Understanding Amps, Watts, Volts and Ohms!

We’re all increasingly dependent on electricity. How many of us know the difference between a 60 watt and a 100 watt bulb, or how the voltage from the wall socket supplies the power to charge our mobile phones?
Let’s get a basic understanding of the terminology and how they relate to each. This will help you to apply them to electric motors and batteries to power your boating adventures!

The Basics

The most basic units or terms in electricity are voltage (V), current (I – that is an uppercase “i”), watts (W) and resistance (R).
Voltage is measured in volts, current in amps, watts in watts and resistance in ohms.

What is a Volt?

Volts are named after an Italian physicist and chemist called Alessandro Volta who built one of the frst batteries.
Volatge is a measurement of the “electrical potential” (or pressure) at which electricity flows thorugh a system or circuit. Voltage can also be described as the speed of the individual electrons as they move through a circuit and is measured in units called volts.
In the United Kingdom, power from the electrical grid is delivered to homes at 240 volts. This runs our lighting and usual appliances – we’ll ignore 415 volt three phase supply as this is mainly for industrial premises.

What is an Amp?

Amperage is another way to measure the amount of electricity running through a circuit. Amperage is the “rate” at which that current is flowing through the circuit or the number of electrons moving through the wire. They are is measured in units called amps or amperes and are named afer the French physicist and mathematician, André-Marie Ampère.
You will probably have come across 3, 5 or 13 amp fuses in plugs for your various devices. The larger the amp rating of the fuse, the more current can flow through the plug to the device. If you try to run a really powerful microwave using a 3 amp fuse in the plug, you’ll find it will blow the fuse.
You might have come across the fuse board in your house showing different circuits of 20, 30 or 50 amps. If you try running 20 microwaves in the kitchen at one time, the breaker for the kitchen circuit will trip to keep the circuit safe. This is becuase the microwaves are trying to use too much power for that specific circuit. Too much power draw will cause a build up of heat which could possibly cause a fire. The breaker tripping stops the microwaves working avoiding the potential fire hazard.

What is a Watt?

Wattage is named after the Scottish engineer and inventor James Watt. He popularised the steam engine and developed the concept of horsepower. He is probably the most well known of the different terms we’re looking at here.
We’ve all seen 40, 60 and 100 watt bulbs over the years (although now LED bulbs are much lower wattage than old incandescent bulbs of course). We know that a 60 watt bulb is brighter than a 40 watt bulb! But why is this and what does it mean?
Essentially wattage is the amount of power a device consumes or uses in performing its function. It can also be described as “electricity at work” or the power it takes to actually do something. This might be running a vacuum cleaner (perhaps 350 watts to 900 watts) or illuminating a light bulb (7 to 100 watts depending on bulb type).
Watts are calculated by multiplying voltage (the pressure or speed) by amperage (volume) and can be shown in this formula:
watts = volts x amps
W = V * I
So the faster each electron moves through the circuit and the greater the volume that the circuit can hold, then the higher the wattage.

What is an Ohm?

So now you understand the terms that describe
  • the amount of current flowing through a circuit and
  • how much wattage is needed to power different electrical devices connected to that circuit.
However, circuits are made up of various wires and they are not perfect conductors. Most home wiring will be made up of copper or aluminium and both of these materials have a certain amount of natural resistance (or friction). This slows down the flow of electricity. Electrical devices and appliances also apply their own resistance as electricity flows through them.
This resistance, or friction, is measured in ohms which are named after a German physicist and mathematician Georg Simon Ohm.

The Plumbing Analogy

These terms can appear quite abstract so a useful way to remember and understand it all is the plumbing analogy.
The voltage is equivalent to the water pressure.
The current (amperage) is equivalent to the flow rate.
The resistance is equivalent to the pipe size.

Ohms Law

Another basic equation that can be used relates amps, volts and resistance:
I = V/R
and this is known as Ohms law – see Georg Simon Ohm above!
This states that current is equal to the voltage divided by the resistance.

So what is the plumbing Analogy?

We have a tank of pressurised water connected to a hose o water your plants in the garden. If you increase the pressure in the tank, more water comes out of the hose. This is the same as increasing the voltage to make more current flow in a circuit.
Let’s now think about increasing the size of the hose and fittings to the tank – again this will make more water come out of the hose. This is the same as decreasing the resistance in an electrical circuit, which also increases the current flow.
Electrical power is meaured in watts and as we saw above is calculated as volts x amps.
The plumbing analogy applies again. Take your hose and point it at a waterwheel. You can increase the power generated at the waterwheel in two ways:
  1.  If you increase the pressure of the water coming out of the hose, it will hit the waterwheel with a lot more force and the wheel will turn more quickly. This will clearly generate more power
  2. If you increase the flow rate with a larger hose then the waterwheel turns more quickly because of the extra weight of water hitting it

Electrical Efficiency

Blue Lightning

So, in an electrical system, increasing either the current or the voltage will result in higher power.
Assume you have a 12 volt bulb attached to a 12 volt battery and the power output of this light bulb is 100 watts.
If we use the formula above
watts = amps x volts
but rearrange it to
amps = watts / volts
we can calculate how many amps will be required to get 100 watts out of the bulb.
In this case the amps = 100 / 12 which is 8.33 amps.
If we have a different set-up using a 6 volt battery and 6 volt bulb BUT we still want to get 100 watts, we see 100 / 6 = 16.67 amps.
So for the same amount of light from a 6V bulb, it needs twice as much current as the 12 volt system.

More from less!

There is an advantage in getting the same amount of power from less current.
The resistance in electrical wires consumes power and the power consumed increases as the current going through the wire increases.
If we now think about the reistance equation I = V / R above and rearrange it to
V = I * R
we can now substitute this equation for V into the first equation W (power) = I * V making it W (power) = I * I * R
What this equation is tells us is that the power consumed by the wires increases if the resistance of the wires increases. This happens, for example, when the wires get smaller or are made of less conductive material. However, it increases dramatically if the current going through the wires increases.
Therefore, using a higher voltage to reduce current can make electrical systems more efficient. This can allow reduced thickness of cabling for example and as copper is very heavy, this can be signficant.

Efficiency gains

This improvement in efficiency led to the car industry considering switching from 12 volt system to 42 volt systems in the 1990’s. More and more cars were built with lots of electrical systems such as video screens, seat heaters, smart climate controls and so on. These all need thick bundles of wiring to supply enough current. Switching to a higher voltage system would have enabled much thinner gauge wiring to be used for the same current (amps). Click here for more details.
This in fact never happened as other efficiencies were boosted such as more efficient pumps and using digital technology. Electric cars now often have electrical systems between 450 and 650 volts to run the massively powerful electric motors. These need huge power to run without using cable as thick as your arm.
Hopefully we can see how this efficiency affects electric outboard motors and batteries. When we use motors that have separate batteries, we must understand the correct voltage, watts and current for your required application. is very important and you must ensure to correctly rate your cabling for
  • the distance between the battery and the motor
  • the power it must be able to transfer without losing too much voltage (see voltage drop blog).

If you need any help with understanding your cabling requirements, please contact us any time and also see here for our voltage drop calculator tool.

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