Many items you may want to use are impossible to power directly from your batteries. Inverters to the rescue! Inverters are cool devices that turn battery power into 110V AC household wall plugs (or 220V if you’re into that kind of thing).
Inverters are common for vandwellers but they should not be the default for every build. If you can get away with powering everything straight from a DC battery, then skip the inefficiency and save some money. For those trying to cook with and induction burner or run a high powered laptop, you might not have a choice.
There are only two things you will be deciding on when looking at inverters: Size and Type
What Type of Inverter to Get
There are two main types of inverters- MSW and PSW. Alternating current travels in waves that “alternate” from (+) to (-). This is different from how direct current power travels through circuits. These two inverter types describe how DC power is converted into AC power. A picture makes this easier:
- Modified Sine Wave (MSW): More accurately called a multi-step wave inverter. These inverters are less expensive as they aren’t as complex. They create a “choppy” approximation of a sine wave. For many electrical components, it makes little difference how the wave looks, such as light bulbs or things with electric motors like compressors and power tools. Other electrical devices such as electric blankets, coffee makers and induction burners won’t work very well because their microprocessors need a clean wave to regulate temperature properly. MSW inverters also create a “buzzing” noise when operating certain devices.
- Pure Sine Wave (PSW): These have a higher up-front cost due to the electrical complexity. There are no limitations as to what AC devices you can plug into them, other than trying to use more power than the inverter can deliver. They are also slightly more efficient, and we recommend this option if you have the means.
What Size of Inverter to Get
Here’s where we see people go astray. There are limits on sizing an inverter. It’s no surprise that you don’t want it to be too small because then you won’t get enough power. But there are also issues with going too big.
Inverters come sized by how many Watts they can output. Most are listed in continuous watts and peak (or surge) watts.
- Continuous Watts: This is the listed inverter size, and the size you base your measurements on. It is the amount of watts that the inverter can output in normal use.
- Peak (or surge) Watts: This is the max output that the inverter can handle for a short period of time. It won’t affect too many calculations in a van build. The purpose is that many devices have a “start” load that is significantly higher than their running load. For instance, a fridge compressor may draw 100W when it kicks on, but settle down to a steady 50W after a couple of seconds.
The first task is to figure out the maximum amount of watts that you’ll be using at one time. This usually means that you’ll add up all the devices that will be plugged in at once. Sometimes you will have devices that won’t ever be used together (AC and electric blanket) so pick the larger of the two. Then pick an inverter that is rated at a comfortable size above that load (120% or so)
Here is an example:
In this case, if all of the AC powered items are plugged in at one time, you’ll be drawing 270 watts. A 300W inverter will do the job, but a 400W inverter will be ideal. Which brings up the natural follow up question,
Why not go even bigger?
- Efficiency. Smaller inverters are more efficient at smaller loads and at idle. Notice the big aluminum fins on inverters? Those are to bleed off excess heat; lost heat is lost power. So if you are powering a 90W laptop with a 1000W inverter, it will be using slightly more power than doing the same with a 400W inverter.
- Cost. While this isn’t a technical problem, when you get into some better quality PSW inverters, cost can add up quickly with no real benefit.
Now that you know your ideal inverter size, you need to check that your batteries are capable of powering it.
This is a big issue, and one that many vandwellers don’t account for. It is harmful on the battery to discharge it too quickly. Batteries can’t let go of all of their energy at once. In most cases, it will shorten the lifespan of the battery but sometimes you completely ruin your battery. The sizing for this is directly related to the size and type of your batteries, which we explain in more detail in the charging section of our batteries article.
For right now, you can be that your inverter isn’t over the max size with this rule of thumb:
Maximum inverter size =
1.5W for every Ah of an FLA battery
3W for every Ah of an AGM battery
6W for every Ah of a Lithium battery
If you have a basic 150Ah FLA deep cycle battery bank, your safe inverter size is 225 Watts (150 x 1.5).
This can open up a can of worms. You might quickly notice that most vandwellers use a massively oversized inverter for their setup. If you end up calculating that you need 1200W but your batteries should only discharge 400W, you have a sizing mismatch. This doesn’t really have anything to do with your inverter, but it means that you have to make adjustments somewhere in your system. It can be as simple as making sure you never have your laptop and electric blanket powered at one time, or as involved as re-sizing everything.
Tips for installing your inverter
- AC wiring is much more efficient than low voltage DC. So place your inverter close to your batteries and run AC power for the long stretches to minimize the amount of DC wiring.
- Try to design your system so it’s easy to turn your inverter off. Inverters draw a bit of power just by being on, so it’s good to turn the unit off when you’re not using it. Some come with remote capabilities.
- Make sure to properly ground your inverter to the vehicle chassis (along with your battery bank). There is a separate terminal on your inverter- separate from the two power terminals- that needs to attach to a ground point to be able to clear any fuses if something breaks inside the inverter.
|Campervan Inverter Recommendations||Size||FLA Min Size||AGM Min Size||Lithium Min Size|
|Multi Step Inverter||300W||200Ah||100Ah||25Ah|
|Pure Sine Inverter||400W||275Ah||135Ah||35Ah|
|Pure Sine Inverter||1000W||675Ah||335Ah||85Ah|