Ron Bentham's Funky Mercedes Hymer S700 Motorhome blog: Inverters in the motorhome ... for beginners!

An inverter is a box of tricks that takes 12V DC from your battery and converts it to 220V AC as you find at home. You can then plug in a 220v item - like you do at home. An inverter is specified as having an input voltage and an output voltage. A typical inverter takes 12V DC in, and outputs 220V AC, and the power of the inverter is specified in watts.

So in very simple terms, you connect your inverter to the 12V supply of your van, plug your device (ie laptop charger) into the socket on the inverter and away you go.

Unfortunately it is not quite as simple as that and you want to avoid many of the problems that can arise if you don't understand the basic principles of power. Many inverters are supplied with wiring to connect to your 12V supply. Usually this will be a twin core wire, coloured red and black. One end is connected to the inverter, and the other end will either be bare, or with spade connectors, or with a cigar plug. In a motorhome, you should never plug an inverter into a cigar socket, unless the only thing you are going to plug in, is a phone or tablet charger. NOT a laptop or TV. If you want to use an inverter for these higher power items, then you must connect the inverter directly to the battery with proper connections.

A good inverter will be supplied with a suitable cable, and a fuse may be in the cable, or installed in the inverter itself. You should attach these cables directly to the battery terminals, using spade or ring connectors - if you don't know how to do this properly, then get help. If the supplied cable isn't long enough, then it is OK to extend the cable using a wire of equal or thicker dimensions. If the ideal cable length for your particular installation is longer than about 2m, then you should use an even thicker cable, because longer cables at 12V DC can suffer from voltage drop. This is the reason that most inverter cables are quite short, and also the reason that most van inverters are located quite close to the battery.

Every van is different, but in a simple installation, you should install the inverter in a convenient place, close enough to the battery, and where you can also get at it to plug in your 220v stuff. If you can't find a convenient place, then you can install the inverter out of the way, and plug an extension lead into it. Remember that you need to be able to get at the inverter to switch it off when not in use - most inverters consume power when doing nothing. If you are wondering if you can have an inverter feeding all the same sockets that are in your van already, that are live when you are hooked up, then yes you can, but that needs a more complex installation using an autoswitching inverter, which is too complicated for this simple guide - contact me if you want to discuss this. You should also be aware that most inverters have fans - which can be quite noisy, so bear this in mind when you locate it.

So now you have a basic simple working inverter setup - there are a few more points about inverters that i would like to touch on.

One of the hardest things to understand about inverters is that you can't just plug anything into them. In a house, you never think about it, whether it's a phone charger, a laptop, a TV, a hairdryer, a microwave or a fan heater - you just plug them in and off they go no problem. This is because in a house, you are being fed by the national grid. which for all intents and purposes is unlimited and lasts forever. There is a maximum though - most plugs are rated at 13amps, which in watts is equivalent to 3000w, or 3kw. This is a lot of power - enough for a 3 bar electric fire on full.

But in a motorhome, the power is not unlimited, it is supplied by your 12v leisure battery, and compared with the national grid, this battery is tiny. So unless you understand the sums, it is very easy to innocently overload a leisure battery, by asking it to deliver too many watts - far more than it was designed for.

This is the point at which most people switch off because they don't like sums and calculations, especially where electricity is concerned. So I have found that it easier to break it down into simple rules. In a basic motorhome installation, with 1 or 2 normal leisure batteries, and normal wiring, then you should never try to power anything through an inverter that generates heat - this means heaters, kettles, toasters, coffee machines, microwaves, hairdryers, cookers, ovens, slow cookers - this list is not exhaustive, but you get the drift.

What you can use is as follows - phone chargers, tablet chargers, anything USB, radios, CD players, small TVs - up to about 23" and laptops. Basically anything low power. Special cases - that are possible but need careful consideration are - e-bike chargers and curling tongs - these are medium power items that are on the borderline, so you need to understand how to use them in more detail.

Now here is the technical reason, for those interested. As the volts go down, the amps go up, but the watts remain constant. Lots of amps need thicker copper wire. To calculate the amps you need at 12v, you simply divide the watts (of the appliance(s)) by 12.

So - at 12v - which is the voltage of your van battery, to supply 100w of power, you need 8 amps at 12v - you divide the watts by the voltage. The wiring of your van, and the capacity of your battery to deliver amps is basically around 20 amps - any more than that and things start to get hot and stressed. As a rule of thumb, a normal motorhome battery and wiring system is capable of delivering about 10 amps of 12v - in classic Hymer vans, the fuses are 16a. The battery is capable of a bit more, but you shouldn't ask for it!

At the other end of the scale if you look at an average coffee machine, the label will say maybe 1200w - that is an easy calculation - 1200 divided by 12v = 100amps. So if you bought and connected a 1500w inverter to your battery, and switched on the machine, it would ask for 100amps. Normal leisure batteries are simply not capable of delivering that amount of power, and the result would be a ruined battery. But in your house - it's no problem - why? because the voltage is 220v, not 12v - so 1200 divided by 220 = 5.5a, which is under half of the maximum of 13a available from a house plug, so no problem, and it only needs a thin wire So that is why the simple rule states that you should only try and power low power items from a basic inverter installation in a van with a normal battery setup. A normal van setup isn't capable of delivering much more than around 15amps at 12v. Now of course you can have whatever you like - if you want a coffee machine and a toaster, powered by your battery, then you can - but it will cost - you have to install big batteries, thick cabling and proper metering and monitoring - this can easily run into thousands of pounds.

