What is the difference between 'smooth' and 'filter' function for smoothing?

Both seems to be able to smooth data using methods like Moving Average.

Which one should be used in case for smoothing power output from Renewable energy?

What if I want to design my own filter? Like using Gauss-based method? Can I use these functions?

Answer 1

[KALYAN ACHARJYA]

Smooth: Smooths the data in the column vector using using a moving average filter. Filter is the oparand and smooth is the result. "A moving average filter smooths data by replacing each data point with the average of the neighboring data points defined within the span".

Which one should be used in case for smoothing power output from Renewable energy?

I dont think there are major difference to use it.

What if I want to design my own filter? Like using Gauss-based method? Can I use these functions?

Yes

But the application methods based on data types also, read here