Is it better to use a laptop on battery or on AC power?

It won't make all that much difference. What will shorten battery life is temperature: If it gets hot, it will shorten the battery life. Best thing to do, if you are able, is to remove the battery while you're at home and keep it somewhere cool. If it's a Li-ion battery, then they don't like to be completely discharged, so make sure you charge them regularly. Wikipedia: Lithium-ion batteries should not be frequently discharged fully and recharged ("deep-cycled"), but this may be necessary after…