The cool thing about the electronics inside a TK product with a battery is that it only draws energy from the charger when it needs it. So if you generate energy from cycling, no energy will be drawn at all. If your battery is full this means that only in certain cases (low cadence, high force see …) energy will be requested from the charger.
This means that only in extreme cases, when you continuously discharge the battery more than you charge it, power will be drawn from the charger. In all other cases, only at incidental moments, your device will draw power from the charger. That is why we recommend you plug it in if you can. It does not consume energy if it does not need it.