or from the ground. Air source heat pumps are not effective when the outside air temperature is lower than about -15 °C, while ground-source heat pumps are not affected. The efficiency of a heat pump is measured by the coefficient of performance (CoP): For every unit of electricity used to pump the heat, an air source heat pump generates 2.5 to 3 units of heat (i.e. it has a CoP of 2.5 to 3), whereas a GSHP generates 3 to 3.5 units of heat. Based on current fuel prices for the United Kingdom, assuming a CoP of 3–4, a GSHP is sometimes a cheaper form of space heating than electric, oil, and solid fuel heating. Heat pumps can be linked to an interseasonal thermal energy storage (hot or cold), doubling the CoP from 4 to 8 by extracting heat from warmer ground. Interseasonal heat transfer A heat pump with Interseasonal Heat Transfer combines active solar collection to store surplus summer heat in thermal banks with ground-source heat pumps to extract it for space heating in winter. This reduces the "Lift" needed and doubles the CoP of the heat pump because the pump starts with warmth from the thermal bank in place of cold from the ground. CoP and lift A heat pump CoP increases as the temperature difference, or "Lift", decreases between heat source and destination. The CoP can be maximized at design time by choosing a heating system requiring only a low final water temperature (e.g., underfloor heating), and by choosing a heat source with a high average temperature (e.g., the ground). Domestic hot water (DHW) and conventional radiators require high water temperatures, affecting the choice of heat pump technology. Low temperature radiators provide an alternative to conventional radiators. Resistive electrical heating Renewable electricity can be generated by hydropower, solar, wind, geothermal and by burning biomass. In a few countries where renewable electricity is inexpensive, resistance heating is common. In countries like Denmark where electricity is expensive, it is not permitted to install electric heating as the main heat source. Wind turbines have more output at night when there is a small demand for electricity, storage heaters consume this lower cost electricity at night and give off heat during the day. Wood-pellet heating Wood-pellet heating and other types of wood heating systems have achieved their greatest success in heating premises that are off the gas grid, typically being previously