frosty said:
Lights off = less electricity. Flashlights on = battery charger always on
The theory is sound. With household lighting you use unnecessarily powerful incandescent lights all the time. Even if you convert to CCFL (at a considerable price) it's still several tens of watts of light, and it's still often way more than you need.
With flashlights (I'm assuming LED lights here) you only use, say, 1 watt lamps for just as long as they are required, which is much more efficient.
Batteries will eventually die, but €8 will buy you a dozen 2500mah off-brand AAs on eBay, shipping included. They won't hold their charge for long, but at the rate you'd use them high self-discharge wouldn't a problem.
Or if you use 123s, €12 will get you four rechargeables and a charger.
So all things included you use a lot less energy if you only use flashlights in the house.
Then again, if I had to only use my flashlights in the house I'd have to squint all the time and I'd trip over stuff a lot.
My personal suggestion to save up: add a few Luxeon LEDs (with AC adapters) to your existing household lights. You use the LEDs when you only need a little light (most of the time), and you can always turn on the incandescents when you need to find that damn little screw that just fell on the floor...
If you are a real pennypincher you could go a step further and also add a few standard white LEDs. Use the standard LEDs to get enough light that you don't bump into things, use the Luxeons for intermediate lighting and fire up the incandescents when you need high brightness.
This way your lighting will still be highly efficient, but you'll avoid all the hassle of having to carry your light around with you.
Edit: hmm, I though bare Luxeon LEDs cost less than they actually do... perhaps the best idea is still to just stick with low-power CCFLs.