Display relays status

central_backend/src/energy/engine.rs

@@ -39,6 +39,10 @@ impl RelayState {
     fn is_off(&self) -> bool {
         !self.on
     }
+
+    pub fn state_for(&self) -> usize {
+        (time_secs() - self.since as u64) as usize
+    }
 }
 
 type RelaysState = HashMap<DeviceRelayID, RelayState>;