Installing an Inverter in a Motor Home

by Bob and Chrissy Eustace

Installing a Small Inverter in a Motor Home

An inverter is really an essential item in your motor home, as it converts 12 volts DC from your battery or solar cell into household style 230 volts alternating current. This makes your motor home even more like "home". Rather oddly very few models come with one fitted as standard. Sunliner is a notable exception and of course most of the $180,000.00 plus models from Winnebago and Swagman. If you know nothing about inverters this article is for you, as even if you have no intention of installing one yourself it will make you more conversant with what you need to buy or have fitted! As usual this is another minefield area, and yes it easy to make expensive mistakes and worse buy an unsafe unit. We have made mistakes along the way and these are detailed here. Generators and inverters both perform the same task. The difference is that an inverter only uses full power when the connected device requires it. A generator chugs away even under no load and is noisy and a polluter! Better to add more solar panels and perhaps another battery, PLUS it doesn't need petrol. Please be aware that biggest is not necessarily the best as inverters use up watts when not running and there is a 10% loss even with top notch units. Wherever possible you are better off using 12 volt DC appliances as despite their slightly extra cost, they are usually more efficient being purpose built.

Please remember safety when messing with inverters.

It can kill you just as quickly as the normal mains, particularly if you don't bolt it into your motor home. Some inverters can be quite lethal if used in the boot of a car for instance and a fault develops. The following instructions relate to fitting an inverter in a Freeway, however with minor adapting you can certainly use it with most other motor homes regardless of manufacturer. At the outset please note that we will be CONTINUALLY stressing safety throughout this article. At one stage we abandoned this story because of all the safety problems we encountered, but then realised if we did nothing no one will find out about the dangers and pitfalls. So please read this article as a starting point in your quest for finding the perfect inverter for you.

Modified Sine Wave Inverters

There are basically two main types of inverters on the market. The cheaper MODIFIED SINE WAVE more or less imitates 230 volts by "chopping" bits out - sometimes called "simulated sine wave". This is a bit like driving on a pot holed road - not at all smooth. This is fine for many crude applications where so called "dirty" electricity works OK (power tools etc.) but is totally unsuitable for things like laptop computers, printers and scanners. Some of the really low cost entry models are not too good on TV either, as you get lines on the screen even though ads can say they are OK - this is actually RF interference. The only way to find out if your gear will work OK, is to take your electrical bits and pieces to your supplier and try them on the inverter you are interested in. An easier alternative is to buy a model the same as one of your mates, but make sure it is safe, does the job properly and is not too large. Modified sine wave models (square wave) start from under $100.00 with the "plug into a cigarette lighter models". Yes folks, as usual we have bought one of these with absolutely disastrous results, so we are totally qualified to say please don't do it, as many are a total waste of money! There are however some very well made units available in the larger sizes and you will even find them in expensive motor homes, but be aware that they may still not work on the more sophisticated electronic devices and indeed reduce the life of your appliances. With microwaves for instance cooking time can be greatly extended. Other weird things can happen like your remote controls wont work, your cordless drill charger fails and your florescent lights all "buzz". Our best advice is to ask lots of questions and thoroughly read what we have put together here as we have talked to a lot of people. Above all though, please do your own research and make an intelligent decision.

Pure Sine Wave Inverters

Photo_Victron Sine Wave

Victron Sine Wave Inverters

The other type is FULL SINE WAVE - also called "pure". These start at around $175.00 for 200 watts (in 2017). ... In our view you are better off going for a quality full sine wave inverter right from the outset, otherwise you will find yourself "upgrading" only a little bit further down the track. Full Sine Wave produces 230 volt AC as good as, or mostly cleaner than that in your own home and it will almost be totally free from spikes and other nasties that can kill delicate electronic equipment. So using a sine wave inverter ensures that all your equipment operates exactly the same as it would when powered from your AC wall outlet at home! A powerful argument for buying the best!


Think before you buy!

Please read this entire article before buying any old inverter from your nearest stockist ... as your safety could be compromised. You do need to do some very heavy thinking before racing out and buying your first inverter. It is one area where BIGGEST IS NOT BEST! This is because apart from higher cost, all inverters consume electricity even when idle - about 10%. This is proportional to size. In our view trying to run a microwave in a small motorhome is totally daft and will only lead to dissatisfaction and disappointment with your system. You can find out the horrific technical reasons for this by reading Motorhome Electrics and Caravans Too! by long time motorhomer Collyn Rivers.

