what is the best device to use to measure

what is the best device to use to measure calorie output on any given day?

I don't ask this in a calorie-obsessed way, but a scientific, curious about my body way. I work PT and on the days I don't work, I am home with my 4 month old. I KNOW I burn more on the days I'm home with the baby and I'm just curious how much the difference is. I work out regularly and am comfortable with the amount I burn and the calories I eat so this is just something I was thinking about and wondering if I can measure it in a way. Or maybe I just want to prove to hubby that it IS hard work being home with the baby ;-) No really, I am interested in what my body does on a regular day, in the two different circumstances.

So, would it be a HRM, fitbit, bodybug..? other suggestions? And I'm talking something reasonable (ie I could buy on Amazon), I'm not going to go to a clinic and do a bunch of tests or whatever. I already have a Polar FT4 HRM, could I use that for an estimate? Put it on when I get up in the morning, take it off when I go to bed? Or should I buy something else for this? Would I stop it and restart it after extra exercise and count that separately? Probably to get a better reading on a regular day. On the days I am home, I usually don't get to exercise other than taking baby for walks (unless hubby gets home early enough for me to run or go to the gym). Luckily the days I'm at work, sitting all day, are the days I get to go to the gym.

TIA for any ideas!