Do I Need An Inverter?
Inverters (Part 1 Of 2)
Power & Water
For Everyone
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An inverter is a crucial piece of kit in any serious van build. It's how you get mains power from a 12v or 24v battery. It means you can run standard appliances light lights and fans, a big fridge, even power tools. But there are many things to know. Lean in, and I'll whisper a few of them to you.
Self-Suction
No, it's something else.
Actually, it's called something else, too, but that's hardly important.
While an inverter appears to magically provide huge amounts of power from a puny power source (230v from a 12v supply? Witchcraft!), what really goes on is simply a conversion from one ratio to another. In any case, an inverter requires power, just like everything else. So, when you consider how much power you need from your inverter, you also need to factor in a few extra percent to power the device itself.
Self-suction, as I like to call it, is the fact that the device providing the power consumes some of it.
The power used by your inverter is sometimes referred to as "self-draw" or "additional power requirements". You can usually find out how much it needs from its technical documentation. More on this in a minute.
Output Signal
Stop looking for a cheap inverter. They're garbage. Here's why:
Electricity, like sound and light, has a frequency. The simplest form of frequency is the sine wave. And when your inverter puts out a sine wave, the electricity is "clean", meaning it powers things consistently. That's the kind of electricity you get from your sockets at home. Great. Let's call that "pure sine". Because that's what it's called.
However, when the frequency is blocky and disrupted, which is the hallmark of cheap inverters, it can cause damage to some kinds of equipment that requires a clean signal. It can also be impossible to power such devices with this "dirty" electricity. Power tools like saws and drills may refuse to function, or function in an erratic way, which is dangerous to you and damaging to the tools.
Why would anyone make such an inverter? Only to cater to cheapskates, by using inferior components. Don't be a cheapskate.
The signal coming from a dirty inverter is called "modified sine", and it's a bad idea, unless you only use very simple devices that don't mind an uneven power supply, like phone chargers and LED lighting (which is terrible for your eyes and may even cause permanent damage).
Calculating Requirements
Although the inverter is only a part of the system, it's really important to know how much power you need to use at peak times (charging everything at once, while using the fridge, lights, fans, and that special "back massager".
For more info about this, see my exquisite guide to installing a solar system: 
Calculating Power Requirements
Startup Draw / Current
When you power up a device, it can draw a little more power than it does during operation. Usually, that's not a problem, but if the device happens to be a 1000W table saw, you might find that power spike to be as high as 300% of operational draw. What?! That's right, bitches. Your 1000W saw might need 3000W, just to start up.
Now, that incredible spike may only last for a hundredth of a second, or even less, but if your inverter can't provide it, that mofo ain't gonna start. And not only that, you might burn out the saw, the inverter, and even the battery, by trying it. Ok, that's a stretch, because you've already:
investigated fuse types → Short Guide To Fuses
and learned how to fit your breakers → Wiring In Your Breakers
so your system is pretty secure from spike-related death, but startup draw demands serious consideration, and here's another reason why:
Overhead
As usual, there's more to it than calculating your power needs and matching them. The truth is, you need to exceed them by at least 15%, in order to keep your inverter from melting. If you have determined that you need to be able to pull 1000W at any one time, you can't expect a 1000W inverter to do the job. Why not? It's like buying a Ferrari F40 and expecting it to run at precisely 211mph at all times. It might be capable of it, but how long do you think it can keep it up (you could say the same for the people who drive them)?
Your 1000W draw requires a bigger inverter. Perhaps 1200W is better for you. Better still, 1500W. That would give you a nice bit of head room. But don't forget about that startup current...
So let's get this straight. If you want to run a 1000W saw from your inverter, you might need to provide a momentary pulse of 3000W, and your inverter might need to be even bigger, in order to satisfy that demand*. Let's go with 4000W. Getting pricey, isn't it? And don't forget, that needs to be a pure sine inverter, too. You could be looking at €300 or more.
* Inverters usually have a momentary power output that far exceeds their "RMS" output (the continuous output level), for exactly this reason. A decent 1500W inverter may well be able to tolerate a 3000W burst for a fraction of a second, in order to power up that saw. It's worth checking.
Final Consideration
Do you actually need an inverter?
If you only want to power laptops and small device chargers, you may be better off with an appropriate DC-DC adapter, which will save you the cost of an inverter and power your devices more efficiently. This will mean using less power and wasting less time on charge. Better yet, many devices run directly from USB3 outputs, now. And with 12v fridges and other appliances designed for use in a vehicle, your need for an inverter may be an incorrect assumption.
At the end of the day, it all comes down to figuring out what you really need to power, whether or not it requires A/C current, calculating the total of those devices, adding 15-25% (at your discretion), and researching the best solution.
On the next page, I'll tell you about the inverter, itself, and how to install it.