If actual prices are used to calculate the tax in both markets, the decision rule is:
By comparing the two scenarios, one can see that the incentives depend only on (gross) price. Producers shift supply between markets only if the price in one market is higher than in the other.
These results apply even if only one market price is above 180 EUR/MWh while the other is below. If a reference price is used, the tax term on each side of the inequality either cancels out (if the reference price is above the cap) or does not enter at all (if the reference price is below the cap). In both cases, the gross spot price determines production incentives.
If, on the other hand, actual market prices are used, we have (for
and
):
This inequality holds for all situations where
and
, which means that incentives are maintained even in situations where one market is taxed and the other is not.
Hence, using the day-ahead price as a reference price for all markets does not affect incentives to shift supply between wholesale markets (from day-ahead to intraday).
However, government tax revenues are affected by the choice of reference price. If the day-ahead market on average has the lowest prices of the three wholesale markets, using the day-ahead price as a reference price will, all else being equal, give a lower government tax income than using actual prices.
3.1.4. Impacts of incentives on the intraday market
In contrast to the day-ahead market, which uses marginal pricing, the intraday market uses a pay-as-bid model (see Text Frame 3.2). In the intraday market, suppliers face a trade-off when submitting their bids because they are paid their bids and not the market-clearing price. On the one hand, sellers are tempted to bid above their marginal costs since they are paid their bids. On the other hand, bidding too high involves the risk of not being dispatched and gaining nothing. The marginal revenues of producers thus depend on their bids and their marginal costs, and their bids depend on expected average prices (and marginal costs).
As implemented in Sweden, a revenue cap uses the day-ahead market price as the reference price. Thus, the revenue cap will not affect behaviour in the intraday market because the optimal bid functions are not affected by the day-ahead price, or the day-ahead price affects the expected prices on the intraday market in the same way for all agents, and this price is common knowledge to all bidders.
The Danish implementation did not have an hourly settlement but used total revenues to calculate the tax base. Thus, it was still optimal to maximize revenues for given marginal costs, which implies that behaviour was unaffected by the tax.
Hence, the implementations in Denmark and Sweden ensured that incentives in the intraday market were not affected.