But if all you want is to charge your devices and maybe a TV and laptop, then you can, with a simple inverter setup that can easily cost around £100 all in.

But remember - there is no free power in a van - whatever charge you take out of your battery, whether it's your lights, fans, TVs, gadgets, through your inverter or not - that charge has to be replaced somehow - either by the engine while driving, solar, or on hookup. So although this article is specifically about inverters, remember that all aspects of a mobile 12v system have to be taken as a functioning whole.

So what inverter should you buy?

Pure sine or modified sine or quasi sine? These are terms that you will see in inverter ads. My simple advice, is don't bother about it - it's a technical discussion and there is lots of rubbish talked. Basically you can use any inverter that is sold either in shops or online. Most inverters nowadays are pure sine, and that's all you need to know.

The next discussion point is what inverter to buy. Ebay and Amazon are chock full of a bewildering array of inverters of all shapes and sizes and at prices from £25 upwards. Again, lots of folk have opinions and love to make statements on Facebook on what is best. A favourite opinion is that cheap "chinese" inverters are "crap", and you get what you pay for. My answer to this is that while there is always a possibility that any electronic product can break, it is bad business to sell a faulty or unreliable product - Ebay and Amazon function on the principle of feedback, and nobody survives very long if half the stuff they sell comes back for refund. The reality is that chinese stuff isn't all that bad, and all you have to do is look at a range of prices, check that the supplier has decent feedback, and choose the middle way - not the cheapest, not the most expensive.

However it is worth bearing in mind that there are a couple of manufacturers who are famous for the quality of their products and the most famous of these is Victron. Victron are a Dutch company who have a vast array of quality products, and are generally regarded as the best available. There is another company called Sterling, who are UK based and have a similar reputation, although as with most things, opinions always differ. Victron make a wide range of inverters, and their small inverters are priced similarly to the bigger models of other manufacturers. When asked, my standard answer is usually, I would rather have a small Victron than a big chinese one. There is also another brand called Epever, who are famous for their solar controllers, and they also make good inverters at decent money.

A common mistake that many people make when choosing an inverter is over specifying. You will see ads that offer 1000 watt inverters, or more, and it is easy to think that more is better. However this isn't the case. You should size an inverter for the purpose you need it for, and big is most definitely not better.

However you also need to understand what the numbers actually mean. The words to look for are "peak" "power" and "constant". The Chinese are very fond of big numbers - so they will put numbers like "1500w" and "1000w" and even more, but when you read the small print, it will say "peak power". This means that the inverter is only capable of supplying that power for a few seconds. This isn't as dishonest as it sounds, because many appliances, especially those with electric motors, do indeed need a lot of power for just a few seconds when you switch them on, so it's not all rubbish - it's just that they tend to overemphasise the "peak" and under emphasise the "constant". Constant is what you want - around 500w constant is what you want for a normal low power installation as detailed above. Quality manufacturers like Victron, on the other hand, always quote "constant". So a 300w inverter from Victron will have a peak value of over three times that - they just don't shout about it!

Then there is the matter of efficiency. Most inverters nowadays are over 90% efficient so effcieincy losses are not as important as they used to be. Big Chinese inverters are less efficient than small quality ones, and even though we have already discussed that efficiency isn't everything, there is another aspect to efficiency that is quite important in a van, especially if you are a "normal" owner with a single leisure battery. This is about "no load" or "standby" power. This is the amount of battery power an inverter uses while switched on, but doing nothing. A big 1000w inverter can use 1 amp of your battery every hour it is switched on. This may not seem like a lot, but that 24 amps if left on all day and night. So a good rule to obey when using an inverter is - switch it off when not in use. I have seen many a flat battery because of this - people think that if nothing is plugged into an inverter, then it is using no power. This isn't true.

So generally speaking, if the main purpose of owning an inverter is to power your 130W laptop, then a 200w or 300w inverter is more than enough, and smaller high quality inverters use less power on standby than bigger cheaper ones. Victron inverters take this one step further by having "auto sensing" - they take hardly any power at all until they sense the need. This is a useful feature to have if you forget to switch it off as it won't drain your battery. Other manufacturers also offer auto sensing.

At the time of writing in 2020 I have started to notice a new type of chinese inverter on the market that not only supplies around 400w reasonably efficiently, it also has built in USB sockets to charge phones and tablets, and it also has a digital display so you can keep an eye on your battery voltage - and all for £40. It is hard to argue with these for the money.

If you need more information or have questions, then ask on the Classic Hymers Technical facebook group. https://www.facebook.com/groups/297054424534823/

A typical Chinese 400w inverter showing plug sockets, 4 usb sockets and voltage displays. All for under £40

How an inverter should be wired to a battery - there will of course be other wires on your battery, but this is the basic idea - no cigar plugs or crocodile clips please!