Ten household air–air heat pumps were used to heat an experimental greenhouse (G hp ) with a floor area of 151.2 m2 at night in winter. The main objective is to investigate system coefficient of performance (COP). An estimation model was developed: 1) the energy balance analysis of another greenhouse (G oh ), identical and adjacent to the G hp but with a conventional oil heater, was investigated synchronously, 2) based on the energy balance analysis, the heat transmission coefficient of the G oh was estimated, 3) assuming the heat transmission coefficient was the same for both the G hp and G oh , the heat generated by the heat pumps was estimated, and 4) the COP was estimated as the ratio of heat generated to the electric energy consumed by the heat pumps. When the inside air temperature was kept at about 16 °C and the outside air temperature (T out ) ranged between −5 °C and 6 °C, the average hourly COP was 4.0, with a highest value of 5.8. Even when T out < 0, the average hourly COP was 3.3, with the heat pumps defrosting asynchronously. The spatial distribution of air temperature in the G hp was more uniform and its fluctuation with time was smaller than in the G oh .