Energy customers receiving details of how much their energy bills will rise in the April price rise are braced for higher costs – but a big leap in standing charges has already taken many by surprise.

For millions of customers on variable rate tariffs the daily charge for electricity, applied whether you use any power or not, will leap, with some set to pay 80% more when they take effect in spring.

While many expected rising unit prices, driven by the increase in the wholesale cost of gas, they are questioning why other charges have gone up so much. One Observer reader got in touch last week about what she described as an “inexplicable” rise in her electricity standing charge.

A customer of SSE (now owned by Ovo), she said the rise in unit price from 20.6p to 28.46p was “reasonable in the scheme of things” but questioned why the standing charge was going up from 24.11p to 43.39p a day.

Her annual bill is set to rise by almost £300, of which the electricity standing charge makes up more than £70. “It’s the price of the energy that has changed, not the system of delivery, so I am struggling to understand how this is justified,” she says. Ovo did not comment.

On social media customers from a wide range of suppliers have posted reported rises of about 20p a day, or £73 a year.

One Twitter user posted that on his mother’s bill the standing charge for electricity had almost doubled, while another complained theirs had gone from £8.43 to £13.43 a month “before I even switch a lightbulb on”.

Another wrote: “I can understand a rise in unit prices, but not the standing charge. Even if we cut our usage we have to pay that.”

Standing charges are a daily fee for gas and electricity customers and are added to your bill regardless of how much energy you use. Ofgem does not set a limit on this part of your bill – it is down to suppliers how they break down unit and standing charge prices below its overall cap.