Surplus
Power you make or store that your home is not going to use. It is the only thing the gateway ever lists.
Every block, the gateway estimates production for the next fifteen minutes and subtracts what the house is likely to use, based on the last few weeks. What is left is surplus, and that is what goes on the board.
If the estimate is wrong, say the oven goes on halfway through, the listing shrinks in real time. You never sell power you needed.