Basics: Understanding Amps, Volts, Watts, and Watt hours.

Have you ever wondered how electricity works in your home or car?
Confused by the long list of terms needed to communicate with others online?

Understanding the basic electrical terms can help demystify the electrical system we all know and love.  Let's break it down into simple terms so you can feel more confident in your understanding.

What are Volts?

Volts is a measure of the electrical pressure or force that drives the current through a circuit. It's like the "push" that moves electricity from one point to another. Higher voltage means more pressure pushing the electricity.

Voltage is the potential for electrical energy to flow, and can exist even when seemingly nothing is happening in a circuit.

A common analogy is to compare voltage to the water pressure in a pipe.  The higher the pressure, the more you can potentially "push" through the pipe.  Now think about a tap on that water pipe.  Turning the tap off means there's nothing coming out, but you know there's pressure behind the tap, just waiting to work it's magic and push that water out the moment you open the tap again.

The most common voltages used in battery systems are 12, 24, and 48 volts.

What are Amps?

Amps, short for amperes, is a measurement of the flow of electrical current in a circuit. Think of it as the amount of electricity that is flowing.  Amps are a direct result of the amount of voltage (pressure) and the amount of resistance in the circuit.

Circling back to our water pipe analogy, amps would be similar to how fast the water is coming out of the pipe.  How fast it can fill up a bucket.

It is the combination of voltage, and amperage, together that lets us get work done.

Amperage only exists when the tap is on.  No water flowing means no amps.  If you turn off a light switch, no current is flowing to the light anymore.

What are Watts?

Watts are a measure of power in an electrical circuit. It's the multiplication of voltage and amperage at a specific moment in time that gives us this value.

Wattage represents the rate at which electrical power is consumed (or produced). In simple terms, watts tell you how much work can be done with the electricity.  

An example is an old 60 Watt light bulb.   It will consume approximately 60 watts of power when supplied with it's rated voltage.   In our homes, this lamp is connected to 120 Volts, and draws about 0.5 Amps.   

To calculate the wattage we simply multiply the volts and amps.

120V x 0.5A = 60W

To reverse that, and calculate the amps (perhaps all we know is the wattage and voltage) we can move the numbers around.  

60W ÷ 120V = 0.5A

 

What are Watt hours?

Watt hours (Wh) are a unit of energy, representing the amount of power used over time. It's a way to measure the total amount of electricity consumed or produced in an hour.

For example, if a device uses 100 watts for 1 hour, it consumes 100Wh of energy.  After 10 hours, it has consumed 1000wh. After 36 hours, 3600Wh.   

As the numbers you are looking at start to grow, Kilowatt hours (kWh) becomes an easier unit of measurement.  But it is just as simple.  It is simply Wh ÷ 1000.  So in our example above, after 10 hours, our 100W load has consumed 1kWh (1000Wh), or at 36 hours it has consumer 3.6kWh (3600Wh).

This is where we really start to get into useful numbers that can help us understand things like battery capacity, solar panels, inverters, and how to estimate what equipment we need for our off grid adventures.

What are Amp hours?

Amp hours (Ah) are a similar unit to Wh, or kWh.   It is a measurement of energy drawn over a specific period of time. 

However there is one important distinction to be made.  Amp hours only considers amperage in the calculation.   Because Wh is the product of voltage AND amperage, it can be readily applied when calculating between different voltages in a system.   This becomes an obvious advantage when you start looking at your 12 volt battery feeding your 120 volt inverter in your RV.   

Let's look at a 600 Watt electric heater, running in your RV.  It uses 120V electricity, output from your inverter.  We know we can calculate the amps by dividing power and voltage. 

600 watts ÷ 120 volts = 5 amps 

So it'll use 5 amps per hour right? 5Ah for 1h, 10Ah for 2 hours, etc. But that's at 120 volt. How do we know what our battery needs to supply? We could do some math to get a ratio of the different voltages between our inverter output and input.  Oooooor we could simply work in Wh instead. 

600W x 1h = 600Wh per hour

At this point we could start to look at battery specs and find one that has enough Wh for the length of time we want the heater to run. 

Or we can work in Ah instead.  So we decide our battery system is 12 volts.  We divide the 600Wh by 12.

600Wh ÷ 12 volts = 50Ah.

So why would we want to use Ah instead of Wh?  Well deep cycle batteries have traditionally been marked with their capacity in Ah.  Newer generation lithium batteries are often marked with both values, but Amp hours is still the most common we see in conversation.  It's changing though, so you'll be fine to use the simpler Wh (and kWh) units.

 

More Calculation Examples:

Let's say you have a heater that operates at 120 volts and draws 5 amps of current. To calculate the power consumption in watts, you would multiply the voltage by the amperage:

Watts = Volts x Amps
Watts = 120V x 5A = 600W

Now, if you run this heater for 5 hours, you can calculate the total energy consumed in watt hours:

Watt hours = Watts x Hours
Watt hours = 600W x 5h = 3000Wh (or 3kWh)

To convert that number to Amp hours, you simply need to divide by the voltage you want to apply it against.   In the example above, that source is 120V.

3000 Wh ÷ 120 Volts = 25 Amp hours

But what about if we're talking about how much battery we need to run that heater on an inverter?  We must remember that we originally calculated the heater's power consumption in Watt hours (or kWh).   This makes the battery sizing estimate fairly easy.  If we're using a standard 12V LiFePO4 battery, we actually use 12.8 Volts as our voltage value (just trust me on this one for now).

3000 Wh ÷ 12.8 Volts = ~235 Amp hours

In real life we need to consider some efficiency angles, like the inverter, cabling, etc.  But for this example, it's close enough.

Using the Math In Real Life:

When we start looking at things like how much battery we need, it becomes clear we need to understand how much power we consume in a day. Or even a weekend.

Understanding these basic concepts of electrical theory can help you make informed decisions about your electrical systems and devices. Whether you're setting up a new circuit or troubleshooting an issue, knowing the fundamentals of DC voltage systems can empower you to handle electrical tasks with confidence.