It's In the Last Line

I usually come in twos threes or fours Usually available in all kinds of stores Spend a lot for me and I will last a long time Who am I you're wondering in this rhyme I make all your electronics come to life A remote control for example or electric knife All different sizes usually with the letter A When I am spent I'm tossed and thrown away If it wasn't for me your camera wouldn't operate To the wall clock in your kitchen I'm his best mate So what am I and what can I possibly be I am dear friend a most humble battery