This paper describes our strategy to operate WSN with or without security based on our measurements of energy consumption using CrossBow and Ember sensor nodes. It was found that the node microcontroller's CPU operates for substantially longer times for both hashing and encryption operations compared to the time for handling messages without any security. However, the associated overall increased energy consumption is relatively insignificant. The longer high-power radio transmission times due to hashing were more costly. We provide design guidelines to apply security for energy limited WSN.