How to generate power in your motorhome and campervan
Words by Clive Mott
Before we get into what sort of inverter and generator you need for your motorhome, it’s helpful to appreciate the difference between electricity generated via a portable device and that produced domestically via a power station. Understanding this difference is key to safety.
The electricity that comes out of a battery is produced chemically and gives us a fixed direct current (DC) voltage of a nominal 12V from a leisure battery. The stuff that comes out of a 13A mains socket is different and, for the most part, is produced mechanically. This is nominally 230V alternating current (AC) and the higher voltage means we can get much more power through smaller cables.
Mains safety in the motorhome
If you put your fingers across the terminals of a 12V battery you are very unlikely to feel anything. If you were stupid enough to put your fingers across a 230V AC mains supply the resulting shock could easily kill you. Consequently, methods are put in place to stop a mains voltage getting connected to people.
This resulted in the development of the residual current device (RCD) circuit breaker, which interrupts both the live and the neutral connections. The RCD will have a maximum current rating and will switch off if this current is exceeded.
In a motorhome this will typically be 16A (domestically, it’s either 60A or 100A).
Any electrical equipment in a motorhome will be subject to much more regular shock and vibration than domestic equipment so earth faults are more likely over time. So, it’s vital to wire them safely.
Portable generators for the motorhome
At temporary campsites set up by the major clubs and at motorhome shows, large portable generators are used. These are typically three-phase diesel-powered machines of many kilowatts, running at a fixed 1,500rpm. In all these applications, earth stakes are used to bond the output neutral to earth at the generator so your motorhome RCD will operate normally.
Budget portable generators used by motorhomers are smaller and simpler in construction. Usually petrol-powered with an engine speed set to 3,000rpm, their output voltage waveform approximates a sine wave and the control of the voltage and frequency can be less than ideal.
For example, with a 1,000W (1kW) generator, if you switch off an 800W load (such as a camping kettle) the percentage change in loading on the generator is large and it will take a short time for the engine to throttle back to no load. During this time the output voltage and frequency of the alternator can momentarily increase significantly before it recovers back to normal. This voltage spike can upset some mains equipment. The kettle will not mind, but your motorhome charger, fridge, etc, might.
More sophisticated generators are ‘inverter generators’ and the key point is that their engine speed does not set the output mains frequency or voltage. The AC mains is generated electronically using an inverter and this responds almost instantaneously to maintain the output voltage and frequency quite accurately.
Their other advantage is that, at low electrical loading, the engine speed can be much slower, which results in less noise. However, at maximum electrical load, the engine speed can be more than 3,000rpm so they can get more power from a relatively small engine. All generators make noise, with Honda generators being probably the least noisy.
You need to consider earthing your generator. All small portable generators provide an isolated electrical output. In other words, neither the live nor neutral connections are bonded to earth. If you are using the generator to power a single bit of kit such as a hedge trimmer then an isolated output is deemed safest. If, however, you are distributing the generator output to several places – just like in a motorhome – then think again.
If you connect your generator output directly to the hook-up of your motorhome then the fridge and battery charger will be powered from the generator, and all your sockets will become live, as will your electrical heating (if the generator has an adequate power rating). What will not work is your RCD as there is no return path via the earth wiring of your motorhome to the generator’s neutral.
If you have a dedicated connection cable just for the generator then the neutral and earth connections of the plug that goes into the generator can be linked together. This will enable your RCD to operate normally. However, this connecting cable must not be used for connection to a domestic mains supply as it would cause the domestic RCD to trip.
Mains inverters for the motorhome
Mains inverters take DC power from a leisure battery and convert it to mains AC power. There are two types.
The modified sine wave or quasi sine wave description refers to inverters that produce an AC output of 230V at 50Hz but not with a sine wave shape. The wave shape of these is square. These are the simplest and cheapest. Equipment that doesn’t include electronics within it will work OK powered by one of these.
The more expensive inverters produce a pure sine wave and should be OK on anything within their power rating. If in doubt opt for a pure sine wave inverter – they’re safer for complex electronic items such as laptops and e-bike chargers. Make sure you buy one from a reputable source – not just the cheapest you find online – as there have been cases of cheap inverters simply sporting pure sine wave stickers.
Both types of inverters take all their energy from your leisure battery so it is important to understand the power demands of such devices.
As the inverter output voltage is about 20 times (12V to 230V) as much as the battery voltage then, for the same power, the inverter input current will be 20 times as much as its output current, plus a bit allowed for inverter inefficiencies. Say a ratio of 22:1.
Wiring in an inverter to your motorhome
Like portable generators, inverters provide an isolated output; however, the last thing you should do is connect the inverter output to your motorhome mains hook-up input. If you did this then the fridge would operate from it (150W perhaps) and the battery charger would operate (400W), for starters. So what’s the best way to install one?
If you have a very small inverter (150W max for a laptop or one bit of medical equipment) then the inverter can be connected to a cigarette-type power socket, providing this socket is wired to your leisure battery.
You will only ever have one bit of equipment plugged into the inverter output so the isolated output is deemed safe.
If you want to charge an e-bike, then a 600W inverter should be used, but this time wired directly to the leisure battery via a 60A maxifuse and using the cables supplied with the inverter.
Any battery paralleling cables must be at least as thick as the inverter input DC cables.
Extend the mains side by all means, but you should never extend the 12V DC cables. You can then use a mains extension cable installed just for the inverter to a fixed 13A socket in a prominent position.
It’s helpful to use one with a neon indicator built in, which will act as a reminder for you to switch off the inverter when it is not in use to prevent draining your leisure battery.
The simplest way to power a microwave oven is to have an inverter-connected socket adjacent to a dedicated mains socket, which should be used for the microwave.
This will need an inverter of at least 1,500W wired with very thick 12V cable, 35mm square for example, as will be any battery paralleling cables. The fuses will need to be rated at 150A or more.
Powering all your motorhome mains sockets
If, on the other hand, you want power to all your existing motorhome 13A socket outlets when you do not have 230V mains hook-up, then a proper install should be carried out by a qualified professional.
With the possibility of running more than one bit of kit at any one time, additional safety measures should be taken.
For a full inverter install you’ll need to fit another RCD to uniquely isolate the inverter output.
For the earth fault amps to have a return path, the neutral connection of the inverter output must be connected to the vehicle chassis (battery negative). This way, the same protection against earth faults is provided as at home.
One method is to use a manual switch to enable your motorhome’s sockets to be connected to either the incoming mains, or the inverter output.
A similar duty can be done automatically using a mains changeover relay.
The relay energises when ‘real’ mains is present and connects the 13A sockets to the incoming mains supply. Without mains the relay’s closed contacts connect the sockets to the inverter output. This is called a priority switching unit.
Some higher-power inverters are made with an input for AC mains as well as the DC supply from the battery.
This feature is often called in-built priority switching.
The inverter output socket provides real mains AC when the inverter is turned off and the motorhome is connected to a hook-up and inverter AC when the inverter is turned on and real mains is not present.
Inverters of this type cannot have the output neutral bonded to the motorhome’s earth (chassis/battery negative) because, when the motorhome is hooked-up to the mains, a false return path to the sub-station would result in the motorhome’s RCD tripping.
These combined units are fine if you just want a single socket to power a microwave oven for example but, if you need the flexibility of powering all your mains sockets from the inverter, then a separate inverter with its neutral bonded to the chassis, followed by an RCD and a priority switching unit will make your installation as safe as the mains supply in your house.