@hek Maybe he would like to know a little more in advance before it gets dry empty

I hate to disappoint you @micah but @Mike-Musskopf has got a point: MQ* sensors are power hungry. You may want to connect everything up to 240V via a small PSU. There is a big thread on mysensors about air quality detection. DHT is not the best sensor for a battery application. Please check BME280 (pressure, temp and hum) or Si7021 (temp and hum) - these would be my favourite sensors for low power consumption. Small tip - your Arduino Pros must be 3.3V and not 5V for the battery usage.