- Doing Without A Generator
- Microwave Ovens
- Generator Types and Usages
- Inverter Starting Ability
- Beware of Exaggerated Claims
- Committed to air conditioning ?
- Honda Generators

Many people seek to use air conditioning and other big electricity consuming devices whilst away from mains power. Generating electricity in the bush is not hard to do - but no matter how you do it, it costs a fair bit of money and some ways of doing it cost a great deal more than others. Back home we pay 10 cents to 16 cents per unit (that's 1000 watts used for one hour), but once you start generating it yourself it is likely to cost from 80 cents an hour.

The main reason for wanting to do this is usually to run an air conditioner, and the cheapest and simplest way of doing this is to run it straight off a 240-volt AC generator. Unfortunately many air conditioners need a lot more power to start them than they do to run them, so this may require a generator that's a lot bigger than you might at first have thought. Further, motor generators vary in their ability to start things. For its 2000 watt MicroLite generator Onan suggests an air conditioner of 650-750 watts or so, and allows a further 300 watts for lights, TV etc.

Some air conditioners use more power than others do for the same amount of cooling. A more efficient unit is likely to get by with a smaller generator to drive it. If you look at air conditioner specifications you may see the letters 'EER' and a number. EER is an acronym for Energy Efficiency Rating. The higher the associated number, the more efficient is the unit. The lowest you are likely to find is 7. The highest is likely to be about 16. (The EER is the ratio of British Thermal Units to watts).

Running a small air conditioner from a petrol-engined generator costs about $1.20 - $1.50 an hour. That's about $100 a week if run for 10 hours a day. Or about $70 a week if you run it from a diesel generator. To run air conditioning full time in the bush is likely to cost up to $1000 a month for petrol or $600 or so for diesel. You keep cool - but it's not cheap.

It may also cost a fair bit of money for the high quality quiet generator that this will need. You are not likely to see much change from $7000 if you have it built in - but you can save a fair bit if you do this yourself.

Doing Without A Generator

The cost of a large amount of electrical power used by oneself can be so high that it really is worth considering the alternatives before jumping straight in.

Leaving air conditioning aside (for the moment) an electric-only fridge gobbles up well over half of a typical motorhome's daily power. This is not a vast amount of power, but providing it may be extremely inefficient from a generator because fridges cycle on and off (at 5 - 15 minute intervals) but generators have to run continuously.

It's not an issue if the air conditioning is running all the time, but if that air conditioner is not needed, the generator is gobbling up fuel doing nothing useful for over half the time. Running an electric fridge as the only load on a generator thus costs about $20 a day. Running it as an extra load on the generator costs only a dollar or two.

A better solution is to use a tropical rated Electrolux gas/electric fridge. These fridges are designed for ambient temperatures up to 43 degrees C. The only way you can be sure the fridge really is rated for this temperature is from the 'compliance plate'. Where it says 'Climate Type', the entry immediately to the right must be a 'T'. If it's any other letter the fridge is all but certain to be rated at 32 degrees C.

Microwave Ovens

Microwave ovens are a big trap away from mains power because they draw a lot more power than many people realise. Even if you plug one in at home, a typical '800-watt' oven actually draws about 1330 watts. If you run it from an inverter it draws 1550-1700 watts. That's around 135 amps at 12 volts.

You can save a huge amount of money by confining the use of air conditioning and the microwave oven to where there is mains power. If you do that, it is best to use the three-way gas/electric fridge mentioned above.

If you do this, your electrical consumption is likely to be 350-450 watts/day. You can obtain that comfortably from two 80-watt solar modules (or three modules if you really must use that microwave oven). Alternatively you can recharge the batteries using a little 50 cc Honda dc generator. This will take a little over an hour every second day.

However this article is about using motor generators - as opposed to not using motor generators - so here goes.

Generator Types and Usages

There are various and different types of generator. Some run on petrol, some run on diesel. Some produce only 240 volt AC, some produce 240 AC plus a small 12-volt DC output. And some produce 12 or 24 volt DC only.

