I don't have a power meter.
I'm not sure any calorie calculation is accurate no matter how it's calculated. The important thing is that whatever device/method is used is precise so I have a reference and then I adapt to that.

Accurate is returning the same number every time with that number being correct.
Precise is returning the same number every time.

For example a scale - If I want to lose 10 lbs. it really doesn't matter if my scale is off by 5 lbs., as long as it is off by 5 lbs. every time I step on. I can still track my weight.