Lighting is a small fraction of your overall electricity usage. HVAC alone uses many times as much. While using efficient lighting is a good idea far too many people get hung up on it when there are better places to conserve.
It depends on how many people are in your family and how frequently they forget to turn off the lights. Plus, having 400W of incandescent bulbs on is 400 more watts that the A/C system has to cool.
Yes and No? I mean, my bill only goes up $20 in the summer to cool the house, and that's in a 35 year old home in Dallas. That's over 90 straight days of 110F temperatures last year.
It's a new, pretty efficient HVAC unit though. Thankfully. First home I've had that didn't struggle to maintain 76F indoors during the hottest part of the day.
Still, there's not a ton you can do around the house as cheap as replacing the bulbs in the high-traffic areas. Anybody can do that. Replacing your HVAC for something more efficient is going to be a challenge for most.
Then there's high-R-value windows to install. Foam insulation on exterior walls (brick, so that's a lot of dry-wall work). Radiant barriers in the attic space. More ceiling insulation. Tankless water heaters.
Outside of maintaining your weather stripping, I can't think of anything that pays off as much, for as little, than replacing your light bulbs. Maybe getting a Nest thermostat? We've got recessed lighting in the Living Room, which had four big 90W halogen bulbs when we moved in, and are almost always on if somebody's home. Replaced 'em with 17W LED bulbs (http://www.usa.philips.com/c/led-light-bulbs/ambientled-17w-...).
That comes out to about $7/month in savings. (360w vs 68w @ $0.0912 per kw/hr). In the year since I installed them they've paid for themselves. They'll last 22 years according to Phillips. The ROI on LED (for your highest-traffic rooms at least) is stupid cool, and I'll effectively never have to bust out the 8' ladder to replace them. I might be off base, but I think I've typically replaced halogens every couple years or so. So even over 5 years the old bulbs probably would've cost me 10X more.
For our situation at least, that's a no brainer. If I can spend $50 today and avoid paying $5/month indefinitely, that's a big win in my book. I may not be able to afford to buy a car cash, and have to pay the financing penalty, but at least I can break out of the "financing" cycle with my home lighting. ;-)
To each his own. I'm comfortable at 76, which is good, because my family will complain about being cold. (They're happy at 70 in the winter, though... go figure.)
I live in a new construction condo in the Pacific Northwest. A couple of times per year I have to turn on a single heater.
My $35 power bill is my desktop computer, refrigerator, hot water heater, and the 7 lights that end up turned on just to light up a 500 or so sqft living space. (Recessed lighting is evil.)