Apart from different types of generators, there's various ways of using generators. Which generator you choose and how you use it can hugely affect costs. No single type of generator is inherently better than another type of generator. What is important is selecting the right one for the job.

A lot of the choice is not so much about generating electricity as storing electricity. Using a basic AC generator is like using mains electricity at home. When mains power is there - it's there. If or when supply fails, it's not. You can't readily store AC for use when the electricity authority cuts it off. If you want to make AC now and use it later you have to store it in some other form like turning into direct current (DC) and storing that as an electro-chemical reaction in a battery.

The alternative is to generate DC directly and store it (as above) in batteries. You can then use it directly with things designed to run on DC. Or you can use bi-sexual things like halogen lights that by and large don't care whether you provide them AC or DC because either turns them on. Or you can use an inverter to turn DC into alternating current whenever you need it and in the amounts that you need.

To recap, if the generator only produces AC, you have to use it there and then - or you must turn it into DC for storing in batteries. You then either use it like that - or turn it back into AC via an inverter. A minor advantage of generating DC is that it can charge batteries directly - it avoids the losses incurred in first turning AC into DC.

240-Volt AC Only

An AC-only generator is usually the simplest and is sometimes the cheapest way of generating power. But you only have electricity whilst it is running. This is fine for air conditioning and the odd power tool, but if you have an electric fridge, the generator must run continuously because it cannot be stopped and restarted every time the fridge cycles on and off.

A further problem is that the generator must be sized for the total load. Air conditioners typically use five or six times as much energy as a fridge and many need that amount again for starting. This is less important if you are running the air conditioning and a few other things 24 hours a day, but if it's not being used like this all the time it is wasting vast amounts of fuel because the generator is well over ten times larger than otherwise required. It's like sitting for long periods at traffic lights in the front half of a Mack semi-trailer that you are using mostly to commute to and from the office.

Running air conditioning more or less continuously necessitates a quiet and ultra-reliable generator. This will cost about $7000 if you have it built in. You can buy cheaper ones for $1500 - $2000, but the more rugged ones built for continuous use will deafen you, and will be unusable within a hundred metres of anyone else. They are seriously noisy. (Honda generator/inverter units are discussed at the end of this article.)

Any generator larger than 1000 watts or so will cost a minimum of 60-80 cents an hour when it is running totally off-load. This will increase to about $1.60 an hour if you are running a small air conditioner and an electric fridge. Or about $1.10 an hour if you are using diesel.

A 240-volt AC only generator is the cheapest way to run air conditioning continuously away from mains power. This is also true of running air conditioning for short periods - but it is not then economic to use it also for a continuously running fridge. If you need power then you must run the generator - even if it's just to watch TV.

240 Volt AC Plus Inverter

We'll have a look next at ways of providing power for people who want air conditioning only occasionally when away from mains power, but have or want an electric fridge. I'm not recommending using an electric fridge - and certainly not a big one - just explaining how to go about doing it.

This way you still use much the same generator as above, but you utilise its spare capacity to charge a battery bank. Even with the air conditioning running there will always will be some spare capacity. Without some spare capacity, the air conditioner is unlikely to start; and it's always there during the fridge's 'off' cycle. So, as noted above, you use this spare capacity to charge batteries, and these drive an inverter that supplies power when the generator is not running.

This is not as roundabout as it might at first sound because this way the generator is always working close to maximum load - and that's usually its most economic load. If the air conditioning is not used, the generator needs only run two to three hours a day to charge the batteries - and those run the fridge and all other electrical loads.

You can set this up so that you always run the generator when you need air conditioning; or you can have an inverter large enough to run the air conditioner. In theory this enables you to have air conditioning whether the generator is running or not, but it necessitates very large capacity batteries - like 1000 amp/hours or more.

