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Understanding Motorcaravan Electrics


switchesModern motorhomes have quite sophisticated electrical systems. Often these will be both 12 volt (battery) and 230 volt (mains) and may include the means to convert from one to the other. The whole subject is of course somewhat ‘technical’ but I’ll try to describe it here in relatively simple terms and will try not to get too bogged down in the technical detail - but I will need to use some numbers to show what’s happening.

The first thing to understand is the entirely different scale of household and portable electricity. Household electricity consumption is measured in Kilowatt-hours and we use about 50 of them per day in our home. Household electricity is limitless - the electricity company delivers however much I want. In complete contrast my motorcaravan battery holds a finite amount of energy – a bit less than just one kilowatt-hour in fact! It’s true that we can re-charge it from time to time but this is a rather slow process, so in practice we have to accept that a battery based electricity supply is somewhat limited. In fact batteries of all kinds are the big modern disappointment, they're always too big in size or weight but too small in capacity. I read a 'QI fact' the other day that said all the batteries in the world would power us all for only about ten minutes!!  Recognise the limitations and life will be easier.

vehicle radioThe technology is very similar to that in a motorcar but with one very important lifestyle difference. When you are in a car the engine is running almost constantly and generating electricity to power everything you need. In a motorhome your greatest electrical needs are when you are stationary and the engine is not contributing at all. This is potentially a real problem if you intend to stay in one place for a long time. Fortunately many motorcaravanners are inveterate roamers! Others provide stationary power sources like solar power.

striplightThe second essential technicality to understand is the amount of electricity consumed by different types of appliance. This is most easily seen by looking at the ‘power’ of the appliance in ‘watts’ - this is almost always written on the appliance somewhere. Typically a motorcaravan light will be 5 to 10 watts whereas a fan heater will be 1000 to 2000 watts or even more. Heating is the most demanding class of appliance having very high power demands, usually far beyond anything that a 12 volt battery can deliver. As a result most motorcaravanners rely on gas for such things. As an alternative one can use a ‘hook-up’, a temporary connection to the campsite’s mains electricity or even to a generator to produce one’s own ‘mains’ electricity from the enormous energy contained in gas, petrol or diesel. Note that not all motorhomes are fitted with major mains electrical appliances like hobs, ovens, microwaves, etc.

12 v batteries & charging

leisure battery installedThe heart of any portable electricity system is the battery. The battery stores energy and turns it into 12v electricity when required. In all but the smallest or cheapest motorcaravans this will be a separate ‘leisure battery’, installed so that it can be charged and discharged independently from the vehicle battery. This sensible arrangement means you can use the living quarters electrics as much as you like and still be able to start the ‘van when you want to!

Although any 12 volt lead acid battery could be used anywhere in the ‘van, in practice special types have been developed for each role. The vehicle or starter battery is designed to give very high starting current for a few seconds and then to be constantly charged while the engine is running. A leisure or deep-cycle battery on the other hand is designed for long slow discharges and occasional re-charging. Each works best and longest when doing its designated task!

Many modern motorhomes will have two built-in charging arrangements, one driven by the engine and one via a mains hook-up. In our own ‘van each system charges both batteries but this is not universally true. A control panel giving a guide to battery condition and charge or discharge rate may also be fitted. The best systems are adjustable for battery type and are set at a slightly lower than optimal voltage to minimise ‘gassing’ (all conventional lead-acid batteries produce small quantities of hydrogen and oxygen when charging and should be vented to the exterior as a safety precaution). There is a newer sealed battery type - a gel battery – that is finding favour in motorhomes due to the complete absence of gassing when charging. My own, admittedly limited, personal experiences with gel batteries have so far all been bad with a 100% early failure rate 

control panel - battery checkIn practice most owners will have little influence over what is provided initially. So, the best policy is to assume that the manufacturers have provided a suitable system but to ask about charging both on the move and on site and then to use the systems fully to find any weaknesses before you set off on that six-month trip to Scandinavia! It was this process that persuaded us that we didn’t have enough battery capacity in our latest ‘van. Battery capacity is usually described in amp-hours – how many hours it will last giving out a set amount of current (amps). Leisure batteries typically have ratings of 75, 90 or 110 AHrs. Naturally enough the higher rated batteries are both larger and more expensive. We now have twin 110 AHr batteries but many people manage quite well with a single ‘75’ and a few intrepid types carry four or even more ‘110’s! It all depends on your lifestyle, installed appliances, carrying capacity and how often you are able to recharge fully.

electrics control unit under front seatRecharging follows a similar pattern, if you’ve taken out 20 amp-hours you’ll need to put the same back again when charging. In practice nothing is 100% efficient so you’ll actually need to put back slightly more – in this case say 6 hours at 4 amps. Again in practice it is not always possible to get the full capacity out of a battery, low temperature and high rates of discharge both diminish capacity greatly; also for long life no leisure battery should be discharged by more than 80% of its capacity, preferably less than 67% (2/3). In practice the performance of a battery is dismal once past the 67% discharge mark so you probably wouldn't want to persevere in any case.

