I wondered if a more experienced grower, or electrical bod, could please comment on the following:

As well as the vaults of knowledge on this excellent site, I have a few books which I read to give myself a grounding in the ways of the doobie.

One of these "Hydroponics: Indoor Horticulture" by Jeff Winterborne, (not a plug, but mods please remove if not allowed), says the following: (selected data only)

white light - 100% theoretical efficiency
HPS lamp - 64% theoretical efficiency
Halide lamp - 45 - 57% theoretical efficiency
Fluorescent lamp - 38% theoretical efficiency

Does this mean, for example, that a 400W HPS will only actually use 256W (64% of 400) generating light, and the rest is wasted as heat?

I have my doubts, as (in the example given) that would mean 144W would be given off as heat - which seems an awful lot to me (almost as much as a small tube heater!). Maybe not all loss is heat - old ballasts hum, so is some energy lost as sound?

OR does it mean that a 400W hps will actually use up ~615W (400W is 64% of 615W), but that would mean an even bigger loss of 215W (615-400).

I am confused , sorry if this is a daft question. Its probably neither of these answers!

The reason I ask, is that I wondered if the books figures could be used as a rough way of figuring out temp rise caused by certain types of lights in various spaces.

Thanks for reading!

Lighting has various ways of quantifying the efficiency.

Plants require light in what is known as the PAR spectrum (photosynthetically active radiation). Do a wikipedia search for a good beginners article. All lights produce different spectral outputs, only those producing peak outputs in the red and blue ends of the visible spectrum are useful to plants. I believe it is this, the book you have referenced is referring to (with regards to efficiency). Jorge Cervantes book is available for preview on google books, I suggest having a read of the most relevant pages on lighting. Nearly all of which are available entirely free and a bit more illuminating (pardon the pun).

Lumens per watt relates to the amount of light energy produced by a light source, but this measurement is not directly related to the heat output of the light. Lumens per watt, simply measures the volume of light energy being produced per watt of electrical power consumed. And although you can work backwards (through lengthy mathematical calculations). It doesn't really tell you much about a lights heat output.

And finally, after I've blinded you with bullshit. Light wattage ratings, are just that, they tell you how many watts of electrical energy the light uses in total. So a 400W light consumes 400W of power. Now, since ballasts aren't 100% efficient, its actually slightly more. But f@ck worrying about stuff like that. That 400W goes into making heat and light, so now you see why a more efficient bulb produces less heat and more bud.

I see what you mean uglykittens - so you think these figures refer to the amount of useful light produced.

It makes sense - many thanks for your reply!