The downside of this approach is that you need a very big battery charger. The more affordable battery chargers are only 70% or so efficient (that is they waste 30% of the energy that goes into them) and they need 20% more current available to them than they actually use. So the output of the generator must be close to twice that of the rating of the charger. For example, a 100-amp/12 volt charger (1200 watts) needs a 2000-watt generator - not the 1200 watts its rating suggests.

You can improve charging efficiency to about 85% by using a smart charger, but a big smart charger will leave little change from $3000. You can do this via one of those big sine-wave inverters that also double as smart chargers, but you are then limited to either charging, or using, but not both at once.

If you use a 15% more efficient charger you can get away with a 15% smaller generator. Further, smart chargers do not require anything like an extra 20% current to be available (they have what electricians call a close to 'unity power factor'). In practice, a 1200-watt smart charger should start and run quite happily from a 1500-watt generator.

Running costs using air conditioning will remain at $1.10 - $1.60 an hour. But, and it's a very big BUT, when the air conditioning is not on, the generator needs only run two/three hours a day. Then the cost comes down from $30 or $40 a day to less than $5 a day, or about $3 a day if you have a diesel generator. This is an enormous difference, but it's still a costly way of keeping the Chardonnay cool. A gas/electric fridge costs less than a dollar a day.

DC Motor-Generator

The final main approach to all this is to use a 12 or 24 volt DC motor generator that is designed to charge the batteries directly. All 240-volt electrical loads are then supplied via an inverter.

I need to stress here that it is not practicable to charge batteries effectively from the DC output of a 240-volt AC generator. This output is 10 amps at best - and you need 50-100 amps to charge a big battery bank. This DC output is usually at too low voltage anyway - even if it's described as being for 'battery charging.' It will bring a battery from flat to half charge - but it will take forever to get much past that. You'll hear them churning away all day long on campgrounds in vain attempts to fill a battery There are also big dc generators designed to run 12 volt loads - and these put out only 12.6 volts or so - useless for battery charging. If you buy a DC generator to charge batteries, it must be designed specifically for that purpose: it needs to maintain up to 15 volts at full output.

Beware of Exaggerated Claims

DC charging generators are often claimed to be far more efficient than the 240-volt AC generators that we've been talking about so far. Some vendors claim they are 50% more efficient - or looking at it another way - they provide the same amount of electricity for half the amount of fuel. And indeed they may well be telling you the truth.

But what they are not telling you is all the truth because they are almost certainly comparing a diesel powered DC generator against a petrol powered AC generator. And diesel engines are 35%-40% more efficient than petrol engines performing the same amount of work. The fact that the electrical generator is DC not AC is responsible for less than a quarter of any gain. Looking at it another way, if the overall gain is 50% then the fact that it's DC is responsible at best for a bit over 10%. For example, Onan's 5000 watt diesel AC motor generator produces 35% more power and uses about 5% per cent less fuel in doing so than Onan's 3600 watt AC petrol driven generator. But, like all diesel engines, it costs more to buy.

Looking at Onan's diesel-engined AC motor generator again - this uses the same Kubota Z482 liquid-cooled diesel engine as many other DC motor generators. And those that I've looked at all use about the same amount of fuel to produce the same amount of power - whether AC or DC. In fact, short of overturning most of what's known about thermodynamics there's no reason why they wouldn't.

Where a DC generator directly scores, is that it charges batteries directly. You can of course charge batteries from a 240-volt AC generator, but to do that you must reduce the voltage and convert the AC into DC. This requires a battery charger and most of these waste about 30% of the power they convert. That is they are only 70% or so efficient.

Worse, because of an AC effect called 'power factor', 20% more current must be produced and made available for that charger. This extra 20% is not actually used as such - it rushes around in the wiring and sort of fills in gaps that come and go, but the generator still has to be 20% larger - and it also burns more fuel making that 20% available. The end result is that the generator must develop about 60% more power than the charger might appear to need. It's one of the reasons why so many people have problems running battery chargers from generators.