I should add here that there is no need to become paranoid about flat batteries. A proper system functioning normally will allow the leisure battery to go flat and still leave you with a fully functioning vehicle battery. It might even pay you to deliberately flatten the motorcaravan battery to be sure that the system really does work properly - I would choose a non-critical time and place to do it though!

There are several different charging systems available, the high capacity vehicle alternator that only functions when you are on the move, a built-in charger that only operates when ‘hooked-up’, solar panels that are only fully effective in bright light, preferably sunlight and small wind turbines that plainly need some wind to function. And of course you can generate electricity with a portable ‘genny’, many of which have a direct charger output. The noise of a genny does however mean that there are restrictions on when and where you can use it. The newer ‘briefcase generators’ are much quieter but still won't make you popular with your camping neighbours!.

12v Appliances

halogen lightIf we accept for the time being that all our heating will normally be provided by gas, just what are we going to run on electricity? Well quite a bit in fact - lighting, music, TV & video, laptop, mobile phone, water pump, hot air distribution, fan, and so on. Motors can be quite heavy users if run at high speed, so blown air heating and other fans should be run at medium or low speeds if battery capacity is an issue. Water pumps are also heavy users but are only on for a few seconds at a time so are not a problem. Even showering tends to be a shorter process than at home because there’s rather less hot water to play with and most people use the pump just to get wet and then again later to rinse off, rather than continuously.

Most modern motorcaravans are well supplied with electrical gadgets so it is quite important to remember to switch off everything that is not required at the time. Some people are very good at this but others aren’t – us included! To help with assessing 'demand', most appliances are rated and labelled in watts – a 10 watt reading lamp for instance. This is not overly helpful when we’ve been talking about volts and amps! Fortunately it is easy to work out one from knowing the other since watts is the same as volts x amps, so for vehicle electrics all you need to know is your twelve times table (see the right hand panel). Calculation does help us to understand and illustrate the influence of lifestyle. For example if we sit and read with three 10 watt lights for perhaps 6 or seven hours in winter, that’s 15 or more amp-hours taken from the battery. If we could read with just one lamp for only three hours in summer that would just 2.5 amp hours used. This is quite a difference when you consider that we might only have a 75Ahr battery and should only use 1/2 or maybe 2/3 of that capacity anyway.

tv and videoSome appliances take a high initial current or ‘surge’ that is higher than their official rating. Appliances like microwaves, pumps, vacuum cleaners, etc., are well known for this, while others may be particularly sensitive to voltage drops - TVs for example. We had a particular problem in our own ‘van where the TV would switch itself off whenever a tap was turned on and the water pump started – this was not very popular as you might imagine! We resolved this by putting the TV on a separate new circuit with heavier wiring which solved the problem completely. To do this work yourself you must understand polarity, wiring gauge, fuses and relays, otherwise get an auto-electrician to do it for you. (We are thinking about adding an additional technical section here to address these issues).

Another high demand application is cooling, not least because it often involves heating! The mobile fridges fitted to most motorcaravans are different to domestic ones being the absorption type. These have a heating cycle as part of their operation, the heat is of course ducted outside so not to negate the cooling process. These fridges can take 8 to 16 or more amps and are usually wired so that they can only be run from 12 volts while on the move and receiving charge from the vehicle alternator. fridge controlsWhen stationary you are expected to run ‘3-way’ fridges on gas or mains hook-up. It’s worth noting here that absorption fridges are rather sensitive to ‘tilt’, about 5º is thought to be acceptable but some manufacturers claim a greater tolerance. Absorption fridges are favoured because they are virtually silent in operation. More conventional compressor fridges are beginning to appear in mobile applications now they are becoming quieter but they are still quite expensive. The benefit is that they run on only a much lower current draw - maybe 1 or 2 amps average, maybe less. Very few air-conditioning units have a 12 volt option due to the very high demands involved, typical roof mounted air conditioning units draw anything from 35 to 55 amps average when on a 12V supply - so another severe battery bashing application!

