Electric motor power consumption and maths question

haaammit · 18 May 2012 at 11:27

We have a borehole for our water, and I want to work out how much per year it costs to pump the water up to the header tank.

Using the average of 150 litres per person per day means an annual consumption of 54750 litres per person per year.

The pump is 800w (this is also the approximate amount shown on our electricity energy metery thing when it starts and stops). It takes approximately 20 seconds to pump up 10 litres of water, so each litre takes 2 seconds.

Maths time.

54750 lits x 2 seconds = 109500 secs /60 = 1825 mins /60 = 31 hours.

The pump runs at 800w, so 31x0.8= 24.8 units per year per person? Have I got that right? Less than £6 per person per year?

Motor consumption time - does the motor use a burst of power to start up, or will it just use 800w all the time?

I hope that makes sense - many thanks

FreeStream · 18 May 2012 at 11:32

Peak consumption I suspect would be higher, but only for the time needed for the motor to come up to nominal RPM.

But your maths seem sound.

dirtychinchilla · 18 May 2012 at 11:33

It looks right to me, but for your power consumption, how do you know that 800w = 1 unit? Assuming you've been given that it's fine.

For the motor, I think you would assume that power consumption is constant. I can't imagine they'd want more...

FreeStream · 18 May 2012 at 11:37

dirtychinchilla said:
It looks right to me, but for your power consumption, how do you know that 800w = 1 unit? Assuming you've been given that it's fine.

For the motor, I think you would assume that power consumption is constant. I can't imagine they'd want more...

I think his 24.8 units per year is actually 24.8 kWh per year

24p/kWh seems a tad high, but reasonable.

haaammit · 18 May 2012 at 11:37

I believe that one unit is a kilowatt hour, so say you have something that uses 1000w of power, it will use1000w of power, or one unit (kilowatt hour) per hour. Something that uses 500w per hour uses half a unit per hour (I think that's right

)

So a pump running at 800w uses .8 units per hour

div0 · 18 May 2012 at 11:40

dirtychinchilla said:
It looks right to me, but for your power consumption, how do you know that 800w = 1 unit?

For electricity 1kWh = 1 unit (kWh = kilowatt hour)

So 31 hrs of 800W => 31(hrs) * 800(watts) / 1000 = 24.8kWh = 24.8 units of electricity.

yer_averagejoe · 18 May 2012 at 11:43

You'll probably pay something like 14p/unit so £3.50 per person, per year.

haaammit · 18 May 2012 at 11:56

Many thanks everyone