Actual Power Requirement

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David B

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My battery charger shows readings as follows on my power meter.

240v 

MAX Watts  161

MAX Amps  1.12

Power Factor 0.6

I want specify a 12v to 240v Inverter - what capacity should I specify - based on the wattage or the ampage?

 
Not enough information there to try to help you.

What system is it?  What do you want it to do? 

An invertor is usually specified for the load it is required to drive and the available power source .

 
My battery charger shows readings as follows on my power meter.

240v 

MAX Watts  161

MAX Amps  1.12

Power Factor 0.6

I want specify a 12v to 240v Inverter - what capacity should I specify - based on the wattage or the ampage?


They are normally sold by wattage, but be wary what you buy. You want pure sine wave, and I usually work on double the wattage you think you need.

 
They are normally sold by wattage, but be wary what you buy. You want pure sine wave, and I usually work on double the wattage you think you need.
Many thanks - pure sine wave certainly also double the wattage - good advice.

My quandary is that following Ohms Law V x A =W  the wattage of my battery charger should be 268.8 but the shown value according to my power meter show 161w. Clearly if this is multiplied by the Power Factor of 0.6 = 161w is correct. So, should I base my requirement on double 161w or double 268w?

P.S. The figure I quoted were what my power meter was showing while charging the battery so actual figures.

 
You can't directly relate the charge rate of a battery to the discharge rate. Time and battery capacity both come into the equation.

For instance you could take 100 hours to charge a 100AH battery at 1 amp, but then discharge it over , say 25 hours at 4 amps.  That's an over-simplification disregarding efficiency and losses, but illustrates the point.

 
Thanks for your thoughts Guys - I think you are misunderstanding my question.

The background is that I have a mobility scooter, the onboard batteries being charged by a 240v charger when I am at home.

My problem doesn't not relate to the charging of the batteries when I can attach the charger to a 240v socket.

My car has a hoist so I can carry the scooter in the back.

What I want do do is use the 240v charger to charge the scooter batteries when I am driving.

I therefore want to buy a 12v to 240v pure sine wave inverter to power the charger - but what capacity inverter do I need?

My quandary is that following Ohms Law V x A =W  the wattage of my battery charger should be 268.8 but the shown value according to my power meter shows 161watts. 

The figures I quote were what my power meter was showing while charging the battery so actual figures.

If I want to buy an inverter of, say. double the wattage required to be on the safe side, should I but one that can provide at least 322watts as per my power meter reading or one that can provide at least 535watts if I apply Ohms Law V x A =W?

 
OK, I understand the question now. I agree with Binky's advice above. Don't skimp on capacity.

Low power inverters don't increase in price proportional to their power and for reliability it's worth getting one well above your nominal load so its semiconductors can have a nice unstressed life. 

I just looked at CPC, and a Duracell branded 800watt one is about £70. You can pitch your rating lower if you want, but you won't save much for a quality one.

You also want substantial DC wiring in your car. Have an auto electrician wire you a fused supply straight from the battery. Don't try to rely on the cigarette lighter socket.

 
Many thanks Geoff - I will take your advice. I'm still confused about the difference between the reading on my power meter which says 161watts and what Ohms Law says it should be based on the Amps X Volts. Not to worry, an 800watt inverter seems to fit the bill if connected to a fused supply directly from the battery. Many thanks again. 

 
Many thanks Geoff - I will take your advice. I'm still confused about the difference between the reading on my power meter which says 161watts and what Ohms Law says it should be based on the Amps X Volts. Not to worry, an 800watt inverter seems to fit the bill if connected to a fused supply directly from the battery. Many thanks again. 


Chinese product manufacturers lie  :^O . With battery chargers many will control the power being supplied to the battery to prevent overheating /  charging too fast. It's a complicated subject and depends on the battery materials, so for example, a fully discharged battery will be trickle charged slowly to prevent damage until it has about 15-20% charge, when it can be charged faster upto about 80% when charging slows down again. 

 
Don't worry too much about the figures. Ohms law, in its simplest form works perfectly for DC.

AC is a whole lot more complex mathematically, and using ohms law gives only a rough indication.

Add to that your power meter isn't a precision calibrated instrument, (though one I have seems quite good), and the ratings plates on things like battery chargers are also not to be relied on. 

Being maximums, they will assume highest possible input voltage  and maximum output current,  -conditions which you probably don't have.   

 
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