I suggest using this much more up to date thread, which will be updated as necessary, Lighting an Aquarium with PAR instead of Watts instead of proceeding here.
I have been wondering how much PAR a typical T12 light produces. Like most everyone else I have just assumed that watts per gallon was a way to guess the light from T12 bulbs, but there is no more reason to expect that to mean anything than there is to expect it to mean anything for other bulb types. So, I decided to do some testing.
I borrowed a new two bulb 48 inch T12 light fixture from one of our local aquatic plant club members, bought a new T12 bulb - a Phillips "Natural Sunshine", 40 watt 5000K, 92 CRI bulb at HD, borrowed our club PAR meter and took some readings. Since I have previously found that I get virtually the same readings with water in the tank and with air in the tank, I omitted the water this time. Then I plotted my smoothed data on a common plot with T5 and PC data:
To compare this with "watts per gallon", I know that a couple of 2 bulb T12 fixtures will grow plants in a 55 gallon tank. That tank is 20 inches deep, so if the substrate thickness is about the same as the height of the bulbs above the top of the tank, each bulb should give about 9 micromols of PAR, or 36 micromols for 4 bulbs. That is right in the middle of the low light range. So my data is consistent with real life results.
The light fixture I borrowed has an acrylic splash shield and a removable back, which is a white reflector. I tested the light with and without the splash shield to find that the shield reduces the intensity about 7%. Testing with and without the white "reflector" shows that the reflector increases the intensity by about 36%. The data used for the chart is with both the shield and the reflector.
Some popular tanks are only 12 inches high. For those tanks T12 bulbs should give about 25 micromols per bulb, so a 2 bulb fixture will give low medium light intensity, probably a good choice for many people with one of those tanks.
I believe T8 bulbs produce about the same amount of light as T12 bulbs, but at a lower wattage, because they are more efficient. The fixture I borrowed uses starters and magnetic ballasts, so I didn't try it with a T8 bulb.
EDIT: Updated chart above and added the following chart:
Another way to use this is to convert it into a simple table, that lets you select a lighting option based on tank height, how high you want the light to be above the top of that tank, and how much light you want. This assumes that multiple bulbs are mounted close together, reflectors are typical for that particular type of light. And, I left out the AH Supply light kits.
EDIT (again) New chart added.
The T5HO line on the first chart has been difficult to apply, because so many cheap T5HO lights with less than good reflectors are now available. To make it easier to estimate how much light you can get from different quality T5HO lights, try this chart:
And, to judge the quality of the reflectors: