Fuel retailers overcharged motorists when the coronavirus lockdown was introduced by failing to fully pass on savings, according to analysis.
Challenger brands to Britain's biggest energy firms have cemented their place at the top of the rankings in an annual customer satisfaction survey.
Energy firms are sometimes keeping customers on hold for more than half an hour before they can get through - and around a fifth typically take over 10 minutes to pick up, according to Which?
Energy firms risk misleading environmentally conscious customers with tariffs marked 100% renewable electricity, according to a Which? investigation.
More than 250,000 customers have been moved on to expensive energy deals after their suppliers went bust, new research has revealed.
More than half of energy customers with a smart meter have found the devices stopped working when they switched supplier, research suggests.
Consumers are increasingly switching to small and medium sized energy providers and shunning the "Big Six", according to new figures.
Cheaper energy deals are vanishing from the market ahead of a new price cap, analysis from Which? has found.
Energy customers are facing "infuriating" waits to get through to call centres, while some online queries are going unanswered, a watchdog has found.
Energy suppliers need to triple their smart meter installation rates to have any hope of completing a national roll-out by 2020, new analysis suggests.