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I believe that the energy generated from solar panels is pushed by excessive potential from the grid. So some of the amount of power generated is wasted as it can't get into the power grid.
The measurement of power is disabled by the force of power already on the grid.
Also when it comes to measuring and passing for power, it is the power distributor that qualifies the amount to bill on statements.
As a power generator using solar panels it is the power company that is the customer. It is then the customer who measures how much solar power to pay for. That customer retailer relationship is unusual in commerce.
On a different topic, the food in supermarkets is sold in all sorts of sizes. One has to read the fine print that is price per 100g and convert to price per kg to get a comparative price between products.