a backup electrical power unit for use in providing a backup power supply to an alarm panel when primary power to the alarm panel is interrupted . the backup unit comprises a solar panel array , including an array output , the array producing an array dc voltage and a charging current at the array output . it further includes a battery including a battery terminal output having a battery output voltage . a first circuit is included , interposed electrically between the array output and the battery terminal output , for electrically connecting the array output to the battery terminal output at a first voltage level of the battery output voltage . this circuit disconnects the array output from the battery terminal output at a second voltage level of the battery output voltage , whereby the charging current stops flowing to charge the battery when the second voltage level is reached or exceeded ; and the charging current resumed so as to charge the battery , when the battery output voltage is at or below said first voltage level . the backup electrical power unit also includes a second circuit interposed electrically , serially with the first circuit to disconnect the array output from the battery terminal output below a disconnect , voltage level . a circuit is provided for connecting the battery output voltage to the alarm panel when the primary power to the alarm panel is interrupted to thereby provide the backup power . status indicators are provided .