The Smart Garden 3 is consumes 8 Watts of power, while the Smart Garden 9 is 13 Watts. Leaving them on for a full month, the Smart Garden 3 uses 3.8 kWh (kilowatt hour) of power each month and the Smart Garden 9 uses 6.2 kWh. The average electricity rate in the United States is 13.19 cents per kWh. The Smart Garden 3 costs $0.50 (50 cents) per month and the Smart Garden 9 costs $0.82 (82 cents). The electricity costs are minimal.