heavy bike, steep hill...more burned calories?

I ride a bicycle pretty much everywhere I go in town, unless the weather's actually dangerous. I'm trying to figure out how many calories I burn on it, but I can't pin it down for a number of reasons. I only average about 12 mph, but there's a couple of reasons for that. The bike I ride is custom built and weighs about 40 pounds, and I normally carry a lot in my pannier bags -- textbooks, changes of clothes, groceries, pretty much anything I have to transport from A to B. That adds some weight to the ass end of my vehicle. In addition, I live in the Appalachian mountains, which means there's nowhere for me to go in town that doesn't involve a lot of sweaty cursing. I normally log my time as "cycling 13 mph," but I'm wondering if I should give myself more credit. I definitely feel like the work I put into cycling is more than "moderate effort." Anyone know how I can go about figuring out how much energy I actually expend?

You could buy a Heart Rate Monitor that allows you to enter your age, weight, gender etc... those are as close to accurate as you'll probably get...