Our best advice is to go with the SMALLEST sine wave unit that will meet MOST of your needs, rather than go for the much larger wattage's offered by the modified units. If you use computers you have an easy choice as there is only one way to go as modified sine wave and computers are a giant no no! Yes you will come across the odd motorhomer running a computer from one but rest assured problems will develop further down the track. A good move is to find a fellow motorhomer with an inverter and try your gadgets with it. (please read the safety notes at the bottom of this page before buying) Just because the appliance you wish to run has a LOWER WATTAGE than the inverter you intend buying doesn't necessarily mean it will work with your appliance. We had a 50 watt TV and a 100 watt cheap and nasty inverter, and yes it wouldn't work as the INRUSH current needed to fire up the telly couldn't be delivered. A separate video on the other hand worked OK. The diagram shows you graphically how the sine wave flows smoothly and the modified or "square wave" is lumpy.

Photo_Sinewave /Squarewave

What we do with ours

Rather than give you heaps of calculations with formulas, here it is in a format you can easily understand with no technical mumbo jumbo at all. We worked out what we needed to run and set ourselves a realistic limit of 150 watts. Ouch I hear you say - I can't do anything with that! Well you can in fact do quite a lot, and you don't need a bin full of batteries either. Below is a list of all the electrical items we run - not altogether though mind you! All of these items can easily be run using a single or dual house batteries without flattening.

  • Stab mixer for soups
  • Tooth brush charger
  • Sony digital camera charger
  • Orion TV video combination (also runs on 12 volt)
  • 8" oscillating fan
  • Sony Vaio laptop computer (also used for watching DVD)
  • Toshiba laptop computer
  • Charger for Psion Revo - email
  • Iomega zip drive - X 2 - USB
  • Yamaha Spyder CD burner - USB
  • Acer 15" LCD computer monitor
  • Orange juicer
  • Sewing machine - Husqvarna

Maximum Loadings

Bear in mind that many items draw far more current than what's on the label . Like most motorhomers at Barcaldine, until the wonderful lectures by Collyn Rivers, we were completely unaware of this simple problem. Thank you Collyn! Our 30 watt fan actually uses over 100 watts as it uses an inefficient induction motor! For a better understanding of this we suggest you have a read of "What's Watt" by CMCA member and successful author Collyn Rivers. This is one of a series of freebie articles written for the Wanderer Magazine and republished on Collyn's website. Our maximum load which we run for around 4 hours a day in bright sunshine is two laptops and two zip drives. Occasionally we run a 15" LCD monitor as well - 100 watts. This is all done on one 65 watt Unisolar shade tolerant panel to a 100 amp/hour AC Delco deep cycle sealed battery. We stop once the battery reads 12.2 volts on the Kyocera Solar Commander controller. At this voltage the battery is 50% discharged. We have been doing this for two years on the same battery. We feel our battery starts from fully charged but we can't positively check this, as it is a sealed battery and you can't insert a hydrometer. In high summer we run one computer and the fan. We only use one computer when running the LCD monitor - used for watching DVD and editing photos. We use all this gear sensibly and use our brains on dull days. A good excuse to veg out! We would stress that as yet we have not tried the ultimate test of trying to run this set up in Tasmania in winter! Our current Unisolar 65 watt panel produces 3.4 amps around midday in full sun in Brisbane. Bear in mind that most solar panels do not produce the wattage stated on the nameplate. This is because of the way that the manufacturers rate them - when short circuited or at their peak output voltage! Unlike other brands the Unisolar seems to give the closest output to stated and we get 49 watts. You really do need to factor the "missing watts" in when working with solar. (watts = volts X amps)

Our electrical set up

We have been messing with solar and inverters for many years, and as usual these articles are mainly about ERRORS of judgement we have made, in an attempt to educate others not to make the same mistakes! Well we got this one method 100% right and it works great! On the Freeway we have a starter battery, an isolated single deep cycle house battery charged only from the vehicle alternator and Winnebago's 230 volt charger (13.8 volts output). We then have a completely separate system consisting of a single Unisolar 65 watt roof mounted panel, an AC Delco sealed deep cycle wet cell battery mounted underneath, a Kyocera Solar Commander regulator FM16 and a TBS 150 watt inverter. Please note that we are not suggesting that the TBS is the best unit you can get. All we are saying is it works for us and we are aware of its safety limitations. (we discovered these short comings totally by accident during a "lead" inspection at a chapter rally - see the notes at the bottom of this story)

