what size battery for a 1500w invertor

[chrisarvor]

am i rightr in thinking a 120amp battery will run the invertor which will only be supplying a small microwave.
the microwave will only be used to heat up pasties etc and on return to the mooring i can charge up the battery on the shorepower.

 

[aluijten]

Should give yo something like half an hours microwave time untill 50% of your battery charge is gone. Take care that you use a battery with current capacity (high drain).

Arno

 

[duncan]

he would be able to heat pasties for the entire fishing fleet in half an hour! - but your point is well made.

 

[pvb]

Yes, but not for long...

You won't get half an hour's use. Your inverter will take about 100A when running the microwave. At this current draw, a typical 120Ah battery will be 50% discharged in about 10 or 15 minutes. You certainly shouldn't discharge it below 50% regularly, or you'll soon have to buy a new battery! So if you can heat your pasties in 10 mins or so, you'll be OK.

If this doesn't seem to make sense, it's because battery capacity isn't a fixed value. The battery you think of as 120Ah is most likely rated on a 20hr rate. So it would theoretically supply 6A for 20hrs. But if you discharged it at twice the rate (12A), it wouldn't last for half the time (10hrs) but only for around 8 hrs - so its capacity would have dropped from 120Ah to 96Ah (12A x 8hrs). If you discharge it at say 60A, it would probably last for less than an hour - so its capacity would have dropped to only about 50Ah. And it follows that taking 100A out of it will reduce its capacity still further.

Depending on the size and type of your domestic battery bank, a better solution might be to add the extra 120Ah battery to the domestic bank, and just run the inverter off the (bigger) domestic bank. The reason for saying this is that the larger the battery bank, the smaller the percentage discharge for a given current draw. Deep discharge cripples batteries. This would also give you a better capacity in the domestic bank for everyday use other than the microwave.

 

[Piers]

That's a hefty inverter since most microwaves are either 1kW or 600W. However, bigger is better, as they say.

First, remember you should not run a battery down more than 40% of its fully charged state.

Second, as has already been said, use one that has a current limiter.

Third, you need to decide if you need a pure sine wave inverter, or one of the less expensive pseudo sine wave systems.

 

[aluijten]

Re: Yes, but not for long...

You also could consider buying a 12 volt Microwave to avoid some losses:

"http://www.ukcampsite.co.uk/classads/forsale/view.asp?ad=4-10-14-1903&subj=12+VOLT+MICROWAVE"


Arno

 

[BlueChip]

Re: Yes, but not for long...

Would it also make sense to use a car starter battery as these are optimised for heavy current loads over a short periods and are much cheaper than marine batteries?

 

[Anonymous]

[ QUOTE ]
the microwave will only be used to heat up pasties etc

[/ QUOTE ]Have you ever eaten a pasty reheated in a microwave? Soggy and horrid - they are better cold!

 

[npf1]

But a good sandwich is even better!!!

 

[Gunfleet]

Re: Yes, but not for long...

Those draw 65 amps @ 12v!

 

[duncan]

Re: sandwiches don\'t compare on a cold wet day\'s fishing

even with good soup!

 

[VicS]

But remember microwave ovens draw much more power than their rated output. My 800watt oven draws 1420watts!

 

[chrisarvor]

so does this mean a car battery of 120 amps would do the job?

 

[aluijten]

Short and sweet:
Yes for short period and dedicated battery
No if shared battery.

If you would pick a 12volt microwave you would at least loose two sources of losses Your inverter will give you a loss of 15-25% and the power transformer in the mircowave another 10% or so. I'm not sure how effective a 12 volt microwave is but you could compare the output wattage versus the input wattage ratio from a 12 volt model to a 230-240 volt model. My guess is that a 12 volt model is slightly better in this respect. That would give you more usability for the 12 volt model and you wouldn't have to buy an expensive inverter.

Arno