The following response assumes a whole home electric tankless water heater is being considered...
1. Energy is not wasted keeping water warm while not being used.
2. No need to install natural gas or propane fuel gas piping
3. Heat exchange, i.e. electric to heated water is more efficient than the typical two element storage tank type electric water heater.
1. There is significant electrical demand when the tankless water heater is in use, sometimes as much as 100A
2. Significant electric demand means large (expensive) electric conductors and appropriately sized breakers
3. Your home's current electrical panel may not have sufficient capacity for a load of 50-100A, thus an upgraded panel and utility service upgrade may be required
4. Should an utility upgrade constitute the main electrical service sub-grid be upgraded you can be responsible for the costs in certain service areas.
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