using a generator inverter

Submitted: Thursday, Mar 21, 2013 at 12:54
ThreadID: 101224 Views:3231 Replies:8 FollowUps:6
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Hi great forum,i have a 4.4kva petrol generator mounted on the front of my 1975 millard caravan and when i am on the road it supplies most of the 240 volt power i need to keep me happy.However i run my 150watt fridge while i am mobile to keep my food frozen and fresh because being a pensioner on a limited budget i find it cheaper than buying take out food while god knows how expensive petrol is today,so it helps.I dont run it all the time i may swithch the generator off for 3 hours or so and then start it up again for a hour or so to bring the fridge back up to cold.I was thinking of gett a inverter because i was given a fairly new deep cell 12 volt battery[$300 bucks worth].Can some one tell me what size inverter i would need to run my 150 watt fridge please,i took the details of it off the plate on the back.
Fridge details are as follows.
rated voltage 230-240 volts.
rated frequency 50Hz.
ratted current 0.42-0.43A.
rated input of heating systems 140-152W.
defrosting input 140-156W
climate class T
I also had this idea of using the inverter that is inside the generator is there anyway i could feed 12 volts into it and get 240 ac out to run the fridge without the gen running?
Stupid idea i supose but i have a lot of time on my hands,ha.
I would like to here everyones ideas,thanks.
Graeme Briggs.
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Reply By: Member - Frank P (NSW) - Thursday, Mar 21, 2013 at 13:21

Thursday, Mar 21, 2013 at 13:21
Hi Graeme,

A 200 watt inverter would be sufficient and give you a 25% overhead so it's not working too hard.

You get nothing for nothing. Running your 150 watt fridge on the inverter which is powered by the battery will flatten your battery just as fast as running the fridge on 12 volt straight off the battery, so your fridge will behave much as it does now - ie as the battery runs down the inverter will switch itself off due to low supply voltage and your fridge will warm up, requiring you to run the generator.


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AnswerID: 507272

Reply By: Ross M - Thursday, Mar 21, 2013 at 14:05

Thursday, Mar 21, 2013 at 14:05
G'day Graeme
For a 150 watt fridge you will need at least 4x the inverter capacity ie 600W to allow for startup loads. It is an electric motor start current you have to cater for, not the running load.

However, IF you use an inverter to stepup 12v dc to 240vac for the fridge and this is running off a battery, it will draw around 20 to 25 amps from the battery.
This will mean a deep cycle 105ah battery, fully charged, will only last only last about 2 1/2 hrs before the battery itself will need a substantial recharge to recover the discharge amount. This takes time and will many hours to do it.
Then the battery and the fridge will both require power from 12v source and 240v source, respectively, to charge the battery and run the fridge simultaneously ie off the generator.

Can't see any advantages there because of generator run time will use a lot more fuel than already using.

There is no way the 12v can be run into the inverter of the generator as they are an energy transfer system where engine power is converted to AC and it is then used by the invertor system to switch and create 240v ac.

If you have solar it may help but that is another system and it would require many watts of power from the panels to accomplish it all.
Ross M
AnswerID: 507281

Reply By: Member - Richard L (WA) - Thursday, Mar 21, 2013 at 14:15

Thursday, Mar 21, 2013 at 14:15
Hi Briggo
A 150 watt fridge probably has a startup current of between 500 and 900 watts.
A 1000 watt inverter would probably be sufficient.
Don't forget when running the fridge off your 12v battery through inverter you will be using about 13amps/hr.

AnswerID: 507284

Follow Up By: Member - Frank P (NSW) - Thursday, Mar 21, 2013 at 15:48

Thursday, Mar 21, 2013 at 15:48
Ross, Richard,

It's not a compressor fridge, it's a heat cycle fridge. Refer to the OP 's opening post:

"rated input of **heating** systems 140-152W."

So he doesn't need the huge overcapacity in the inverter that you have suggested to start a compressor because it doesn't have one.

He needs something that will safely deliver 150-odd watts continuously without being near its limit. 200watts would be enough.

However I don't think there'd be much of a gain, if any, going that route.


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Follow Up By: Member - Frank P (NSW) - Thursday, Mar 21, 2013 at 15:53

Thursday, Mar 21, 2013 at 15:53

Disregard the above follow-up. I've found some more info that suggests you are correct. Sorry 'bout that.


