How heavy is heavy? 

Mostly it is simply a cost vs weight exercise. 

Krys alloy electric bicycles with lithium batteries can be as low as 14Kg while older steel electric bicycles with lead acid batteries can be up 48kg. You will find that Krys bikes offer the lightest alloy bikes on the market with the best balance of power/weight to get you where you are going easily.

When selecting your bike you need to think about the terrain you will be cycling over and the frequency of use, purpose of the bike etc.  If you are using the bike off-road then a strong but light weight frame is important, however, if you are commuting on flat roads a heavier load carrying frame might give you more carrying capacity and not be an issue for your cycling effort, most of which will be fighting wind resistance at speed, not against gravity going up hills.