People thinking that they are saving money from more energy efficient products

[DanceswithUnix]

Wonder no longer. Randall Munroe has dealt with this very question: third one down on this What If page: http://what-if.xkcd.com/23/

It's not very much at all, sadly

He seems to be mixing square feet with meters, you lose space probes that way!

So I think he has worked out the case for a 1500m2, which is huge and still not effective. Oh well

Edit:

OK, he is converting the water hitting the roof into litres, which makes sense, so he can calculate the potential energy.

1500sq ft is about 140sq m according to an online unit converter.

So 4m of rain across the roof is 4x140 = 560 m^3 of water. That is 560000 litres of water. That is 1kg per litre, so 560000kg of water.

Potential energy available is m.g.h so 560000 x 9.81 x 5 = 27468000J of energy, or 27468 kJ or 27.468MJ.

A Watt is a joule per second, so kJ = kWs. We want Wh to get normal domestic units.

1kWh is 3.6MJ, so we have 7.63kWh of power, which at the 11.5p I pay is nearly 90p a year

So his result is correct, he must be skipping some steps in the working?

[scaryjim]

... his result is correct, he must be skipping some steps in the working?

Yeah, he just lines up all the figures he's previously mentioned then gives the answer without necessarily showing all the conversions. That's why his equation has all the units listed with the figures. Americans mix imperial and metric with wild abandon at times

It's well worth a read through the back history of What If if you've got a spare Sunday at some point - a surprisingly large number of then end catastrophically

[peterb]

Peterb, do you happen to be familiar with the official electric meters process? Are they still just simple electromagnets turning cogs due to the hot wires current draw?

Sorry for the long delay replying.

The older mechanical meters basically induction motors - the disk rotates at a speed proportional to the power consumed by the load, and that disk drives the dials or display.

Modern meters use electronics to measure current, voltage and phase angle to give the same result.

There is an article here: http://en.wikipedia.org/wiki/Electricity_meter