Electric helicopters range from the smallest available to typically about 4kg/ 600-size at the largest. As the size goes up, the size (and therefore cost) of batteries goes up very quickly.
Electric helicopters have a number of advantages, compared to internal combustion powered helicopters. They are clean (generating no exhaust), quiet, require less tuning and maintenance, and have very little vibration (making them very suitable for aerial photography. The power system (excluding battery) is also very compact, making very small helicopters a reality. The most common electric helicopters for aeromodelling are those around 450 to 500 sized, which can often be used as 'park flyer' helicopters, able to be flown in many large public open spaces.
The downsides are primarily the price of the batteries, the rate at which the batteries recharge (typically 30 minutes to an hour), and the limited endurance (5-10 minutes) of most batteries. As technology advances, these disadvantages may be mitigated, but is seems unlikely that any battery will ever reach an energy density anywhere near liquid fuel.
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