Running a pure sine wave inverter from VZ commodore

[Comrad]

Hi guys,

I was wondering if anyone could provide me with some insight into a little problem I have been thinking about lately. I apologise if this has been discussed in a previous thread, I didn't see it anywhere else.

So, here's the situation. My family has a holiday shack on a beach, in a community that has no mains power. Typically we use a generator to power the lights and what not in there, however, I was planning to go up there by myself in the near future, and, I do not have a generator. I do however own a 1000 watt pure sine wave inverter.

I have calculated the power draw from the shack to be a maximum of about 750 watts, and, my pure sine wave inverter would be able to handle the load, however, I wish to run this inverter off my car.

Now, I know a little about this stuff, so, here are the calculations I have done, I was wondering if anyone can point out any pit falls in my working, or, let me know if I am on the right track. I am also using a lot of estimations, so, if someone had real values, that'd be great as well.

Ok, so, I have a 2005 VZ executive, and, I figure that my battery would be about 45 amp hours, so, given that draw would be 750 watts, at 240v, I would have a current of 3.125 amps. Given that I don't want to damage my battery, I wouldn't want to take it below half charge, so, assuming that there are 22.5 amp hours available, that means I should be able to run the shack for 7.2 hours at full load before getting to half charge just on the battery.
I then though about having the car running. I am assuming an idle current of 30 amps being produced by the alternator so, provided I turned of the stereo, lights, etc... I should be able to get close to that delivered to the system. Assuming a charging voltage of 14v, means it would be producing 420 watts, so, my net loss is 750 - 420 = 330 watts.

Again, using the 240v supply at 330 watts means that I would be drawing, in effect, 1.375 amps, and using the 22.5 amp hour value from before, I would only get about 16.3 hours at full load before reaching half charge.

Does this sound right? Or am I completely missing something? Any help on the matter would be greatly appreciated.

 

[FXST01]

You are forgetting about is:
the start up amps required for your equipment?
What gauge and length wiring are you using to power the inverter (from car to inverter)?
What gauge (normal extension cord?) and length wire are using from the inverter to your equipment?

go to Inverter FAQ - DonRowe.com - Frequently Asked Questions about Power Inverters about half way down the page is a calculator. You may not like the answer when you enter your figures.

 

[Comrad]

You are forgetting about is:
the start up amps required for your equipment?
What gauge and length wiring are you using to power the inverter (from car to inverter)?
What gauge (normal extension cord?) and length wire are using from the inverter to your equipment?

go to Inverter FAQ - DonRowe.com - Frequently Asked Questions about Power Inverters about half way down the page is a calculator. You may not like the answer when you enter your figures.

Thank you for the link, and I appreciate your thoughts on the matter, I do get your point about start up amps, but, they all are covered by my initial assumption of a peak load of 750 watts, we're only talking some fluro lights here, and maybe a battery charger. And yes, I was going to use an extension cord for the most part, as it is the same set up that is used for the shack when the generator is attached, I was just substituting the inverter for the generator, the devices should draw the same load, so, I'm not too concerned about that.

What I was really chasing was whether or not my figures and assumptions were correct, notably the current produced by an idling alternator and a rough figure as to the amp hours of a VZ commodore battery as a standard.

 

[wamboin23]

for starters, the inverter would be drawing more like 60-70 amps from the battery. 750/12=62.5.

even a small 150watt inverter would draw more than 3.125 amps. would need large gauge cable, at least 4 gauge, but if running the cable for more than 2 metres, have to go bigger.

and on a side note, it's a really good idea not to use your starting battery as a deep cycle battery. Starting batteries shouldn't be run down below about 80%, and you run the risk of been stuck with a flat/faulty battery.
To do it properly, you'll need a good quality deep cycle battery or two. A good quality 100Ah deep cycle battery goes for around $350-$450. Also would look into solar to keep it charged, and a dual battery system to charge the battery while travelling.

 

[WhiteLightning]

Whats the model and brand of the inverter? So we can get proper specs for it.
Seeing as it is turning DC into AC there may also be losses in the inverter that should be taken into account.

Generally speaking though wattage through a transformer stays the same. So if you want 750W of power at 240V you will need 750W of power (plus losses) at 12V as well

Ohm's law tells us that you need 53.5 amps at 12VDC to run 750W. a general spec deep cycle battery is maybe 100 150 AH so your looking at full charge to dead flat in three odd hours.

Ohm's law also tells us that you will need 3.125 amps at 240VAC to run 750W. An extension lead from your inverter should handle that current if you dont get carried away with length.

 

[WhiteLightning]

Whats the model and brand of the inverter? So we can get proper specs for it.
Seeing as it is turning DC into AC there may also be losses in the inverter that should be taken into account.

Generally speaking though wattage through a transformer stays the same. So if you want 750W of power at 240V you will need 750W of power (plus losses) at 12V as well

Ohm's law tells us that you need 53.5 amps at 12VDC to run 750W. a general spec deep cycle battery is maybe 100 150 AH so your looking at full charge to dead flat in three odd hours.

Ohm's law also tells us that you will need 3.125 amps at 240VAC to run 750W. An extension lead from your inverter should handle that current if you dont get carried away with length.

Start-up current shouldn't be of concern when it is just lights. But if it extends to fridges or anything with a motor then you run into trouble with start current. I wouldn't like to have every light on in the house and the 50 inch plasma and the xbox only to have the fridge come in and you lose the lot.

 

[cracker]

Whats the model and brand of the inverter? So we can get proper specs for it.
Seeing as it is turning DC into AC there may also be losses in the inverter that should be taken into account.

Generally speaking though wattage through a transformer stays the same. So if you want 750W of power at 240V you will need 750W of power (plus losses) at 12V as well

Ohm's law tells us that you need 53.5 amps at 12VDC to run 750W. a general spec deep cycle battery is maybe 100 150 AH so your looking at full charge to dead flat in three odd hours.

Ohm's law also tells us that you will need 3.125 amps at 240VAC to run 750W. An extension lead from your inverter should handle that current if you dont get carried away with length.

Start-up current shouldn't be of concern when it is just lights. But if it extends to fridges or anything with a motor then you run into trouble with start current. I wouldn't like to have every light on in the house and the 50 inch plasma and the xbox only to have the fridge come in and you lose the lot.

i agree with above comments, start up current wouldnt be a consideration... maybe look at running some bigger deep cycle battrys charged via solar and a generator?

 

[boomnvn]

Start-up current shouldn't be of concern when it is just lights. But if it extends to fridges or anything with a motor then you run into trouble with start current. I wouldn't like to have every light on in the house and the 50 inch plasma and the xbox only to have the fridge come in and you lose the lot.

Dont forget anything with a heating element will also be high wattage.. for example, my sunbeam 4 slice toaster is rated at 1500-1630w.
Power tools also heat up alot like angle grinders, drills etc....

cheers,
spudley.