Start with three physical quantities
Record the installation's generation, electricity exported to the grid and electricity imported from the grid. Generation is usually visible in the inverter's monitoring system, while imports and exports are recorded by the distribution operator's metering system. These are different measurement points. Do not compare their readings without checking the period, units and aggregation method.
For an installation without storage, subtracting exports from generation helps approximate electricity used locally. With a battery, changes in its state of charge, losses and charging flows also come into play. Simple subtraction may then describe a different scope from the one you expect. Your own energy balance should clearly state what it includes and which flows you cannot yet separate.
Two value accounts rather than one money counter
In the first account, analyse electricity fed into the grid and the rules used to assign its value. In the second, analyse the cost of imported electricity under your contract and the remaining bill components. A price displayed in a market app does not automatically replace either complete settlement.
This example is deliberately simplified: a home exports 10 kWh valued at PLN 2 in total, then later imports 10 kWh for which the energy component alone costs PLN 6. Equal volumes have not cancelled out the difference in value. After distribution and other charges are added, the result also depends on the supplier's and operator's documents. This illustrates a mechanism; it is not a market offer.
Read a month without mixing settlement periods
Place the invoice, the prosumer deposit statement and the exported metering data side by side. Note each document's start and end dates. If one file covers a calendar month and another the period between meter readings, align their scope first. Otherwise a timing difference can easily be mistaken for a price calculation error.
Next, identify the items that depend on imports, exports, the tariff time band and simply having an active contract. Moving a washing cycle will not reduce every cost. A fixed charge remains the same if the conditions on which it depends do not change. Assess the energy effect and the total bill separately.
Better questions when planning your day
Instead of asking only when wholesale electricity is cheapest, ask whether your home has its own surplus at that hour and how that surplus is settled. Using electricity locally can reduce imports, but it also means giving up the option of exporting it. Include that opportunity cost when comparing different options.
Prepare a small decision table: available hours, the appliance's expected consumption, possible PV generation and the relevant price component. If a parameter is unknown, enter a range or a question mark. An honest balance like this is more useful than a result calculated to two decimal places by mixing retail and wholesale prices.
Sources & verification
Event dates and data coverage are separate from publication dates. Archive analyses are not backdated.
