Households on dynamic contracts are feeling a surge in electricity prices as fixed-rate energy contracts also become more and more expensive. The average wholesale price in September is more than twice as high as in the same period last year, NU.nl reports.

A dynamic contract follows the market, meaning that the rate can shift every hour or even every 15 minutes. Shifting power usage to cheap hours can, therefore, cut what you pay.

According to NU.nl’s review of jeroen.nl data, from September 1 to 25, the average wholesale price on dynamic contracts was 15.14 cents per kWh. The same stretch last year averaged 7.29 cents, excluding tax, VAT, and supplier fees. The rise has overall been gradual.

Prices jump when the sun comes down. “Daytime renewable output is ample in good weather,” energy analyst Jeroen Bakker told NU.nl.” After dark, gas plants have to run, and gas is expensive.”

On cheap hours, a dynamic rate can still beat a newly signed fixed deal, while it can also exceed that fixed price on other hours. “Households that cannot shift usage gain less from a dynamic tariff,” Koen Kuijper from Energievergelijk.nl told NU.nl.

“Whether the contract works for you relies on total use, how much you export, and how much demand you can move,” Kuijper said. “People who need power mainly at peak times feel this year’s increase more than those who can shift a large share of consumption.”