From a recent group ride recorded by a Garmin 705, uploaded to three different apps. The Garmin records once every second. The kilojoules are pretty consistent.

For the whole 1 hour, 40 minute ride:

Avg watts 165, 808 kJ, calories 940, on ridewithgps.

Avg watts 129, 783 kJ, calories n/a, on Golden Cheetah

Avg watts 131, 788 kJ, calories 878, on strava, weighted average watts 151 - strava says "our best guess at your average power if you rode at the exact same wattage the entire ride."

Golden Cheetah charts, power over time. Here's the raw power data, one data point per second, from the first part of the ride. 1000 seconds is about 15 minutes. And the same data with a lot of smoothing applied.

The yellow dots were a 5 minute 193 watt average. Power really changes second-by-second!