A use of an electric outlet by a consumer forces the outlet manager to pay for the consumer's power usage in current electrical power systems. Even if a consumer uses an outlet managed by another person, one bill for both indoor and outdoor charging information should be required to the consumer in their contract with the utility company. For this purpose, we define a model for the Smart Grid security and propose a Secure Payment Protocol for Charging Information over Smart grid, SPaCIS for short, as a protocol satisfying the model. Our model provides for the unlinkability of consumers as well as for the undeniability and unforgeability of billing information using digital signatures and identity federations. SPaCIS is also efficient in the sense that time complexity is constant relatively to a trivial use such as an individual verification for each signatures, unless a verification error happens. We furthermore evaluate performance of SPaCIS via cryptographic implementation, and simulate SPaCIS in a case that one thousand users generate thirty signatures. Then, we show that SPaCIS with ECDSA can be executed within 6.30msec for signing and 21.04msec for verification of signatures, and conclude that SPaCIS is fairly practical.