How many calories do you actually burn cycling?

I'm confused by all the calculators and information out there! Just started cycling on a stationary bike to lose weight. Does anyone have an accurate estimate of caloric burn? My bike calculator is telling me 150kcal per 10km. Is that about right? Go easy on me, I'm a noob.

For background, I'm 240lb, male, 6ft, cycle for 75 mins at 125 BPM.