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Reply By: briggo - Thursday, Mar 21, 2013 at 14:16

Thursday, Mar 21, 2013 at 14:16
Thanks for your input guys i think i will stick with the genny and maybe the esky thanks a lot and i may see you on the road one day i am in clermont qld just now and am thinking of traveling south down dubbo way after the easter rush so look out for me in my 1999 gold falcon and blue and white mighty millard i try to camp on the side of the highway in spots back a bit from the road its so much cheaper when i cant afford the c/parks.
Bte for now happy travels.briggo.
AnswerID: 507285

Reply By: TerraFirma - Thursday, Mar 21, 2013 at 14:24

Thursday, Mar 21, 2013 at 14:24
Hi Briggo, What you really need are solar panels if you are going to use an inverter or batteries and want cheap power. Generators are noisy and mechanical and a form of temporary power, inverters require plenty of battery power and batteries require charge and that's where solar panels come in. Solar is the most efficient form of generating long term power as you can imagine.
AnswerID: 507289

Follow Up By: Honky - Thursday, Mar 21, 2013 at 16:20

Thursday, Mar 21, 2013 at 16:20
Whilst I have solar and generator I am not technically savvy so if I am wrong please advise.
I disagree, solar would be very expensive.
In my books the cheapest option would be a 2 stroke 600 watt generator for around $100. Tank of fuel would last about 6 to 8 hours.
Please note that I have not factored in noisy, smelling only cost.
FollowupID: 784504

Follow Up By: briggo - Friday, Mar 22, 2013 at 10:38

Friday, Mar 22, 2013 at 10:38
I already have a 4.4 kva generator which is more than enough to run everything i need although i have to unplug some things to run others i can put up with it.I spend a lot of my time when traveling camping away from other folk so the noise does'nt worry them i tend to stay at places off the side of the road because it costs nothing this is the only way i can afford to travel like i do.
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Reply By: TerraFirma - Thursday, Mar 21, 2013 at 16:49

Thursday, Mar 21, 2013 at 16:49
Long term Honky solar is the way to go. Fuel for generator at $1.50 per litre is hardly cheap is it mate? That's why solar is being used on houses and farms all over the country
AnswerID: 507291

Follow Up By: Honky - Thursday, Mar 21, 2013 at 22:32

Thursday, Mar 21, 2013 at 22:32
I thoroughly agree with you as I have set up solar systems for use around my home. I have all outside lights running of solar/batteries.
The systems cost around $600 each and I have two. I also have a solar system set up in the car to run an 80 litre fridge with a dcdc charger.
The generator is going to produce up 40 amps per hour if running through a battery charger for 12 volt.
To get 40 amps out of a solar is going to cost a lot more taking into consideration that you are only going to get maximum ampage for only for a few hours a day.
House solar systems are subsidised and is one of the main reason electricity is so expensive.
It does feel good when after the set up cost shock you get power for nothing but at $100 plus $1 an hour to run for a generator it is going to be a long time until you regret not getting solar.
FollowupID: 784552

Reply By: briggo - Friday, Mar 22, 2013 at 10:33

Friday, Mar 22, 2013 at 10:33
Hi guys,i borrowed a 200 watt inverter from a mate and although it wouldn't run my fr idge it is at this time running my 60 watt flat panel tv,when i started the battery voltage read 12.25 volts on my meter and after 2 hours of running reads 11.46 volts.
I don't know what this has to do with anything just a experiment.
So a thousand watt inverter should be big enough with power to spare to run the fridge a?
AnswerID: 507342

Follow Up By: TerraFirma - Friday, Mar 22, 2013 at 12:12

Friday, Mar 22, 2013 at 12:12
1000 watt would be handy but I know you can pick up 600 Watt Pure Sinewave for around $100 with peak power at 1200w,.

Ebay Link
FollowupID: 784595

Reply By: Janos K - Thursday, Mar 28, 2013 at 18:01

Thursday, Mar 28, 2013 at 18:01
Three way , Two way fridges and Compressor fridges. - Differences???
With the three and two way fridges the best performance is on the 240V heat element, hence the main reason of using the inverter. You lose efficiency in power consumption but you get the additional performance of the fridge.
The DC element is generally smaller and with constant current they generally do not last as long and the performance is terrible (temperature wise) . Its more as a back up. Hopeless in long trip conditions.(but see my last tip\trick to improve this)

If preferring to run a three or two way fridge off an inverter then all you need is a 300W Modified\MSW Inverter,Generally under $100.00. But .. look for the best efficiency.
This is an 8ZED Brand one , up around the 90% efficiency. Less power is lost from switching and heat.

When discussing the compressor type fridge freezers, If they rate it with "A\amps" then this will be the max it draws, If in watts then generally times this by 3~4 for PSW Pure Sine Wave, And times by 5 if powering it via a modified sine wave inverter.
I wouldn't run a compressor fridge freezer off a MSW Inverter, 1 - you get the motor shuttering hence a great nights rest, 2- It's terrible efficiency as the MSW draws more power to run the compressor hence a larger load on the batteries. The money you save on the Inverter you end up investing in batteries.

Tip\trick - When at powered sites,and you have any avail space, try and fill the freezer as much as possible, If any spare space then add bottles of water or freezer bags with water or sauces. then when on the road the fridge \ freezer will not start up as often. When the ice begins to melt then move it to the fridge. The more the fridge freezer is populated the denser it is and the less it starts up.
Sorry gents - not good for beer fridges.
AnswerID: 507747

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