My new elliptical trainer can show both Watts and Calories

I could not make sense of the numbers so I pedalled it for 100 seconds while keeping the Watts display as close as possible to 40.

By my reckoning I should have expended 40x100 watt seconds or 4000 Joules.

Based on the conversion factor that 1 calorie=4180 Joules, I would have expected roughly 1 calorie. But the machine said 7 calories.

Is the machine out of calibration somehow? Or have I misunderstood something?

While 40 Watts is easily sustainable, the highest reading I could get in an intense burst was 210 Watts and the machine was flexing quite a bit. What is a resonable peak Watt reading for a beginner?

40 watts is nothing....

Check out this site: Bicycle Speed and Power Calculator (http://www.kreuzotter.de/english/espeed.htm)

imho as a beginner focus on 100 rpm and stay there forget watts and crap if you are
a beginner you need to figure out how to pedal. get to 100rpm and stay there for 2 hours
and do this

in a year then talk about watts

The body's efficiency is not 100%. For cycling, use a factor of four (do the math yourself with an assumed 20% efficiency) to convert from power (W) to energy (kcal) for a full hour of work.
Working it out based on 40 W for 100 s, comes to roughly 4.5-5 kcal.

However, the assumed efficiency for an elliptical trainer might be, say, 13%, in which case it would come to 7 kcal in your case.