especially when you need to understand voltage, wattage and resistance. But you do need to understand how these terms work. Knowing the power demands of your vessel and its accessories is crucial. This will ensure you don’t blow any fuses or drain your battery.

Understanding Your Electrical Systems

When dealing with electrical systems you’re going to run across three terms pretty frequently – amps, volts and watts. Any electrician can explain the difference between these three units but you’d be surprised how few lay people can. Most of us know they have something to do with power, and batteries are measured in volts, but how are they different? If you’re not sure, don’t feel bad. Let’s take a quick look at the differences.

Ampere: Most people refer to these as amps. It’s the base unit used to measure electrical current. The amount of electrons that past a given point per second is measured in amps. All electricity is the flow of electrons.

Volts: A volt is a unit of electric potential and we measure them with voltage. Voltage deals with how much potential, which you can think of as force, is behind each electron. You can consider amps how many electrons there are and volts how much pressure is causing them to move. People often use a plumbing analogy here. Amps describe the amount of water flowing in a hose but volts describe the water pressure forcing the water out. When you compare voltages, a higher voltage indicates the potential to make use of more energy from the same current. A higher voltage battery could potentially operate more appliances or operate than longer than a lower voltage one, but the amps do need to be factored in.

Watts: Watts are actually a combination of the volts and amps. It doesn’t actually measure electricity, but the work that electricity can do. It is the flow rate of power. When one amp flows through an electrical difference of one volt, that is defined as a watt. So the higher the wattage of an electrical device, the more it is able to do. For instance, a 1200 watt microwave can cook your food faster than an 800 watt one.

What About Electrical Resistance?

Electrical resistance is a feature you don’t want in your electrical gear, but you can’t avoid. You want to minimize it as much as possible, however, Resistance limits the potential of your electrical system and overall performance of the power your battery generates. More resistance equals wasted potential.

Ohms: Electrical resistance is measured in ohms. Resistance is much like the name suggests, and it is how much a given substance will be opposed to the flow of electricity through it. A copper wire is a great conductor and therefore has minimal resistance. A piece of rubber will not conduct electricity at all and can be considered to have high resistance. But even in a wire, there is still resistance. Bends or frays in wire create resistance, as does corrosion. The longer a wire travels from the power source, the more resistance it experiences. Likewise, wire size is important. Thinner wires offer more resistance than thicker wires. If resistance becomes too great, power flow can be interrupted. This can lead to things like brown outs.

Nearly every electrical device you use on a boat will have wattage, voltage or amperage listed somewhere on it. But if you don’t know what this means, the terms are kind of pointless.

What is an Electrical Circuit?

Circuits are another term we come across frequently when discussing electrical systems. You’ve heard of short circuits and circuit breakers. So what does all of that refer to?

In the simplest terms an electrical circuit is a closed loop. A single wire that is looped in a circle could be considered a circuit. It would allow for the flow of electrons to continue along that wire endlessly as long as a current can continually be applied.

In more complex systems, a circuit is part of an electrical network. This is where you have a battery and things like resistors, inductors, transistors and switches that allow the electrons to flow in the directions you need them to flow. They go from the battery to a light bulb or a bilge pump or a refrigerator.

The important thing to remember about a circuit is that it’s closed. That loop metaphor still stands. The power runs through the entire circuit, there isn’t a place where it stops or ends. That’s why there’s no safe place to grab a bare electric wire in a live circuit, the current is always flowing.

Everything on your boat that uses power should have a label on it detailing its power requirements. This should detail its demand for amperage and its output in watts. If not on a label than in the electrical equipment manual.

Circuits on a Boat

For the most part, your boat will operate with one of two circuit designs. This keeps things simple and can be easy to figure out once you’re familiar. Basically you’re looking at either a series circuit or a parallel circuit.

Series Circuit: Unsurprisingly, if you connect components in your circuit in a series, it’s called a series circuit. Current through every component in the circuit is the same and voltage supplied is the sum of the voltage across each component. Think of this like Christmas lights. If one light in the circuit shorts out, then all the lights after it go out. That’s because the flow of the series has been stopped.

Parallel Circuit: In this case, instead of being joined sequentially, your components are all wire parallel to each other. The voltage across each component in parallel will be the same and total current applied is the sum of current through each component. The current is divided between the components.

Differences Between Series and Parallel Circuits

Flow: In a series circuit, the same amount of current flows through every component. In parallel circuits, the current flowing through each part combines to form the current through the source.

Resistors: If you place a resistor in a series circuit, the voltage across each will be different but the current will remain the same. However, in a parallel circuit, if you place a resistor, the voltage across each will remain the same.

Interruption: If a component part of a series circuit breaks, the circuit will break. This is that Christmas light example again. If one fails, all fail. However, in a parallel circuit, because they are arranged parallel to each other, the circuit will not be broken. The other components will still function.

Arrangement: In a series everything is arranged in a single line. In a parallel circuit they are arranged parallel to one another.

When Do Circuits Matter on a Boat?

All your electrical components need to be wired in circuit or in parallel. It’s important to know how things are wired, especially if something goes wrong. Plus, if you want to do any of your own wiring you’ll need to decide how to handle it. This is most often something that boaters installing solar panels deal with. Is it better to wire a solar panel or a solar array in a series circuit or parallel?

As with everything else electrical, it depends. If there’s a chance your panels are going to be experiencing frequent shading because of a boom or something else, consider wiring them in parallel. This will help maintain a consistent flow of power, even when one panel is in the shade.

If shading is not an issue, you may want to wire your solar panels in a series. This will not change the current but it can increase the voltage. How does that work?

In a series, total voltage is added up. So if you have three solar panels then the total voltage would be panel 1 plus panel 2 plus panel 3 voltage. However, if you wired them in parallel then total voltage would be equal across the board.