Smart battery chargers are 85% - 90% efficient. By using one of these instead of a conventional charger, the loss through using AC charging would be halved. Thus, whilst DC direct charging is inherently more efficient, if one used an AC generator and smart charger, the gain would be less than 10%. A big smart battery charger will cost about $2000-$3000 dollars, but as DC generators tend to cost about that much more than AC generators - the smart charger's cost will be largely cancelled out. If you were to use one of those inverters that double as smart chargers, you'd have the charger for virtually nothing (except that it would not be possible to both charge and use current at the same time).

In essence, smallish DC motor generators are likely to be a little more efficient than most smallish AC motor generators, but if they are substantially more efficient it will be mainly because they are powered by diesel not by petrol. If driven by a diesel engine, an AC generator of the same capacity will use almost exactly the same amount of fuel.

Inverter Starting Ability

If a sine wave inverter is big enough to run something, it's usually big enough to start it. So here we score once again. If everything is run from the inverter, the generator no longer has to cope with starting loads - or even for the additional load of a microwave oven, because the batteries and the inverter handle that. However if a conventional charger is used with an AC system, it does need to be at least twice the rating of that charger.

Another benefit of this approach is that the generator is mostly working at its optimum load. There is also the huge advantage that electrical energy is only being used when an appliance is used (the inverter is otherwise more or less asleep).

Equipment costs are about the same as the AC generator/inverter approach, but running costs are likely to be about half that of a petrol-driven AC motor generator, and about 80%-90% of that of a diesel-powered AC motor generator.

Running Air Conditioning Overnight

A minor problem with running air conditioning away from mains power is if you want to use it all night.

It is possible to do this from battery power but it needs humungous battery capacity. Whether or not it's practicable I leave for you to decide. Here are the numbers: a motorhome-size air conditioner draws about 750 watts. That's 62.5 amps at 12 volts. If run for eight hours, and allowing for battery and inverter losses, that equates to 600 amp/hours and will need a battery bank of 1000-1200 amp/hours. A battery bank to do that will weigh 600-700 kg and cost about $1500-$2000 and you'll need a coach to carry that amount of weight.

It makes more sense to have a quarter of that battery capacity and to run the generator when air conditioning is used for more than an hour or so. This is perfectly feasible with the battery/inverter systems described - except with the combined inverter/chargers - which can do one thing or the other - but not both at once.

Summary

Firstly, doing without a big generator. Unless you are committed to using air conditioning away from mains power, you do not need a big generator. I'd also strongly advise you not to use an electric-only fridge - use a tropical rated gas/electric fridge instead.

If you still need 240 volts, use solar energy and/or buy one of the 50 cc Honda numbers - these will put up to 50 amps into a 12-volt battery. Use solar energy or the generator, via a battery, to drive a 250-watt sine-wave inverter. A really good one will set you back about $350-$500. That will not be big enough to drive a microwave oven - so either only use that when you have mains power - or buy one of the several 12 and 24 volt units now commercially available from companies like Samsung.

(Incidentally it's not only a big inverter you need to run a microwave oven - it's also a big battery bank. Pulling the 130 plus amps these things draw out of a 150-amp/hour deep cycle battery hugely shortens that battery's life. You should not charge or discharge a deep-cycle battery at greater than 25% of its amp/hour capacity. So the minimum size 12 volt battery bank for a 135 amp microwave oven is 520 amp/hours - and that's twice what most people have. (In such situations, a starter battery may well outlast a deep-cycle battery).

Committed to air conditioning ?

If you are committed to air conditioning 24 hours a day away from mains power, go for a straight 240-volt AC motor generator. If doing this for a substantial part of the time, consider using a diesel-powered AC unit such as the Onan CMQD. This is larger than needed for most purposes, but it's fine for mega-motorhomes with a couple of air conditioning units plus a big electric fridge. A diesel-powered unit will be a lot more expensive up front but will last forever and 40% or so less to run.

If you need air conditioning, but not all the time, go for the battery/inverter approach. It costs more initially but is hugely cheaper in fuel costs. Nor do you have to buy a motor generator large enough to start the air conditioner - because the inverter will handle the overload.