Circuit diagram


The circuit diagram shows you just how simple this project is. Make CERTAIN that you put an inline or blade type fuse or DC overload breaker very close to the battery in the POSITIVE cable. We also installed a 15 amp self resetting overload to be doubly sure as these are under $10.00! We put this very near the battery as well. If you don't have terminal crimping pliers your only alternative is to use an inline fuse and electrical connectors. The TBS also has its own built in safety features. It turns off with under/over voltage as well as overload and short circuit. Be very careful not to reverse polarity. We did not test what happens but Collyn Rivers assures us it would wreck the inverter! Dearer inverters such as the Selectronic LD range are reverse polarity proof. Wire colours shown below are for illustration purposes only. It is very important to ask your supplier if the inverter case should be earthed or not. In the case of TBS the advice we received from Holland was to keep the case ABOVE EARTH (in English this simply means do not connect to the metal frame of the vehicle - explained in detail below).

Overloads and Safety

Many larger motorhomes have expensive rotary switches and sometimes relays to swap from mains 230 volt AC to inverter supplied 230 volt AC. This is big time stuff to retrofit and can cost megabucks, as it can only be done by a licensed electrician - be aware that many of them may not have experience on inverters in RVs! The picture below is the current method being used by Winnebago in the Leisure Seekers to run a 300 watt inverter. This is a nice installation, but there is no pilot light warning you that you are running on battery. This is important as an inverter still draws current even when there is zero load. A 2000 watt unit will draw at least 2 amps. Please do not attempt this installation as you will be breaking the law and possibly jeopardising your family safety wise. This switch also protects electricity workers whilst working on power lines. The last thing we need is 230 volts being fed into them when they think the power is off. Another point you should be aware of is that many low end inverters are no longer electrically safe when hard wired (permanently connected) into a motorhome and in many the RCD WILL NOT TRIP under fault conditions. This article is not concerned with this type of installation at all but our research indicates that the Selectronic and SEA units (both made in Australia) are very good choices for this application. ... The installation MUST be done by a qualified electrician. Don't forget to ask for a small pilot light if you cant see the front panel of the inverter.

Photo_Winnebago switch

For these reasons we always keep our system separate from the motorhome 230 volt system, BUT we do not neglect safety at all. There are a few options available with the TBS. Mount it so that the ready to plug into outlet is easily accessible. The TBS comes with an IEC320 computer style socket. You find these sockets on computer monitor leads that plug into the computer case. A plug is supplied with the TBS. You can either get your electrician to fit it to a short lead or a small powerboard if you cant find a lead ready made. You could be better off avoiding these hassles by buying an inverter with a standard 3 pin plug such as the Piccolo .

NOTE:- Be aware that many inverters (the TBS in this case) will not trip an RCD if you buy a special safety lead with one built in. We are not too happy about this at all despite assurances from TBS, so we ONLY use double insulated appliances with this particular inverter. We make it a Golden Rule not to use appliances outside the motorhome.

The TBS has an on demand function they call "automatic standby mode" (ASB). It stays in standby mode until a load is detected - drawing just 4 watts. Works great, but we don't use it, as it is actually still using valuable battery power when waiting for load. It also has a self diagnostic mode. Different flashing light combinations indicate specific faults. The only other unit mentioned here that has this feature is the SEA Tempo. If using a sewing machine a lot, this is an excellent power saving feature.


Photo_Battery in box?

Select a location where the DC low voltage cable run will be the shortest possible distance to the battery, as the longer a DC cable run is the greater the voltage loss. Select a cable at least the same diameter as the fly leads coming out of the unit. In our case we used the cable already fitted to the unit - about a metre long. This also eliminated the need for joins. If you MUST have longer leads you could use 6mm automotive cable by Tyan. It would not be overkill to use light 8 mm starter motor cable. Ventilation is an important factor to consider. The TBS unit does have an inbuilt fan, but as with even the best inverters you already lose up to 10% just to run it, so you don't want the fan cutting in all the time. We normally surface mount, but in this case the little lady insisted on a "prettier" installation, so it went into an under seat locker! Sadly we stuck it in the same compartment as the HWS - not one of our brightest moves! Under our seats is a bit like a submarine. However, we don't run our HWS for hours on end, so it really isn't a problem. Those with a motorhome with metal framing MUST MAKE CERTAIN that the case does not contact any metal surface (above earth) - all Leisure Seekers and Alpines for instance. There is a technical reason for this explained in the safety notes below. Now you are probably wondering why the inverter is mounted so far back from the wall. Well the original plan was to run it into a safety switch mounted outside the seat. We scrapped that idea as the TBS doesn't work well with a Clipsal safety switch.

