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Amps, Amp-hours, Watts  What's that all about?

Article by Val Rigoli ©, free advice freely given, my personal thoughts and advice gathered from in-excess of 40 years of practical hands-on experience, learned skills, and industry knowledge.
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G'day folks, lets see if we can make these muddy waters a little less muddy.

So let's start with finding out what the difference is between
Amps, and Amp-hours!

I think it's easier to understand if I give you some easy examples to work through.

For the sake of this explanation lets say you have a 12 volt fridge with a compressor motor, and this fridge draws 5 amps while it's compressor motor is actually running, now if the compressor motor ran continuously for the whole hour, the fridge will have drawn 5 amps in total for that hour, and we express this as 5 Amp-Hours (5Ah), and so if it ran like this continuously for 24 hours, it would have drawn and consumed a total of 120 Amp-Hours (5Ah x 24h = 120Ah).

However the fact is the compressor motor should not ever be running continuously for the whole 24 hours, it should cycle on and off as required to keep the fridge at a constant temperature, to whatever you have set it to in the fridge, so now if it was running on say a 50% duty cycle, i.e. running on and off for a total of only 1/2 an hour running in total within that hour, it would still be drawing 5A while it's running, but seeing it's running for only half of the hour, it's total power consumption for the hour would now be just 2.5Ah, and just 60Ah over a 24 hour period (2.5Ah x 24h = 60Ah).

So along these lines lets look at say an electric 12 volt water pump, while it may draw 10A when it's running,
but if it only runs for a total of 15 minutes each day, it's total draw for the whole day (24h) would be just 2.5Ah, so not much at all.
Really, given the very limited amount of water that can be carried on-board by most folks,
we can expect that in most cases water pumps get used for less than 5 
minutes a day,
so hardly worth worrying about what 
water pumps consume power wise at all! 

                              Watts that you say?                           

Ok some electrical appliances are rated in Watts rather than Amps, but that's ok, it's easy  to convert these Watts to Amps.

You simply take the stated Watts, and divide that by the Voltage and this gives you the Amps that the item will draw

So lets say for example you have a 120 Watt light globe (like the type you find in some car spot lights) and divide this 120W by the 12V that is used to power it and you end up with 10A at 12V (120W ÷ 12V = 10A).

This also works the other way around, if you have the amps and the voltage of an item and you want to know the Watts, simply multiply the amps by the voltage and you have the Watts.

So for example remember that water pump that was drawing 10A at 12V, well 10A x 12V = 120Watts

Now this works for any Voltage, Watts and Amps combination.

Like a 100W household light globe at 240V = 0.41A.

So if you had a 2400W room heater, at 240V = 10A, and this just happens to be the most you can draw out of a standard household power point, 10A or 2400W.


When trying to work out how much power will be drawn from your batteries by an inverter, a very rough but easy calculation is for every 100W at 240V, you can expect the inverter will draw about 10A at 12V from your batteries, it really works out a bit less than this but it's close enough for doing quick calculations.

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