I am planning to buy a washing machine. I see online that front loaders use more electrical power (around 1500 watts) than top loaders (max 500 watts).
If that's true, then why are front loaders called energy efficient?
That 1500W is the maximum motor power of a front-loading washing machine, relative to the maximum of a top-loading washing machine. Front-loading washing machines only use that much power when doing high-speed spinning, and that's only a small part of the cycle; generally they're just gently turning the wash over. More significantly, front-loaders use much less water, and specifically much less hot water, saving heating energy as well as clean water.
Another reason for a high peak load may be that it's a cold-fill-only unit. These are designed to only draw cold water, using their own heating unit for when a warm or hot wash is desired. Of course, this uses energy, but in return saves the energy that would be used by your hot water heater. This may seem like a wash [sorry about that] but the hookup is simpler, which can be important in large buildings. More importantly, if the washer is a long way from the hot water supply then there may be more water in the pipe than the washer uses to fill itself. In that case, a hot-and-cold unit would try to fill itself with hot water, but just get the cold water from the pipe, leaving the new hot water slowly cooling in the pipe, ready to be cold for the next load. A cold-fill-only unit would just heat the water it needed, leaving you with the desired temperature wash and no wasted heat.
Finally, with that powerful motor the final spin is much faster, and so they extract much more water from the clothes before they go into the dryer, so the dryer will use far less energy (electricity or gas) to dry them.
Front-loaders also use more power if they heat the water. We're starting to see washing machines which only take a "cold" water connection. If you ask for "warm" or "hot", they employ an internal flash-style water heater. This is only practical on front-loaders; top-loaders take far too much water.
The idea is to reduce the number of utility connections to reduce labor in large developments. With that same goal, they make "condensing" dryers with an built-in dehumidifier so they don't need a dryer vent pipe, and washers which power off the dryer's 240V feed, to eliminate the need for a 120V laundry circuit.
A top loader fills with 20–40 gallons of water three times for a standard cycle. Our front loader fills five times for a standard cycle but it takes less than four gallons to "fill".
60–120 gallons vs. 15–20 gallons. That is an amazing energy savings....
After our front loader finishes its cycle with high-speed spinning, there is very, very, little moisture left in the clothing. This makes drying considerably faster and much cheaper. A full load from our old top loader would take 50–60 minutes to dry. With the front loader, it rarely takes more than 20 minutes (for a full load of jeans).