7 x 2 x .04 x 16 x 30 x .097 plus add 5% for ballast loss - depends if you have conventional ballast or electronic.
My suggestion: go on ebay, get used T8 4 lamp ballasts, such as advance or sylvania (advance is slightly better than sylvania), look up overdriving t8 fluorescent, and re-lamp with t8, especially Literonics 5000K
90 CRI - you will get way more light, your bill will be basically the same (running 45w instead of 40) - i did this myself, and I measured the actual current (0.75 amp for 2 x t8) You can get used ballast - in a lot of 10 for 3 dollars apiece plus shipping - this is what i did myself, and the light output is insane! (I measured it with an actual PAR meter, trust me you will get at least double the light output from what you are getting now) - you can come to my place and have a look if you are in this area - best regards, overdrive.

Look up
http://reocities.com/Heartland/Pines/7557/overdrv1.html
http://www.ebay.com/itm/Lot-5-Advance-120-Volt-4-ft-T8-4-light-Electronic-Ballasts-REL-4P32-RH-TP-/221150508933?_trksid=p5197.m1992&_trkparms=aid%3D111000%26algo%3DREC.CURRENT%26ao%3D1%26asc%3D14%26meid%3D3316118300864059412%26pid%3D100015%26prg%3D1006%26rk%3D1%26sd%3D221150508933%26
you can run 4 fixtures and get more light than you are getting now with 7 and your electric bill will be 30% less.
that is why T12 is no longer manufactured and has been replaced with T8 - too inefficient, wastes power.

Overdrive, thanks for the info. I am living on social security these days, and am trying to cut my light bill. I don't have money for my monthly expenses as it is, and can't replace the fixtures I already have, or bulbs that are still good. I have to use what I already have.

I'll work on the calc you gave me, and see what I get, thanks! I'm using the cheap 48" fixtures from HD or Lowe's. I have some T8 bulbs, btw, but again what I could get at the big box stores.

Well, using your calc, it looks like it's costing me ~28/month to run those lights! I was estimating 20. No wonder my bill has been so high!

I looked at those ballasts. They would have to be installed in the fixtures, right? Take them apart and put these in instead of the ones that are there?

Gee, I was thinking T8 would be pretty efficient...

It is very easy. You unscrew the screws to get at the ballast. Throw the old one out, and replace with those from ebay (those are the ones I use). Follow the instructions on the overdriving website, and put in new T8 lamps. The litetronics are available from Newegg at 3.40 plus shipping, and is the cheapest online site. There are some cheaper lamps but I bought the following, and the shipping is only $7. for however many you need. your other option is you have no money and need to save on electricity is to buy the used 2 lamp advance T8 ballast, but you will be much happier with the overdriving, at least you will be able to grow more bigger faster. Using a 2 lamp ballast will save you 25% on your hydro, or you can overdrive 4 fixtures which will still save hydro and get more light.
http://www.newegg.com/Product/Product.aspx?Item=9SIA0BD0785091

Or there is a lamp for $3 Litetronics 28260 which will be just about as good (100 lumen less).