230V Hook-ups & appliances

electrical 'caravan' connectorsA mains hook-up is available on many sites at extra charge. A special outdoor cable with 16 Amp connectors is used to connect the ‘van to the site mains. Numerous sites here and throughout Europe use these standard connectors but adaptors are also available to fit the domestic outlets found in France, Germany, UK and elsewhere. Many ‘vans will switch over to mains automatically once plugged in but some require you to switch on internally. Mains outlets in campervans and motorhomes are protected by circuit breakers or fuses in the same way as they are at home. The more modern ‘elcb’ units are preferred due to the higher level of protection against electric shock.

Once hooked up the 12v leisure battery will probably be put on charge automatically and the vehicle battery might be too, all internal mains sockets go live and you can use electrical appliances in much the same way as you do at home. Some motorcaravanners carry miniature ovens or microwaves to take advantage of this. The fridge can also be switched over to mains and in some ‘vans you’ll be able to heat water using mains too.  

hook-up connected All this is not quite the same as it is at home though because there is a limit on the amount of electricity you can use in practice - on the available amps. There is always a limit and although top sites like those run by the Caravan Club may offer a ‘full 16 amp’ supply you will sometimes find you are limited to a much lower level, to 6 amps or even to 4 amps or less on some rural sites abroad. 16 amps will allow you to run just over 3½ Kw of appliances. Even this relatively high level of supply limits what we can do - if this is winter and we’re just come in from the cold wanting to run the fan heater, put the kettle on, dry our hair and heat some food quickly in the microwave, we can’t because the supply will trip out (switch off) if we do! Just add up the watts to see why, 2Kw heater plus 2Kw kettle plus 1500 watt microwave plus 1800w hair dryer = 7+Kw, more than twice the allowed supply!! You can of course use all these items one at a time unless you find yourself in ultra-rural France limited to 4 amps in which case you would have trouble using any of these appliances at all since 4 amps means we'll only be able to use appliances rated at around 1Kw (kilowatt); this is why people buy camping irons, camping hairdryers, etc. You might like to consider too that ‘tripping out’ the electric supply may affect just you, but it could mean everyone connected to your electricity post or even the whole campsite - what a way to meet your neighbours!

mocrowaveMicrowaves in particular cause confusion because their description usually includes the energy output of the unit in watts. This is actually very different to the total number of watts consumed. Our own domestic microwave is rated at 900w but it also has a 900w grill; add to these the fan and the normal inefficiency in converting electrical input into heat output and the total input is 2550w. Even worse they are normally subject to input surges when first operated so if needing the grill at the outset it would be wise to allow a little over 3Kw for this particular appliance! (13 amps). This high demand is the reason that many domestic appliances are not suitable for use in motorcaravans. Fortunately special ‘low wattage’ versions are made for ‘camping’.

Fridges and rooftop air-conditioning units run well on mains hook-ups but be sure to check on the real ratings for air-con because some have both heating and cooling built-in but one never normally uses both together. If retro-fitting air-con it’s worth checking that it will start and run on low power on one of the more restricted hook-ups since they often have high start-up currents. All manner of lesser gadgets might be used too, laptop computers, mobile phone chargers, gps systems, etc. Some of these will run either on 230 or on 12v but normally via separate adapters that are optional extras and most make very modest demands on the motorcaravan battery.

Converting 12 to 230v

Conversion of mains to 12v is a fairly straightforward process via an inbuilt battery charger giving a 12 v supply. Conversion in the opposite direction is also possible using an inverter but this process is somewhat problematical. Clever electronics are used to lift the voltage and to convert the steady dc voltage of 12v systems into the alternating waveform of mains electricity. Not all inverters are created equal, there is a good reason why the ones 'on special' in garages are 20 and the professionally used ones are 1000!!  The process has imperfections and so not all appliances will run satisfactorily on cheap inverters, indeed some will fail to start, some will make strange noises and some will burn out; many more appliances and gadgets will now run on what are called ‘modified sine wave’ inverters but just about all will run on the more expensive ‘pure sine wave’ devices. A particular problem is 'inductive loads' like rechargeable toothbrushes, these will not tolerate the cheaper inverters and damage may be done to either appliance. Inverters are rarely fitted as standard.

