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%.
I believe bitcrack does exactly this but uses a GPU card.
I believe Bitcrack is designed to bruteforce the
puzzle transaction with outputs' addresses derived from private keys of limited range.
If you want to use your CPU to something more viable, I'd suggest you to bruteforce the puzzle's keys instead.
Good Luck!