You can't get electrocuted by a 12 volt DC battery anyway, the voltage is not high enough to overcome the resistance of your skin.
When we talk about wattage, we talk about ACTUAL wattage, not equivalencies. So if your 150 w equivalent Fluoro bulb is actually 40 watts, then you need to figure the cost of 40 watts, not the equivalent 150. YOU NEVER USE the equivalent wattage for anything. Its just like saying "Hey this light bulb puts out the equivalent of 500,000,000,000 watts of light, but only uses 1 watt" Guess what? the power actually used is what your paying for (1 watt), not some farcical equivalent rating done by some light bulb manufacturers advertising department.
I run 2000 watts of power, it costs me approx $60 per month at a rate of 6 cents per KWH (Cheapest power in the nation where I am).
Appliances and electronic items DO use power when plugged in but not used. It is said that by unplugging all items not used will result in a 30-40 % energy saving on your elec bill.