It is most likely noise and pickup, possibly within the emonPi itself, possibly from adjacent circuits.
It’s quite hard to measure zero - that implies absolutely perfect analogue electronics, which unfortunately we don’t have - so the usual ‘cure’ is to add a step in the processing to ignore the output (set it to zero) when it’s less than a few watts. In your case, I’d wait until you’ve seen a day or two’s generation, and then choose a number that’s only just above the ‘nighttime’ value you see.
There are two ways you can tackle this:
If it looks as if the 25 is reasonably constant, you can subtract 25 always. If it isn’t, then you can subtract a smaller number, and then zero the value if it’s below your new threshold.
In emonCMS, on the Inputs page, look down the processes. For the first option, you want to add (+) -25.
For the second option, further down you’ll find “if >, skip next” and you skip if greater than (say) 25, then the next step is of course “Reset to ZERO”. (What you’re actually doing is: if ≤25, set to zero.)
One thing you should check - is the c.t. the right way round? Your generation should be a positive value, and in the dark, most inverters consume a small power, so it should read a negative value at night. That it read higher with the inverter alive suggests something to be checked when it’s working.