Wiring in series is also a good idea if you have long power cables. The voltage from a parallel circuit may drop too significantly if you have longer wires for it to travel through.

If you do opt to wire solar panels in a series, consider splitting them up. Half on one side of the boat and half on the other. That will maximize current and voltage to ensure you get the most out of them.

What About Ampere-Hours?

Your battery will be able to tell you the amount of ampere-hours it has available. This measures the capacity of a battery to meet the electrical demands being placed on it. Ampere hours measures the current in amperes multiplied by the amount of time it flows. If you have a battery that claims 100 ampere-hours, it doesn’t mean it can operate for 100 hours, that’s not how this works. What it means is that it could supply 100 amps for one hour. So if you have a trolling motor, a handful of lights, and a phone charging you’d need to calculate how many amps they all require to determine the demand they’d put on your battery.

A 100 amp-hour battery gives 100 amps for one hour. But that also means you can expect 50 amps for two hours. Or 25 amps for four hours. If you were only drawing one amp, it’d last for 100 hours.

It’s worth noting that this is on paper. You’d do well to never expect battery rated at 100 amp hours to run at one amp for 100 hours. Discharge rates are often considerably different than how they are listed. It’s a little bit like gas mileage in a car, it just never measures up to what the technical specs advertise because there are so many variables.

Electrical System Terms

There are a few terms that come up a lot when dealing with electrical systems on boats. It’s worth knowing what they mean so you don’t get lost in trying to figure out certain aspects of how circuits and your boat’s electrical systems work.

Current: This is the movement of electricity through a conductor, typically a wire. This is the one measured in amps. Current in most electronics is measured in AC or DC. AC current means alternating current and refers to a flow of electrons which reverse direction at regular intervals. DC electricity is direct current and refers to a steady flow of electrons moving in the same direction from a point of high potential to one of low potential. Boats operate on DC power. It is supplied by the battery and is low voltage overall. Higher power appliances typically need AC power to operate, but there are converters available.

Voltage: The force, sometimes called electromotive force, which causes the electric current to flow in a circuit. Higher voltage doesn’t equal more power, but increases the potential of the already available current.

Resistor: A device that is used to provide resistance to the flow of electrical current. This is necessary sometimes to reduce current flow or divide voltage among other uses.

Switch: A switch is a device that is able to connect or disconnect the flow of electrical circuits. A light switch on your wall, for instance, can either complete the electric circuit when you turn the light on or interrupt the circuit when you turn the light off.

Transistor: This is a semiconductor that works like a relay. A transistor can switch or amplify signals. A smaller current can be amplified to a larger current inside.

Inductor: Sometimes called a coil, an inductor is able to store electrical energy in a magnetic field.

Capacitor: Like an inductor this is also able to store energy. It’s used to filter voltage spikes so that your components don’t suffer damage from sudden voltage increases.

How to Calculate Amps, Watts and Volts

Because you may be doing a little improvisation with your electrical system, you need to know what can handle operating where. That means understanding the equations of how to convert amps, volts and watts. If you can’t do this you risk killing your battery. That’s not something any boater wants to experience. Let’s take a look at those basic equations than can help you get through this.

Watts = Amps x Volts

Example:

15 Amps x 120 Volts = 1800 Watts
5 Amps x 120 Volts = 600 Watts

Amps = Watts divided by Volts

Examples:

1800 Watts / 240 Volts = 7.5 Amps
4400 Watts / 160 Volts = 27.5 Amps

Volts = Watts divided by Amps

Examples:

3600 Watts / 25 Amps = 144 Volts
2400 Watts / 15 Amps = 160 Volts

Be Cautious

Invincible Boats 42’ Center Cabin

Never take chances when dealing with your boat’s electrical system. Make sure all power is off and the battery is disconnected before you attempt any wiring. AC power is far more dangerous and can be fatal. But the DC power on most boats is no joke, either.

If you are doing work on your boat’s electrical, make sure everyone on board knows it. If people may be coming on board and leaving again, put a note near any switches and breakers to let people know not to touch anything.

Take safety precautions seriously. Wear goggles and rubber-soled shoes. Never do electrical work in an area that’s wet.

Make sure you use wires intended for a marine environment. This included insulation and backing materials as well. Marine wire is a very specific kind of wire. You don’t want to skimp on this. Look for tinned copper wire. It will be a silver color. Check with a marine supply store if you aren’t sure. You want multi-strand wire and the insulation should be able to resist oil. If you plan to use the wiring near the engine, it also needs to be rated to handle high heat. That means around 70 celsius. Good quality wiring will detail all of this on the package so you know you have the right stuff. If these details are not included, you may not want to risk buying it.

When you have the power off, it’s a good time to do an inspection. Check all of your wiring for weak spots. Look for cracks and breaks in the plastic sheaths. Check for any corrosion at connection points. Also look for any place where the wires may have melted or discolored from overheating. These wires should all be replaced. They’re a risk for wasting your battery, and potentially causing fires.

The Bottom Line

Everyone wants things to run smoothly on their boat. Unfortunately, figuring out electrical systems can be difficult, especially if you’re not experienced. No one wants to have to call an electrician just to figure out the basics, though. Always make sure you’re paying attention to how much your battery and/or solar panels can handle in total. When you need to, do the math and then double check to make sure you know whether or not you have the power to handle new components and for how long they can run. Always keep your battery charged and consider having backups just in case of emergencies.  A reliable battery charger should be something every boater has on hand. A good multimeter can help you figure out how much power you have to use and allow you to figure out what your components are drawing when you’re not sure.

About Ian

My grandfather first took me fishing when I was too young to actually hold up a rod on my own. As an avid camper, hiker, and nature enthusiast I'm always looking for a new adventure.

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