It carries "an instrument to observe water-related targets such as precipitation, water vapor, sea surface wind speed, sea surface temperature, soil moisture, and snow depth."

To be launched May 18. The name "Shizuku" means "drop of water". See this link for a Mission overview.

What impressed me was it's power budget of 4000 watts. Yowza.

Very ordinary for an earth observing spacecraft. Aqua, Terra, Aura - all are over that.

The sadly dead Envisat was 50% more than that. Many geostationary communication satellites have 5 times that much.

Communications satellites I can understand, but this one is passive. Maybe for keeping things warm?

No - you didn't read my whole post - there are several other earth observing satellites that have that much ,or more power. Not to keep themselves warm - just to power their system, power their instruments, power their telecom gear etc.