A 3000-5000 watt diesel generator costs about $0.80 for 1 kilowatt for an hour, and about 30 cents for every kilowatt an hour more: ie. running one air conditioner costs about $0.80/hour, running two at the same time costs about $1.10/hour. A further bonus is that many diesel motor-generators are water cooled - so you also have semi-free hot water!

A petrol-driven unit is likely to cost $1.20 for the first kilowatt/hour and about $0.50 per each kilowatt/hour over and above that. Running one air conditioner will cost about $1.20, running two at the same time will cost about $1.70 an hour.

If you start comparing AC generation with DC generation remember that much of the claimed fuel consumption difference may simply be due to the fuel they run on.

Honda Generators - (see also this additional article on Honda Generators)

I've left mention of using Honda 20i or 26i generators until the end. These are excellent and very quiet petrol engined generators that have an inbuilt inverter. This improves economy on light loads because it eliminates the need for the engine to run at a constant speed regardless of load.

There is little or no gain at the higher outputs, but fuel consumption is lower, and the unit runs even more quietly, on light loads.

The question arises time and again as to whether you can use one of these units to run an air conditioner. You probably can with the bigger 30I, 3000 watt generator, and some people claim to be running small soft-start air conditioners from the 20i and 26i units (2000 and 2600 watts respectively).

But I am not at present able to recommend them for this purpose because so many members report that Honda dealers advise against it. They say that these generators are not intended for long term continuous usage and, particularly, are not designed to cope with heavy starting loads.

Inverters are generally good at that, but that's mainly because they have a big battery bank that can sustain the heavy current draw - here, the energy reserve is limited to that provided via a small flywheel. In all cases, these Honda generator ratings include their overload capacity. Their running ratings are about 10% less.

If you do consider using one of these generators as a cheaper alternative for occasional usage, you will need to be extremely careful that it will comfortably start the air conditioner. Do note that the rating of these units includes their overload capacity, there is nothing in reserve.

Nevertheless if the generator can start the air conditioner it will certainly be able to run it. The big problem you will encounter is that air conditioner makers are usually wary of quoting exact starting current data. And with the very small margin you are likely to have, that data must be spot on. (see also this additional article on Honda Generators)

12v dc output from Honda generators - the reality

I have just received the following response from Michael Golland - Honda's National Service Trainer for power equipment. "The power supplied from the 12 V dc output of the Honda EU101 supply is unregulated. It is a dc power supply that can be used to charge batteries. but is not a battery charger. If it is left on a battery that is nearing full charge it can reach high voltages and can cause damage to a battery due to overcharging."

"When a battery is nearly discharged, the 12 volt supply from the generator can be used to charge batteries without any damage to the batteries. As batteries become charged, the amperage that can be accepted by the batteries is less, and the voltage of the unregulated dc supply will increase. When batteries are nearing full charge, the power supply from the generator should not be used as a battery charger, and a dedicated three stage charger should be used for the final stages of charging."

So there we have it. I think Michael's response is quite clear. The output can, with care, be used for battery charging but it is not as most people appear to believe, a battery charger. Technically, it has apparently zero regulation - hence its ability to reach high voltage levels as load drops. The BIG warning is for heavens sake don't even think of charging a gel cell battery from one of these devices. I think the oinda is probably still an exception as a quick check shows that most gennies will not put out more than 13.6 volts. I trust this will help - but doubt if it will gladden too many hearts!

NOTE: SEPT 03- NEW CLUB REGULATIONS REGARDING USE OF GENERATORS

A recent Board meeting in Brisbane tackled this problem as it has been raised by many members. The Board has resolved that generators may be operated only between the hours of 9am - 4 pm, by agreement with neighbouring campers. It is felt that consultation with neighbours will resolve many difficulties. However noisy generators over 65dB will be separated at future rallies. The Club now has a dB meter and a licensed operator.