If this is completely mis-informed rubbish please accept my apologies and I would welcome some educational feedback!!.....Here goes:

For simplicity I'm taking standing charges out of the equation - when I got one of those monitor things from my provider they told me to average it out over the month which got me a flat 16 pence per kilowatt hour. (and apologies to the purists if I get capital and lower case initials wrong)

So, if I had a 1 kW heater for example - leaving it on for 1 hour would be 1 kWh used and would cost me 16p (for 1 kWh). So if I left it on 24/7 it would cost me

Per Day - 16p x 24 hours = £3.84

Per Week - £3.84 * 7 = £26.88

Per Month - £3.84 * 30 = £115.20 (other numbers of days per month are available )

But the Hummy does not use 1kW per hour - the manual says max 28W down to <1W in standby with power saving on. I'm guessing that if the disk isn't spinning then it's probably the fan that draws the lions share of that(?) But lets assume if it's on it was using the full 28W all the time - you could think of that as 0.028 kW so for an hour it would be 0.028kWh which multiplying that by my 16p would give me just under 0.5p per hour. So IF (and I'm crossing my fingers about this) we apply that to the factors above I get just under 11p per day or £3.22 per month and therefore slightly over £38 per year total (remember that is assuming the box is using the full 28 watts all the time 24/7).

If you switch off at night and use idle and timers to control on/offs wherever possible during the day then that figure can only get lower.

Granted on standby at <1W you'll consume even less than that but by my reckoning the most money you could save would never be more than the £38 - and given that you will have be box on for some amount of time most days then you wont save all of that.

OK that's my interpretation at least - I'm sure someone will be able to put a formula up that cuts through all my words and sums it up in less than 10 characters but I'm not a numbers man so I couldn't have done that I'm afraid.

Now for the dissection (please be gentle with me)....

[EDIT - corrected use of capitals to appease the Head Master, or is that Headmaster, or Hm or HM?]