The computer is on for other reasons 24hrs a day, so no extra electric being used.
If the processor had been running @10% load all day before, it'll increase its power consumption if your brute-force software bumped it to 90~100%.
There's also concern of lower lifespan of the PC due to 24/7 usage and high temperature (if no maintenance is done).
I'm not guessing one particular number. I generate an address and then compare that to 60000 known addresses with balances, thus increasing the chance by 60000 on every attempt.
Why limit yourself to only a few addresses, while you can easily use all 40 ish million Bitcoin address with a balance?
Not that it matters, you won't find anything anyway.
I agree. I'd recommend bloom filter rather than database, which is commonly used on some brute-force software.