Don't use boiling water with hot water bottles; the ones you can buy have warnings about this (it can damage them). Just use hot water from the tap (which hopefully should be more efficiently heated than however you boil water as well).
Don't worry about energy consumption; it's negligible for the hot water bottle, and for the electric blanket as well, especially compared to keeping your house heat up high at night. Not really worth debating, though do note: the hot water bottle has a fail-proof, gradual, automatic shutoff, it fits any size bed, requires no electric outlet, requires minimal storage space during warmer months (so you can have an extra for guests), and has an initial investment of, like, $4. :)
Don't worry about energy consumption; it's negligible for the hot water bottle, and for the electric blanket as well, especially compared to keeping your house heat up high at night. Not really worth debating, though do note: the hot water bottle has a fail-proof, gradual, automatic shutoff, it fits any size bed, requires no electric outlet, requires minimal storage space during warmer months (so you can have an extra for guests), and has an initial investment of, like, $4. :)