This post is written to help a beginner understand basic electric terms including Watts, Volts, and Amps. If you already know what these mean, head over to our electricity calculation worksheet, and fill it out to get a good estimate of your ideal electric system size.
Pro Tip: We recommend that if you’re fresh to electrical systems – or even if you’ve wired in a couple of components to your Honda Civic before – that you get the eyes of a professional to look over your work. There are plenty of electrical forums where experts will help you for free. So make use of those resources to save you some dollars and perhaps a good shock or fire.
What we’ll cover:
- What is a Watt?
- Watt hours and Amp Hours
- Alternating current (AC power) and direct current (DC power)
What Is A Watt?
The most important equation that you can know when calculating your electricity consumption is:
Watt = Amps x Volts
The tried-and-true method for newcomers to visualize electricity traveling through wires as water flowing through pipes. In a pipe there are two things that measure how much water is going through.
- The speed of the water: This is the Volts.
- The width of the pipes: This is the Amps.
If you know the width of the pipe and speed of the water, you know the total water passing through.
Watts are measure of instantaneous electricity passing through something, just as gallons per minute is the instantaneous measure of a river flowing.
Watts is the total measure of electricity passing through a system. You can have different combos of Amps and Volts to achieve this. If you have a 200W device plugged into a 110V outlet, then it’ll be drawing 1.8A through. If it’s plugged into a 12V battery, it’ll be drawing 17.7A through to get the same amount of power.
Amp Hours (and Watt Hours)
If you want to know how long a battery will last, you need some way to measure how much electricity it can hold. This is done by measuring the storage or capacity of electricity.
Fortunately, the folks naming all of the electrical jargon made this one pretty easy for us. One Amp hour is the quantity of electricity that it takes to use one amp for one hour.
Amps x Hours = Amp hours
This means that if you know how many amps something uses and for how long you’ll be using it, you know how much it will drain your battery. For instance, if you run a 2.5A light for two hours, you’ve used 5Ah. If you use it for three hours, then it takes 7.5Ah.
Knowing all of this, I bet you can now figure out what a Watt hour is! 1Wh is the amount of electricity storage it takes to use 1 Watt for 1 hour.
Why have two ratings, Wh and Ah?
Deep Cycle batteries are traditionally rated in Amp hours. It’s been done like that for decades and most likely you will need to know the Ah of your campervan system. With large household battery systems becoming more common, Wh are starting to become a more mainstream way to measure capacity. This is because a Wh is the same amount of energy regardless of if we’re using a 5V, 12V, 110V, 220V or other electrical standard. It’s more consistent way to measure because it takes voltage out of the equation.
We recommend you calculate your entire system in Wh and then calculate your Ah at the end of the process.
Watt hours is the easiest measure to base your calculations because there are probably going to be a couple of items that will use different voltage than 12V. Watts are also how solar panels and inverters are rated. Less mix-ups can happen if you stick with Wh until your final calculations for system sizing.
But before we get to the good stuff, there’s one more concept to tackle.
Alternating Current (AC) and Direct Current (DC)
There are two different types of electricity to manage. One is called direct current (DC) power and one is called alternating current (AC) power.
Direct Current (DC) Power
Direct current refers to a type of power that flows in one direction. Most digital electronics such as your cell phone use direct current power. Anything you plug into a cigarette lighter or USB cable is run off DC power. Most importantly, batteries store power using direct current which is the main reason it is used in vehicles.
Alternating Current (AC) Power
Alternating current power is a different type of electricity where the flow is constantly changing directions. This is the type of power running through power lines and what all the outlets in your house or office building are delivering. The main reason the power grid is AC is because it is efficient to transmit it over long distances compared to DC. Due to this, household appliances are all designed to be plugged into AC outlets.
If your van is based on 12V DC power, then how do you plug these appliances in? The first step is to use as many DC powered items as you can. Lights, chargers, refrigerators, and many other gizmos come in both AC and DC. More often than not, DC items are more efficient than their AC counterparts because they are designed for RV’s and boats.
For those items that can’t use DC power, such as induction burners, you use an inverter. You can read more about these in our campervan inverter post. For now, all you need to know is that there is roughly a 10% efficiency loss when converting DC to AC power.
Bonus tip: For laptops, phones, and anything that uses a battery, you can often find a DC → DC charger. This avoids needing an inverter if you can get away with it. Because laptop chargers are stepping up 12V → 20V they usually have some efficiency loss as well, but can be convenient for some campervan builds.