The comparison with the fridge is not a good one; the HPS bulb consumes the said watts for as long as it is on. The fridge and fridges on the other hand have a thermostat; as long as the desired temperature is reached, the cooling stops and the fridge relies on good insulation to maintain the temperature as well as transient boosts from the cooler.

A better comparison would be a HPS bulb vs a fridge with the door open at all times.

just to plug some numbers in, a 400 bulb burning 16 hours is 6.4kwh / day or 192kwh/month. This costs roughly £25 per month or $40 per month where SWIM lives. That's shit loads, far more than a $5 estimate. With $40 a month you can order 2 feet of live cactus!

Furthermore, after checking SWIM's bill, he burns around 350kwh/mohtn. An increase of 192kwh/month are actually an 64% in electricity consumption!! Now, that is really a lot!

I am talking about the amount of electricity used in one month. Not at a given time which I have no clue about. The price and watts used for the month is the same for an HPS 400 watts system and a mini fridge.

The $5 ($15 at most depends on how much it is used) is not an estimate but from experience by SWIM and many of her friends. SWIM was suprised her self. Maybe it depends on the system (ballast) one gets and obviously the duration of use but the increase in bill is barely noticeable. Everyone always says it is going to be so much and ... but from experience of many people it is not much at all.

Also some people want to grow these cacti and are not interested in how much they can get to eat in a year. There are other reasons to keep these cacti including hobby, scientific research in the chemical content and so on ...

Hmmm now that Ive read this thread Im not so sure anymore about those LEDs.
http://www.bcss.org.uk/forum/read.php?1,122440
Maybe when they have perfected the technology, in a couple of years..

Anyway, just to get an idea, see picture.
Of course this setup looks like shit, but you can see the surrealistic colour of the light, and the use a spot.

Woow these guys are pro.
Some nice pictures there, I like the blue colour of the LEDs. Thats different from the pink colour I have seen earlier.
The two colours would allow for some creativity when designing a little dessert.

About the fluorescents you use, they are long or compact?
Do you use spots?

Another picture of a LED spot growlight. Or should I say grow LED spotlight?
Anyway, as far as I have understood it fits regular light bulb fitting, and will last at least as long as most common fluorescents. The colour of the light might be a problem, I dont know how intense it will be in real life. But since it is fairly focused (as in spot) it would not necessarily dominate the room. HPS probably would. In another situation the cactus and spot could be placed out of sight, lets say on the window sill of the attic. So to supplement with winterdaylight. Perhaps some neighbours will start spreading rumors, but they would not be taken too serious anyway.

Some of the people growing MJ are already trying the LEDs. They put them in big panels.
I will find out about their results. And also I would like to know more (not just the salestalk) about the amount of light the different spots provide (9W - 15W - 25W - ...), and for how long without significant loss of intensity. I have some more homework to do.

The more I think about it... these spots could be just perfect for SP in wintertime in the Netherlands.
Soon I will have some cuttings to plant, and also it is winter here now. So perhaps santa will buy one spot for me this christmas.
We will see.

9W:

LED Spots sure look cool

My Fluorescents are long . Look at this setup , it's by a biologist friend of mine .
He had good results with it .

Observant, growing cactus hydroponically? That must be fun!

"As far I as know the best LED and fluorescent bulbs for growing plants consume just half of the electricity compared to the ~same intensity~ HPS, which is still considerably high consumption."

Just to dispel this - they definitely don't. HPS bulbs do require considerable energy but the output is one of the most intense available, that's why they're categorised as High Intensity Discharge lamps. One can clearly see the difference of intensity between a fluorescent bulb and an HPS bulb of the same wattage. Although fluorescents may potentially still be more effective for seedlings, I would definitely go with the HPS for older cacti, although I haven't tried this myself. Now that I think about it, the costs of running a powerful bulb for so long may be offset by the number of cacti one could squeeze underneath it...
Wikipedia sums it up

"Compared with fluorescent and incandescent lamps, HID lamps have higher luminous efficacy since a greater proportion of their radiation is in visible light as opposed to heat. Their overall luminous efficacy is also much higher: they give a greater amount of light output per watt of electricity input."

oh I forgot to mention Metal Halide lamps, they are also HID, hot and expensive, but put out more light in the blue spectrum. For certain kinds of plants, this is more effective for vegetative growth than the 'redder' HPS, which is preferred for flowering, but you can also get dual spectrum HPS that have a little more blue than usual. It depends what spectrum cacti like best. What's the spectrum of light like over in texas and mexico where the cactuar villages lie? My peyote on the windowsill doesn't seem impressed by the British summer

^ well the problem with those is that they (let's say regular HPS 250W or 400W) put out some much light and warmth, that one has to use a very big closet or a small room, possibly even ventilation. One will have a big surface covered by light. If the plan is just to grow some/few cacti, then HPS of HID is much of an overkill (=waste) so to speak.

Is it not right that when it is not been watered and geting little sun light that this improves the potency of the cactus? as it generates the mesc/other alks as a defence mechanism? I'm sure i read this somewhere... therefore growing them all year round may not even be worth while?

Maybe stress does improve potency. It could raise the % of mescaline. But i think in the end, when compared, growing for biomass (as a result of continuous optimal growing circumstances) will yield more mescaline. As far as i am concerned, i would like some pretty big cactus growing, and some cuttings now and then.

The problem with winter is that there is not allot of light. The cactus will grow skinny. Not ideal for cuttings, and certainly not a pretty big cactus : /

search out "closet cactus care" by m.s.Smith
its online
he has perfected outdoor gowth/indoor dormancy