The Grid Trade Master Agreement (GTMA) is an agreement for trading electricity within the United Kingdom.
It is normally used either when a power station has excess electricity, which it wishes to "sell back" to the power grid, or when the electricity company wishes to buy surplus power to meet a surge in demand, or reduction in supply.
Although electricity is a non-tangible asset, these agreements are used in order to trade guarantees to provide power for a certain length of time.
References
Grid Trade Master Agreement Wikipedia(Text) CC BY-SA