# POWER

Discussion in 'Tech/Mech General' started by funflair, Apr 14, 2015.

1. ### funflairFunsterLife Member

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OK this idea for a simple lesson on power came to me whilst reading a thread about inverters and a comment was made that the OP would be well served to learn the basics of power, I am in no way an expert but will give it a go for those that understand even less than I do.

First I googled the definition (so that shows how much I know) the answer that suited this little lesson was,
the rate of doing work, measured in watts or less frequently horse power

The rate of doing work or the potential to do work as expressed in watts or horsepower is quite familiar to a lot of us when we talk about engines, this is mechanical energy produced through burning fuel, but the same applies to electrical energy and as motorhomers and users of electrical power we should try to understand some basic principles.

watts (W) volts (V) and amps or amperes (A)

So starting with the basic principle that the amount of work we want to do can be measured in watts we can easily understand that a 2000 watt (2kw) kettle will do more work (in a given time period) than a 1000 watt (1kw) kettle BUT both kettles will boil the same amount of water although the 1kw one will take twice as long so they actually use the same amount of energy.

OK why does this matter, well it could be the difference between sat inside a nice warm motorhome on a winters night and rummaging around outside trying to reset the trip on the hook up post, the trip will be set to a given number of amps which is the rate of flow of the electrons that you are using, to ensure that you do not upset the trip you need to be able to work out how many Amps you are using so that you can switch off the heating for a couple of minutes while you boil the kettle if indeed that is required. To enable you to make this decision you need to remember one basic formula.

W=VxA, watts = volts x amps, simples. You can now work out how many amps you are using if you have a 2kw kettle and are running from the electric hook up which in the UK should be pretty close to 240 volts, so you have 2000(2kw)=240xA now you just need to remember the basic rules for transposing the equation to be left with amps as the answer, you take the Volts to the other side of the = but it must now go under the W as it has crossed over the = so 2000/240=8.33 Amps, so you now know that if you also have a 2kw heater running as well and only a 10 Amp trip it would be a good idea to switch off the heater for a few minutes while the kettle boils (must be easier than running outside in your PJ's).

If you are still with me I said at the start that this was prompted by a thread on Inverters and batteries, up to this point we have assumed that the motorhome is plugged into the site electric and that your only potential problem is the trip on the electric post. If you you dont have the luxury of site electric you can still get 240 Volts from your 12 Volt batteries using an "inverter" this is great if you want to be totally self sufficient BUT it makes it even more important that you understand the basic rules of power as your only supply now is what you have stored in your batteries and once you have used this you need to be able to put it back in for next time.

Now going back to the 2kw kettle but running off an inverter the same equation W=VxA or A=W/V will still apply BUT the voltage is now supplied by your batteries so it becomes 2000/12 which means that you are taking 166 amps (plus a bit more for efficiency losses) from your batteries, a decent battery bank will supply this and indeed has been the subject of other threads so I will leave it alone other than warning that any battery bank has a limited capacity measured in amperes per hour or ah.
If it takes 6 minutes to boil the kettle that is 1/10th of an hour so you have used over 16 ah from your battery which sounds OK but you do need to put this back so that you can do it again and again, you might have a 100watt solar panel and your read out tells you it is making 4 A as the sun is shining so it will take at least 4 hours to put back what you have taken out in 6 minutes if you have 4 panels and make 16 amps it will only take about 1 hour, now if you also use the microwave for 5 minutes and somebody (not me) uses the hair dryer for another 5 minutes you could easily have used 50 ah which you will not get back from just one panel giving 4 amps but you would stand a good chance with 4 panels as long as the sun stays out.

This is not meant to be a guide to matching your Inverter and battery bank size to your solar panels capacity that could be another thread, it is simply meant to give you the ability to be able to judge the power that you are using and be able to equate this with the rate of the hook up trip or the capacity of your batteries and the rate that your solar installation recharges your batteries. There are other ways to charge batteries but we can leave that for another day as they are likely to involve engines of some sort.

So, hopefully I have made this look quite simple, just remember W=VxA so A=W/V and if the V comes from your batteries you have to use 12V not 240V, it might be worth making yourself a list of your electrical equipment and the amps that it uses at 240V and at 12V if you intend to use an inverter, the wattage should be marked on kettle hair dryer etc.

If you have managed to read this far you deserve never to trip the site electric or have flat batteries because you have left the inverter on for too long BUT if it happens dont blame me.

As a last note of warning I will add that most batteries dont like being discharged below 50% of their nominal ah capacity so 50ah as in the above example would flatten your 100ah battery, there again you would not run a 2kw inverter off just one battery of 100ah.

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Errrrrrr.
Yes

If it were so simple !

3. ### DavidG58Funster

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jeepers Martin, I would recommend not even starting to read if you have consumed any alcohol, I know so much less than you and still do

will try again in the morning

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4. ### funflairFunsterLife Member

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It might not be that simple but it looks like an edition of war and peace already, and I am a mechanical not electrical engineer.

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5. ### funflairFunsterLife Member

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I wrote the second half after a couple of glasses of Red so possibly I should read it myself in the morning.

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6. ### John LaidlerFunster

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A valiant attempt! Full marks. Seriously.

I suspect some might argue this is a subject so important it should be made a "sticky" but I wouldn't agree. It needs to be discussed at length until the arty types begin to understand the sciences!

Now revise it explain why the term VA is sometimes used instead of W when discussing AC. Hint: phase shift.

'tis a bit complicated but it will be explained at Cern when they manage to hit the Bosun from HMS Higgs.

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7. ### funflairFunsterLife Member

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I read up on VA some time ago and decided it was easiest to just call them watts and certainly close enough for my mental calculations.

The site we were on in Spain had a 13 amp trip at the post (great) but a 5Kw/h per day usage, imagine me trying to explain that to my wife. And why it takes 3 minutes in the microwave off the inverter to heat milk for two frothy coffee but 4.5 minutes on the crappy Spanish electricity.

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8. ### John LaidlerFunster

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As a lapsed member of the Institution of Electrical Engineers, former President one Michael Faraday - I agree with you!

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9. ### JJFunster

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Knowledge is POWER.

JJ

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