Newbie and a calories per hour question

G'day all
Greetings from sunny Mount Isa, Queensland. This is my first post and ive got a bit of a query regarding calories. I only recently got into cycling, firstly through mountain biking but now ive taken up road cycling. Im 31 years old and i stand 183 cm's. The down side is i weigh 102 kg's. I started using a product called Optifast a couple of weeks ago to help bring down my weight. It seems to be basically a meal replacement protein powder. Im down from an original weight of 106 kg's so far.
My question is i want to take it to the next level and start acurately measuring my calorie intake vs. expenditure. How do i find out an average value for calorie expenditure for an hrs riding. Ive got an older Polar HRM. Its a smart edge model that gives a Kcal measure. How accurate would this be and what is the relationship between Kcal and calories.
Apologies if the questions are a bit basic but thats the level im at. Cycling for fun and fitness and with a view to going competitive once i reach my target weight (85kg's).
cheers
Mick