Effectiveness of solar charging

A few days ago, I set my watch to power-saving mode same put it under the sunlight for about 4 hours. The battery went up from 66% to 70%. Then I left my watch in a drawer (still in power-saving mode) for about 12 hours. The battery went down to 64%.

Yesterday I keep my watch in power-saving mode for nearly 18 hours, and the battery level just dropped 2%, from 43% to 41%.

Solar charging dues not seem effective. Anyone has the same experience?

My watch is an APAC version (Traditional Chinese) with firmware 13.30, the most updated version for APAC.