inverterOf more concern at a practical level is the change in amps drawn. Given that the power of an appliance in watts will not change and that we are stuck with the change of voltage, the only other variable is amps. Add to that some inefficiency in the process and suddenly we're looking at a rule of thumb that says 1Kw = 90 amps! Not only does this require substantial wiring the size of starter cables but would also completely flatten a normal leisure battery in less than half an hour! (the expected usable capacity at a 90A discharge being much lower than at a normal rate of discharge). Note too that amps as high as this are quite capable of starting a fire if pushed through inadequate cabling. Anyone wanting to use inverters to power high demand appliances like these needs to make very good provision for battery reserves and for constant re-charging via solar power, wind energy, on-board chargers and the like.

Not all is gloom and doom, however, small inverters rated at 150w or 300w are very useful to run laptops, mobile phones and camera battery chargers, where only 230 volt devices are owned and no hook-up is available. One caution is about the newer lightweight chargers for these devices and for those very popular electric toothbrushes, these use 'switched mode' chargers and some are quite sensitive to the quality of electricity supplied. They may even eventually burn out if used with the wrong inverter (check that temperatures remain normal in use). For this reason it is wise to have a 'pure sine wave' inverter for these appliances. Although they are more expensive, particularly in larger sizes, it makes sense to have one of the smaller pure sine wave devices just for those small but expensive gadgets with the new sensitive lightweight chargers. Smaller units also draw less current for their own purposes so it is usually a good idea to have a small high quality inverter even if you already have a big one for other purposes.

Relax & Enjoy it!

All this information is intended to help us understand what is going on and where the limitations lie. It doesn’t mean that motorcaravanning is difficult and uncomfortable but quite the reverse. Once you accept and adapt to the limitations you can enjoy a very comfortable and relaxing time whether hooked-up on site or running on batteries away in some remote spot far from civilisation!


First - don't get bogged down by the terminology. Watts, volts and amps are just names like wallflower, veronica and aster.

These names describe in formal terms how electricity works; 'volts' is the pushing force or pressure, 'amps' is how much is actually flowing and 'watts' is a measure of the amount of power in the system. Add the essential time element and you get amp-hours and watt-hours to tell us about total availability or consumption.

12x Table

watts = volts x amps
so for a 12 volt vehicle system all you need to know is your twelve times table. For example an appliance taking 5 amps at 12 volts is 5x12=60 watts and a 6 watt bulb takes 6/12=½ amp.

Ohm's Law

For those of you interested in the relationship between volts, amps, watts, etc., I’ll remind you about Ohm’s law for simple circuits.

V=IR and P=VI and the derivatives, where V=volts, I=amps, R=resistance and P=watts.

All this information could be useful in assessing the effect you are having on the battery, for example if we sit and read with three 10 watt lights on (i.e. 30w), we take 2.5 amps (30w/12v) from the battery for perhaps 6 or seven hours in winter, that’s 15 or more amp-hours (2.5x6) taken from the battery. If we could read with just one lamp for only three hours in summer that would just 2.5 amp hours used.

16A & Hook-ups

Note that the 'full supply' of 16 amps is not available everywhere and in any case might be shared with your neighbours.

To see what it can provide our old friend P=VI comes into play again here telling us that 16 amps will allow us to run just over 3½ Kw of appliances in the UK (16ax230v).

Even this relatively high level of supply limits what we can do. A fan heater plus kettle plus hair-drier plus microwave, might amount to 7Kw and would trip out the electricity supply!

Just add up the watts to see why, 2Kw heater plus 2Kw kettle plus 1500 watt microwave plus 1800w hair dryer = 7+Kw, more than twice the allowed supply!!

You can of course use all these items one at a time unless you find yourself in rural France limited to 4 amps in which case you would have trouble using any of these appliances at all since 4a x 220v = 880 watts only!


Converting 12v to 230v is an appealing process but there are limitations. The thing to remember is that the wattage of the appliance remains substantially the same but the volts and amps change dramatically.

Let’s take a simple 1Kw device that will draw 4.35 amps on mains (1000w/230v) and look at its performance on 12 volts via an inverter. First we calculate as before by dividing watts by volts then we add on about 10% for the inefficiency of the inverter (1000w/12v x 110%) = over 90 amps!! That's nearly as much as a starter motor draws and we all know how long you can use a starter before flattening a battery - just a few minutes!




All material copyright Motorcaravanning.com and/or Neill & Dilys King 1999 - 2013 unless otherwise stated.