The tradeoff with the tree preservation setting is lower values are more productive but lead to deforestation. The deal is you can choose to manage the forest sustainably or thin it out and then move the camp.
Wind and sun are on the same scale in that a fully upgraded solar farm has about the same output as a fully upgraded wind farm if their weather numbers are the same. Without upgrades, a wind farm is about 50% better.
Rain is a different matter. Dams are meant to be used as batteries for other power plants. When only powered by rain, they can be good early game, but building only dams is generally a poor choice.
The KWh unit is calculated under the assumption that a cycle is a month. This means with 1 MWh you can run a 1 MW engine for roughly one real second. Maybe there's a better way to present this information, but it's just the amount of energy stored in the dam.
In map generation, wind is likely to be lower than sun and rain, while sun and rain are inversely correlated. You are meant to choose your power plants based on a several factors including the climate, your resources, the terrain, and what cards appear in the shop.