Can you clarify how I've gone wrong on my math.
Surely its just...
36w x 12 hours = 432watts per day.
432 x 90 days = 38,880 / 1000 = 38.8Kwh
38.8kwh x 0.25p = £9.72
so a 1x400w HPS would mean over 90 days a cost of.... £432. Thats not really 30-40 quid a month.
400 x 12 = 4,800 x 90days = 432,000 / 1000 = £432
Where you're going off with your calcs is in the idea that it's actually 36w X 12 hrs. to begin with. You're not correctly factoring in your kw/h costs from your utility company, which would give you a more accurate idea of your totals. The bottom line without going into all the details is regardless, you'll get far more high quality results and larger yields that will figure much less in power consumption and related 'grow' costs if you go with a HID (MH or HPS) setup of any wattage range. Heat issues aside, those who bloom out their plants with enough CFLs wind up using as much or more power consumption/costs over a single higher lumen output HID light that would deliver superior yield, taste and potency results anyway. And it's not just a Kelvin (light spectrum color) issue. Overall lumen output per watt is in fact more important than anything, as long as the spectrum is reasonably balanced. Even a cluster of (6) 35w CFLs right in the face of the tops just can't put out as many lumens as a single HID using the same watts total as the (6) CFLs combined. The lumen per bulb output isn't greater simply because there's more CFL bulbs. The same is true even with HIDs. The lumens per watt of power usage is higher for a 1000w HID than say a 150w or even 400w HID. A cluster of (6) or (7) 150's that about equaled the wattage of a 1000w HID still wouldn't put out as much light in lumens overall as the 1000w.