Most modern boats today operate on basic 12 volt electrical systems with direct current (DC) power. Throughout the boat, a 120 volt outlets may run off an inverter to convert DC to AC with same voltage of American homes.
Understanding Basics of 12 volt Systems
Watts -Watts are often misunderstood. The term watts measure how much power a device uses, or can supply, when turned on. If something uses 120 watts, that is simply the voltage x amps. If it draws 10 amps at 12 volts, or 1 amp at 120 volts, it's still 120 watts. "Watts per hour" is NOT the same as watts.
Watt-hours - A watt-hour (WH) or a kilowatt-hour (kWh) is a way to describe how many watts are used by something during a period of time. This is what most people mean when they say “watts per day”. If a light uses 100 watts, and it is on for 9 hours, that is 900 watt-hours. If a microwave uses 1500 watts, and runs for 10 minutes, that is 1/6th of an hour x 1500, or 250 WH. When you buy power from your friendly utility (look at your last bill), they sell it to you at so much per kWh. A kWh is a “kilowatt-hour”, or 1000 watts for one hour (or 1 watt for 1000 hours).
If you want to conceptualize how watt-hours plays a role in your boat you might consider this; add up all the amps used by anything on your boat using electricity, multiply that by the volts of your system (12v and/or 120v in USA) (amps x volts = watts) and multiply that total number by the total time everything was turned on in a day and you get the watt-hours of electricity used. An electrical meter at your slip measures this to tell the electric company (or marina) how much to bill you each day. Multiply that times the days in a month and you get your monthly electric bill.
Amps - An amp is a measure of electrical current at the moment.
[An amp is defined as 1 Coulomb per second. A Coulomb is the charge of 6.24 x 1018 electrons. Therefore, 1 Amp is equal to the charge of 6.24 x 1018 electrons passing a point in a circuit in 1 second. Great information for your next cocktail party conversation.]
Amps do not come in "amps per hour" or "amps per day" like a watt does. An amp is a measure of energy, so imagine amps as a storage container for a specific amount of energy. Amps are important because they determine wire size and battery size on a boat. Plumbing and electrical work very similarly (although only one can kill you). Think of it is as the amount of water you want to push through a pipe, and "amps" are your water. Pushing a lot of water in a small pipe creates very high pressure. Electricity works the same way. Try to push a lot of amps through a small wire and you get very high temps (heat). Push a little water through a large pipe and you get very little pressure at the end. Same with amps. Push a little current through a large wire and you get very little energy at the other end, so correct wire size for the amps needed to power something is important.
Amp-hours (AH) - Amp-hours are what most people work with when determining energy for their house battery banks, energy used by appliances and other electronics, and overall energy usage. Along with Reserve Capacity (RC), it is one of the most common terms used to discuss energy available from batteries. Specifically, this is the number of amps x time
A Common Misunderstanding Associated with Amp Hours
Most batteries will use 20hrs as the time over which the battery will deliver the AH listed.
A 100 AH hour batter should be able to deliver a total of 100 amps in 20 hours, so it can theoretically deliver a steady 5 amps per hour (5 amps x 20 hours = 100 AH) or any multiple that arrives at 100 AH from the battery.
A common mistake is made when it is assumed that the 100 AH battery will also provide 100 amps for 1 Hour, or 50AH for 2hrs, etc. It won’t. In fact, a battery of this type may only provide about 40 minutes of continuous 100 amp service or 80 minuets of continuous 50 amp service at best.
This is due to a well known characteristic associated with lead acid batteries. Specifically the capacity will decrease as the rate of discharge increases. The faster you try to use the available energy in a battery, the faster the energy will dissappear.
In other words the relationship between battery capacity (how much energy is available) and the rate of discharge is not a linear one.
The phenomenon being described here is known as Peukert’s law. The the "load" on your batteries is a big factor in how much actual amps you need available to power your house bank for periods with no charging energy being returned to the batteries.
If you have a very large energy demand over short periods of time, your house bank needs to be significantly bigger than just adding up amps x time and using that number as your battery needs
Batteries are the "Heart" of the boat electrical systems. We put together a page that discusses the basics of deep cycle batteries and give you a good foundation for determining battery type and usage.
12 volt Wiring Basics
If batteries are the "heart" of the system, then wires are the circulation system that keeps our boats running. Here is a page that gives you the basics of 12 volt marine wiring: