Households with solar panels cost energy companies several hundred euros a year more than those without, according to preliminary research by Dutch consumer authority ACM.

The ACM decided to look into the cost implications of solar panels now most energy firms have begun charging extra fees to solar panel owners to cover the cost of feeding energy they don’t use back into the grid.

It is not illegal to differentiate between clients with and without panels, the ACM said.

Some firms are charging solar panel owners more for electricity and even gas. Others charge higher delivery costs and most are using an extra fee on top of the normal charges, the ACM said. Other providers are also refusing to offer long fixed contracts to solar panel owners at all.

Follow-up research will now focus on Budget Energie, Eneco, Engie and Vattenfall, all of which use different methods to calculate the additional fees, the ACM said.