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Electrical Units

Explained Badly

It has become paaaaiiiiinfully clear to me, whilst researching solar panels and power systems in general that no-one on the internet really knows the difference between watts, volts, amps, amp-hours, watt-hours, and a hole in the ground. The internet also doesn't know the difference between an amusingly shaped root vegetable and a viable marital aid but that's a discussion for a different website. (Spoiler: bravery.)

So without further ado, I'll try to explain…

Volts are a measure of potential. A bit like water or air pressure in a pipe or container, volts do not “do” anything by themselves, they're a measure of how much “push” there is to force electricity around a circuit when you turn it on. Lots of volts does not equal lots of work done - imagine filling a swimming pool using only a pressure washer, sure there's loads of pressure (volts) but there's not a lot of water coming out of the end.

Amps are a measure of flow, how much electricity is flowing through or past something. This is like the amount of water flowing in a pipe - it doesn't matter what the pressure is in the pipe, the water could be sitting still (no amps flowing) or rushing past (lots of amps flowing). This is why, to measure amps, you have to break a circuit and put the ammeter in-line with the flow, so all the current flows through the meter.

Watts are a measure of power or work done. In electrical terms they are a product of volts times amps (V * A), so the amount of work your water can do.

A few simple examples, if you imagine trying to turn a big water wheel with these you'll hopefully see how they relate to work done (Watts):

  • Hosepipe = low pressure (volts), low flow(amps), will never move the wheel (zero work done, zero Watts)
  • Pressure washer = high pressure, low flow , might eventually get the wheel turning but won't generate much power (Watts)
  • The big pipe at the bottom of a dam = High pressure, high flow, tonnes of power (Watts, or more likely, MegaWatts, fuck yeah!)

Yeah, you're not going to put your finger in that to stop it:

The smart ones of you may have realised that you can do the same amount of work different ways:

  • 1 Amp of current at 10 Volts = 10W of power
  • 10 Amps of current at 1 Volt also = 10W of power
  • 10 Amps of current at 10 Volts = 100W of power
  • …Hopefully that gives you the picture!
    • If not, go off and read some more, or just give up and trust me - I'm on the internet, why would I lie?

This is a good illustration of the difference, here is some low-voltage high-current cable: See it has lots of copper to carry all the current, and relatively thin insulation as the low-voltage doesn't try too hard to escape.

…and here is some high-voltage low-current cable: See how it doesn't need anywhere near as much copper to carry the current but it has very thick insulation to stop the high voltage from jumping out (arcing) through the insulation, a bit like a pipe carrying very high pressure would have thick walls.

Amp-hours or as seen on your iPhone battery, milli-Amp-hours (mAh) may sound a bit like they should just be called Watts but they aren't, they're a subtly different measure of capacity. In battery-land they're a measure of ability to supply an amount of electricity based on various standard assumptions about how far the battery voltage is allowed to drop before it's either no longer useful to us, or the battery suffers damage and loses its ability to hold as much charge next time round. In theory, a 100Ah battery should be able to supply 1 amp for 100 hours after being fully charged, or 100 Amps for 1 Hour, or 10 amps for 10 hours. In reality it may not almost certainly won't manage all of those depending on what sort of battery it is, and amp-hour ratings are really just a rough guide to how much a battery holds.

Watt-hours or more usually (as seem on your electricity meter) kiloWatt-hours (kWh), are also a subtly different measure of an amount of electricity. They are (in electricity meter terms) a running total of the total amount of work done.

Further reading for the terminally curious

Your kettle may be 1kW (1000W) but it only takes a few minutes to boil, whereas your energy-saving light-bulb may be only 10W but it gets left on for many hours, in the course of a day they may both do exactly the same amount of work (use the same number of Watt-Hours) despite being very different power ratings.

Note that there is a lot more to it than this, but this is the basic bare-bones imperfect-reality adjusted for the modern hard-of-thinking internet denizen of limited attention span. There have been at least a couple of books written about electricity, and there may even be website about it, if only there was some way of finding it.

electricals/vir.txt · Last modified: 2019/05/15 23:33 (external edit)
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