Photo_Battery in box?

We used a separate battery to the house and starter batteries. We mounted it in a new bin under the motorhome - covered in another article. You do not need to go to these lengths but this system works well so it could pay you to at least think about the merits. With the Freeway, if you look at the photo below, you will see the inverter is mounted on an open wooden frame and well away from anything metal. (above earth) As it is inside a locker we have protected all cables using 20mm plastic solid conduit. You can buy conduit (very cheap) at electrical wholesalers or Bunnings. It is held together with adhesive. Acetone works great also. Looking at the side wall in the photo at left below, the "tee" in this is to allow the run from the solar panel to take the same route to the battery. Our golden rule is to try and look ahead! Sadly we don't succeed too often but we did with this job, as we have done it many times before! Experience is certainly the best teacher. The photo below right, clearly shows the tee. We used 20 mm flexible conduit to get around the HWS as space was very limited. If you have a Freeway please note that you WONT have the partitions shown here, as we fitted them for specialised storage - covered in another article in this section. All you need is a couple of stringers across the top similar to what we have done on the left of the TBS. These MUST be WOOD . All Freeways and Leisure Seekers come as standard with zero compartments under the seats. The 2002 Freeway 1754 has all metal frame work under the seats. In this case you MUST use a wooden frame.

Photo_Wall vent

The first photo shows the conduit passing through the floor. You will need a 20mm hole saw to do this. Remember that the floor is very thick, but is mostly foam insulation. Be aware that Winnebago use a very strong RHS steel frame around the outside perimeter, so it is best to err towards the inside to try and miss this. Make CERTAINyou completely seal this hole when finished even if it is a very snug fit. The position is dictated by the rear wheels. Measure three times - drill once!

The photo to the left shows an easy way of mounting the TBS - this one is the 225 watt model with the switch on the front. The photos below show a neat way of providing ventilation to the inverter if you like a concealed mounting. Regardless of where you mount it, we strongly suggest that you also provide ventilation. All we have done here is use a snap in ventilator from Bunnings.

Photo_Wall vent

Be careful with the cut out size. We have found that the hole needs to be smaller than what the instructions indicate if you want it to stay in without screws or adhesives. You don't want to make errors here as it is in full view! Again, ours looks like it was meant to be! You can get these in either fawn/honey or white. We found that fawn looks nicer. If you have lots of cash to shed, $45.00 buys you a magnificent Western Red Cedar louvre in varnished timber. Looks fantastic! An easier option is to mount the inverter with the plug end sticking out of the wall. We just poke our lead through a hole under the seat but a neater approach is to use a computer cable insert - Ikea have these for $3.50 for 5. (fawn or white and called Montera)

Summing Up

We cannot emphasise too much that you are working around 230 volts. Please observe the law for your own safety and have a licensed electrician check your finished installation even if you haven't connected into the motorhomes 230 volt supply.

Bob Eustace

Acknowledgements - Special thanks to Collyn Rivers - Geoff Adcock (CMCA) - Daniel Shoulton (TBS Holland) - Michael Rush (Selectronic) - Dave Lambert (Rainbow Power Company).

Safety Notes - It is important that you read this!

Whilst it is legally OK to install an inverter with a 230 volt socket attached to the inverter case into the 12 volt supply, we cannot stress how important it is for fellow motorhomers not to attempt to install an inverter into the fixed motorhome wiring. DO NOT buy an inverter that doesn't have a three pin socket built directly into the housing unless you are having it installed by a licensed electrician who is familiar with RVs. ... When buying an inverter we strongly suggest you ask the supplier the following:

  • Is the inverter double insulated (electrically isolated between the battery and secondary voltage)?
  • Can the inverter DC and AC circuits be earthed?
  • Will the inverter work properly with an RCD?

If the answer is NO to ANY question please don't buy!

Please note: Some product descriptions and recommendations from the original article [no longer available] were omitted in this edited version. The article was written in 2002 and many of the mentioned products are no longer available.