4 ms·

"So, what a kilowatt hour of high-grade electric energy can do for you? Many useful things, i.e.: deliver 4-5 kWh of low-grade heat (or cold) into your home (d

• monochromatic 10y ago
  Heat pumps are more efficient than just using a resistive heater.
• semi-extrinsic 10y ago
  Nope. Heat pumps have a COP > 1. It's quite literally a pump moving heat from a cold place to a warm place. If it takes 1 kWh to pump 3 kWh of heat from outside your house to inside (in winter), you've heated your house by 4 kWh. Note that the electric energy used to drive the pump is transformed into low grade heat, which in this case is useful.
  • SilasX 10y ago
    Good point! It kinda threw me off too, since heat energy isn't usually measured in kWh. But yeah, heat is a kind of higher-entropy form of energy than electricity, so you can, in fact, turn one unit of the latter to more than one unit of the former. The "exchange rate" depends on the temperature; the maximum ("Carnot") heat pump efficiency is T_high/(T_high - T_low), where T_high is the temperature of the room being heated and T_low is the surrounding environment the heat is being pumped from (all on the absolute scale). When T_high = T_low, the max COP is infinity, which has the physical meaning that "You don't need to spend any energy to keep a room the same temperature as its environment; that happens automatically." https://en.wikipedia.org/wiki/Coefficient_of_performance#Derivation https://en.wikipedia.org/wiki/Coefficient_of_performance#Der...
• jrockway 10y ago
  It's an air conditioner in reverse. You run your AC at 2kWh (or so) and you've heated the outside environment by more than 2kWh; otherwise your house wouldn't be cold, energy and heat don't magically appear or disappear.
• witty_username 10y ago
  A light bulb/resistor converts electricity into heat. So the total thermal energy of the world is greater. A heat pump just transfers heat from one location to another. The total thermal energy of the world is the same.