I ride with a power meter, and here are some stats from my end. Firstly, power is work in kj, so stuff like gender, age, HR etc. is not a factor. In cycling, we essentially take kj of work measured by the power meter to be equal to calories expended.
If I'm just riding at a steady endurance pace, I burn about 700 calories an hour, and I'd consider myself pretty highly trained. On rides with my 10 year old (where we average 13ish mph) it's like 300 calories an hour. So if you're pretty novice, I'd guesstimate on maybe 400 calories an hour at the pace you indicated.
I started riding in 2013 and weighed 225-230lbs, and my strategy (besides limiting my eating) was to ride some sort of combination of longer duration or more intensity. I'm not going to delve into what to eat, there are so many self proclaimed nutrition experts. But you can use cycling as a tool to facilitate weight loss (and of course improve cardiovascular health).