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A Frequently Asked Question is can I run my (insert electrical appliance of your choice) from a mains hook up or from my battery?
To answer the question you need to know something about the particular device that you want to run. The first thing you want to know is the items Wattage (a measure of power) or its Amperage (a measure of the current flowing through the device).
You can convert between the two by knowing one other fact, and that is the operating voltage. The three are related by this simple formulae which I will show in two useful forms.
Watts = Amps times Volts and Amps = Watts divided by Volts.
So for example an 800 Watt heater rated for 240 Volts will draw around 3.3 Amps
Or a mains hookup that can supply 16 Amp will support up to 3,800 Watts.
This gives an easy way of seeing what you can have switched on at the same time just add up all the Wattages of your appliances having calculated what wattage your mains can supply.
Two more examples
A 1200 Watt hair-drier will draw around 5 Amps from a mains supply.
The same 1200 Watt hair-drier if supplied from a 12V battery using an inverter will draw 100 Amps, this sounds a lot and you are right, it is!
Forgetting about power for the minute and thinking just about current. If you pass current through a wire, the wire gets hotter. The simplest electrical items the ordinary lightbulb, the convector heater and the fuse make use of that fact, using special wire.
Ordinary copper wire in your cables also gets hot when you pass current through it and this is an effect we want to minimise, you don’t want them to melt or catch fire. In simple terms the thicker the cable the more current it can pass without getting too hot.
There is one other factor to consider and that is volt drop. As current passes along the wire some of the volts don’t make it to the far end. Again the thicker the cable the less this volt drop will be. Also this volt drop is related to the length of the cable. So the thinner or longer the cable or the greater the current the higher the volt drop. Moral of the story keep high current cables short and thick.
Going back to the 1200 Watt hair-drier you can see that the battery is supplying 100 Amps to the inverter at 12 Volts but the inverter is only supplying 5 Amp to the hair-drier at 240V.
The wires between the inverter and the battery will need to be very thick and very short. Also ponder the question: How long will an 85 Amp hour battery last with 100 Amps coming from it?
Some typical Powers (Watts) and Currents (Amps).
In all the examples above the UK mains voltage (nominally 230V) is assumed to be about 240V.
Please feel free to comment - I managed it without Ohms law but I found it difficult to know what level to pitch it at.
Last edited by sallytrafic on Fri Mar 13, 2009 8:30 am; edited 1 time in total ______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
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Excellent piece of work.
Don't want to complicate matters but shouldn't one allow for extra current on startup?
My hoover uses more than its nominal 1200 watts when it starts up. Surely if you're on EHU and using near to the max amperage anything with a motor, for instance, will momentarily take you to the point of tripping out the supply.
Well done! That is clear and non-technical - EXACTLY what is needed.
As a science teacher it is the sort of basic information that GCSE students SHOULD know!
Fortunately my "speciality" is biology! I did basic electricity at school and must confess I wish I had read what you have written - it makes much more sense than my memory!
May I suggest one slight modification? If you read through quickly (as many people do) the line;
Watts = Amps times Volts and Amps = Watts divided by Volts.
makes it possible to run it together, if you put it as;
Watts = Amps times Volts or alternatively Amps = Watts divided by Volts.
it becomes much harder to run the two together. Just a suggestion which others may or may not agree with.
Strangely enough, amps and so on is one technical aspect of motorhoming that I do actually understand. None the less, this post does make light reading that is easy on the brain.
Nice work!
Russell
______________________________________________________________ Our long trip around Europe - blog now online at www.swift-kontiki.co.uk
Might also be worth pointing out that some items such as microwaves quote a power output not a power used. For example a 750Watt microwave will actually draw quite a bit more. For example my 750W microwave actually pull 1080Watts.
Excellent piece of work.
Don't want to complicate matters but shouldn't one allow for extra current on startup?
My hoover uses more than its nominal 1200 watts when it starts up. Surely if you're on EHU and using near to the max amperage anything with a motor, for instance, will momentarily take you to the point of tripping out the supply.
Start up current would not be easy for a lay person to take into consideration as it depends on all sots of ifs and buts, inertia, capacitance, inductance, ramp up time and for small motors tends to be more of a spike than anything more sustained. Most overcurrent devices are fairly tolerant to this kind of effect unless current demand is already high. Franks piece does what it says on the tin.
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but I found it difficult to know what level to pitch it at.
Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray 
