Quick answer
What to do first
Restore a layout that already survives active waves, record your before state, and compare one fixed offline session after you return.
The official description confirms that bases can fight while players are offline, but it does not publish a stable hourly earnings formula. To test build a base rng offline fighting, use your last proven layout, record the visible resource state, leave for one fixed interval, and compare the same fields after returning. Repeat only after restoring the same arrangement. That makes two sessions comparable without promising a coin-per-hour rate that may change with updates, wave progress, or base state.