The International System of Units, or SI from the French Système International is a wonderfully clever refinement of the original metric system.
Took me a while to get all this clear, but it really helps to understand electrical “units”:
- power says something about intensity: volts times amperes, the unit is watt
- energy says something about effort: power times duration, the unit is watt-second
- current says something about rate: charge per time unit, the unit is ampere
- charge says something about pressure: more charge raises volts, the unit is coulomb
Of course, some units get expressed differently – that’s just to scale things for practical use:
- a kilowatt (kW) is 1000 watts
- a watt-hour (Wh) is 3600 watt-seconds
- a kilowatt-hour (kWh) is 1000 watt-hour
- a milli-ampere (mA) is 1/1000 of an ampere
- a micro-coulomb (µC) is 1/1000000 of a coulomb
But there are several more useful equivalences:
- When a 1.5 V battery is specified as 2000 mAh (i.e. 2 Ah), then it can deliver 1.5 x 2 = 3 Wh of energy – why? because you can multiply and divide units just like you can with their quantities, so V x Ah = V x A x h = W x h = Wh
- Another unit of energy is the “joule” – which is just another name for watt-second. Or to put it differently: a watt is one joule per second, which shows that a watt is a rate.
- A joule is also tied to mechanical energy: one joule is one newton-meter, where the “newton” is the unit of force. A newton is what it takes to accelerate 1 kg of mass by 1 m/s2 (i.e. increase the velocity by 1 m/s in one second – are you still with me?).
- So the watt also represents a mechanical intensity (i.e. strength). Just like one horsepower, which is defined as 746 W, presumably the strength of a single horse…
- Got a car with a 100 Hp engine? It can generate 74.6 kW of power, i.e. accelerate a 1000 kg weight by 74.6 m/s2, which is ≈ 20 km/h speed increase every second, or in more popular terms: 0..100 km/h in 5 seconds (assuming no losses). But I digress…
The point is that all those SI units really make life easy. And they’re 100% logical…