FRIDGES

G'day folks, lets see if we can make these muddy waters a little less muddy.

So let's start with Amp-hours verses Amps....

I think it's easer 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 compressor motor fridge, and this fridge draws 5 amps while it's compressor motor is running, now if the compressor motor ran for a whole hour the fridge would have drawn 5 amps in total for that hour, or 5 amp hours (5Ah), and if it ran like this for 24 hours it would have drawn and consumed 120 Amp-hours in total, 5Ah x 24h = 120Ah.

But the fact is the compressor motor should not be running continuously for the whole 24 hours, they should cycle on and off as required to keep the temperature constant in the fridge, so if it was running at say a 50% duty cycle, i.e. running on and off for only half of the hour in total, it would still be drawing 5A when it's running, but for only half the time, so that would now be 2.5Ah it draws from the battery each hour, and 60Ah over a 24 hour period.

So along these lines lets look at say an electric 12 volt water pump, it may draw 10A while it's running, but if it only runs for a total of 15 minutes each day, it's total draw for the whole day would be only 2.5Ah, not much 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.

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 = 120 Watts

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

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

So 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.

Inverters

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.

Val Rigoli Copyright ©