An easy way of calculating temperature exposure (usually called heating degree-days) for greenhouses is required for both long term (years) energy consumption estimates, and various short term (months) calculations. The proposed method utilizes monthly means of daily temperature minima and maxima to obtain estimates of temperature exposure to any base (i.e. minimum inside) temperature. In terms of exposure intensities (temperature exposure divided by the period of time under consideration) an accuracy of 0.3 K on a monthly basis can be achieved.
For transparent structures, such as greenhouses, correction to account for the heating contribution from solar energy is also proposed. The correction is of the order of 2070 for California, and the accuracy of the final estimate for greenhouses is about 0.6 K on a monthly basis.
The model is based on the empirically verified assumption that the cumulative distribution of the outside temperature is approximately normal on a monthly basis. Estimates of the mean and standard deviation are obtained from the mean maxima and minima and used to obtain the appropriate cumulative distribution from which the exposure is calculated by integration.