In the current political climate knowing the precise amount of money you pay for energy is the equivalent of knowing the price of a loaf of bread or a pint of milk.
Unfortunately for Boris Johnson, he forgot to cast an eye over his electricity or gas bills this month before appearing on his monthly LBC Radio phone in show. Tackled on the topic by host Nick Ferrari on Tuesday morning he said: "I leave such things to higher powers. I'm sure it's outrageous but I can't give it to you, I am happy to give it to you later."
He added sheepishly: "I'm sure its exorbitant, my energy bill. The only way to get it down in the long term is to increase our ability to generate lea and abundant electricity."
However following the revelations that some wealthy MPs were charging their energy bills on parliamentary expenses, Boris was keen to add: "My energy bill is certainly not being paid by the taxpayer."
The high cost of energy has become a key battle ground between Ed Miliband and David Cameron. The Labour leader has called for an energy price freeze until 2017, while the prime minister has urged people to switch suppliers in order to get a better deal.
Asked when he last switched his energy supplier, Boris admitted again: "I leave all that key strategic decision to higher powers."
It is not the first time Boris has been tripped up on the price of an every day item that a lot of less wealthy voters will be familiar with. In October the London mayor was tripped up by Jeremy Paxman over the price of bread and milk.