Six houses heated by passive solar energy were monitored during the 1983 - 1984 heating season to determine auxiliary heating fuel used. Predictions were made using the Solar Load Ratio (SLR) method for the expected use of auxiliary heating fuel. Comparisons were then made between actual performance and predicted performance.

The SLR method is used for predictions because it is a widely used tool in the design of passive solar houses, and questions have been raised as to its usefulness as a predictor of auxiliary heating fuel consumption.

Variables used in the SLR predictions and methodology for monitoring actual energy consumed are examined to explain differences in the predicted and monitored energy used.