Interest in the royal wedding is set to trigger an energy demand big enough to boil 500,000 kettles.
The National Grid said the biggest surge of 1,500MW, which is expected at the end of the ceremony and again as the carriage with the newlyweds enters Windsor Castle, would also be enough to open 100 million fridge doors.
It expects viewers who have been glued to their seats to see Prince Harry marry Meghan Markle to celebrate with a cup of tea.
The surge compares with a pick-up of 2,400MW following Prince William and Catherine Middleton's wedding in 2011. This would have been equal to boiling 800,000 kettles.
The key moments which are expected to boost demand include the first glimpses of the royal family and guests, at 250MW, along with a 400MW boost when Prince Harry and the Duke of Cambridge arrive.
The arrivals of the Queen and also Miss Markle could see a 600MW demand, according to the National Grid's expert forecasters whose estimates are based on detailed data, past events and behavioural trends to work out which parts of the day will be the most popular.
These demand increases, called TV pick-ups, often follow key points in live sporting events as fans get up during the break and begin using electricity at the same time, causing a surge.
Football fans are also set for a big day as the FA Cup final takes place in the evening but a much smaller pick-up of 200MW, which is equivalent to powering 67,000 kettles, is expected at full time.
Duncan Burt, the National Grid's acting director of operations, said: "Our control room team will be watching the wedding live for exact timings and managing the UK's supply and demand second by second.
"For big events such as this we expect to see increases and decreases throughout the afternoon as people tune in at key points and take breaks to boil kettles or open fridges to get another drink.
"As a nation, the way we watch TV has changed greatly in recent years with more on-demand viewing, which means we only tend to see large pick-ups for live events such as this which bring the country together."