A solar water heater system has been built and installed at a farm scale operated digester of dairy cows wastes. The solar heating system (solar collectors and storage tank) is connected to a heat exchanger situated inside the digester in order to provide and maintain the wastes temperature at the mesophilic temperature range (37^oC ± 2). Over sixty one days the solar heating system collected 3135 kWh of heat energy. The obtained results showed that, the contribution percentages of the solar heating system were 65.30 and 51.61% and the error root mean square in predicting the digester temperature was 1.66 and 1.83^oC in cold winter months November and December, respectively. While the total biogas production rate and net energy production were increased by 71.4% and 24.1%, respectively.