Buying inverters for motor homes can confuse. Prices vary for products that seem similar but are not. Here’s what to buy.

The major differences between inverters for motor homes (and caravans) are in their ability to run any type of load. It also relates to their over-load capacity and safety. Modified square wave (also misleadingly called modified sine wave) inverters are the cheapest. Not all electric and electronics appliances etc will run from them. They are likely to damage equipment such as laser printers. Sine-wave inverters run anything that will run from mains (grid) power. Unless you know what you are doing, and are buying to power a specific load, buy a sine wave unit. Good inverters for motor homes cost more, but being more efficient, they require less solar and battery capacity.


Sine wave square wave

High quality sine wave units are >90% efficient and will run equipment without risk of damage. Sine wave inverters are now often referred to as ‘pure’ sine wave inverters. Most in fact introduce minor distortion, but too little to matter (it is usually cleaner than mains 230 volt power).

Transformer or switch-mode

There are now two main inverter technologies. One uses heavy iron-cored toroidal (doughnut-shaped) transformers. The other uses lighter and cheaper switch-mode technology. Each can be designed to produce modified square wave or sine wave output. Their surge capacity (short-term overload) however can be very different.


Transformer type inverters

inverter copy

This graph of an actual 3000 watt inverter shows it can output 7500 watts for 10 seconds and 4600 watts 5 minutes and 4000 for 15 minutes.


Like camels, good inverters for motor homes will carry two or three times their continuous load for a few seconds. They are likely to carry 50% more for a few minutes, and 10% or so more for 15-20 minutes. This ability is inbuilt. It is not an overload. If that which they can safely carry is exceeded they sleep to cool off. A good 800 watt transformer type inverter will run a TV/DVD that draw 75-150 watts plus the odd kitchen appliance such as a blender (typically 600 watts), and a coffee grinder (300 watts) at the same time.


Switch-mode inverters

These are far less camel-like. Switch mode inverters are relatively cheap, light and compact and ultra-efficient. But most have only nominal surge capacity. Most have limits on how long they can maintain their rated output. Caution is essential, particularly if buying via eBay. Only a few produce their ‘rated’ output for more than seconds. Most have a continuous output of 80% but some a mere 40%- 50%. They may however be a good buy where space is short, weight carrying capacity is limited and overload capacity is not required.


Temperature versus output

Most high quality inverters for motor homes and caravans are rated at an ambient of 40 degrees C. They can be be used continuously in that temperature. Cheap ones are likely to be rated at 25 degrees C. This may cause them to appear to be more powerful and better value than they are. This is important for use in hot areas. A top quality 500 watt inverter rated at high ambient temperature may well outperform an 800 watt eBay cheapie. Be careful when buying. Some vendors quote ‘rated’ output at low ambient temperature to boost the numbers.


Standby current draw

For powering appliances such as TVs, DVD players, mobile phone battery chargers that may be left on ‘remote’, it is vital to have an inverter with low so-called ‘quiescent’ current – i.e. that required to run the inverter when it’s virtually off-load. This varies considerably but switch mode units usually have the (desired) least. This is usually shown in makers’ technical literature, but not sales brochures. Most post-2012 electrical approved for use in Australia must (legally) draw less than one watt on standby. This is not likely to apply to private imports.


Electrical Isolation

Many low-priced inverters have an ‘auto-transformer’ that has one side of its output voltage connected to battery input. This can be dangerous in some circumstances. Good quality inverters for motor homes are electrically-isolated. They cost more but not having that is a serious safety issue. One major Australian/New Zealand electronics supplier (Jaycar) has, since 2001, sold only (double insulated) and electrically isolated inverters.


Buying inverters for motor homes – caravans

Switch mode inverters are smaller, lighter, more efficient and cheaper, but have less surge capacity. It is likely to be necessary to use a 2000 watt such inverter to start a 500 watt induction motor.

Transformer based inverters are larger and heavier. They cost more to buy and ship. Most are marginally less efficient but have considerable surge capacity. They are suited for loads that require high surge current: such as big power tools.
Be aware that a microwave oven ‘rating’ (typically 800 watts) is a measure of the heat it generates – not its draw in watts. Most are only 50% efficient so an ‘800 watt’ oven is likely to draw about 1200 watts.

If you can afford the price, and can accommodate the size and weight, a good choice is still a conventional transformer sine-wave inverter. If not, then choose a good quality switch mode sine wave unit known to be large enough to start and drive the planned load/s. As that may need to up to five times the rating of the former, think hard before making a purchasing decision.

Far more than with most electrical products, with inverters for motor homes and caravans you by and large you get what you pay for. Price is a reasonable guide but be aware that some imports pass through several levels of distribution – each making a profit.


Connecting Appliances

Inverter Projecta

This 2000 watt inverter has two 230 volt ac outlets. Appliances are plugged directly into these outlets. Inverters of this type must not be connected into any fixed wiring systems. Pic: Projecta.


Some inverters have power outlets on the actual unit. Appliances plug directly into those outlets. These (mostly low wattage) inverters must not be connected to fixed mains wiring. It is not only illegal, but may also prevent the RCD safety circuits from working.

Inverters intended for connecting to RV fixed wiring have no power outlets on their case. There is provision only for direct connection (by a licensed electrician) to mains wiring. The inverter generally relies upon that system’s safety protection. This is a complex issue. The rules (for RVs etc) are in AS/NZS 3000:2007 and in AS/NZS 3001:2008 – both Amended in late 2012. Electricians are respectfully advised to consult the latest (2012 Amended) Standards.


Current Draw

Even small inverters for motor homes draw high current at full load. A 2000 watt 12 volt unit may draw 400 amps on surges.This is much the same as a 4WD starter motor. Generally speaking, if more that 1500 watts is required, it is better to use a 24 volt system, above 3500-4000 watts requires 48 volts. Heavy cable must be used, and cable runs kept to only a metre or so from inverter to battery.

Caravan and motor home electrics, particularly inverters for motor homes and caravans is a complex subject and beyond the scope of an article. All is covered in depth in Caravan & Motorhome ElectricsAll you need to know about solar for cabins and Rvs is covered in my book (now in its third edition) Solar That Really Works! These two books are bought globally – even auto electricians use them as working guides. Solar Success covers home and property solar systems. My all-new Caravan & Motorhome Book covers every aspect of their purchase, use and even self building. The Camper Trailer Book does likewise in that area.

I seriously advise buying one or both books as their cost will be repaid multiple times by getting the system right first time. They are unusual in the author (Biography) has both an engineering and a writing/publishing background of over 50 years. They are technically competent and written in plain English. The few technical terms used are likewise explained.