There are two causes for this:
1) A small number of solar inverters draw a small amount of non‐sine wave current at night. This shows up on the Current Cost CT Clamp (and other, far more expensive equipment including Fluke meters) as a 0.3 Amp current which equates to approximately 75W.
2) Some inverters generate high-frequency noise at night (the kind that will disturb AM radio reception near the inverter). This can be transmitted through the active wire, which will cause the CT Clamp to show this reading. Some of our customers have reported being able to prevent this by installing a ferrite suppressor (available from Jaycar for $10) on the active line.