H3S offers a power alert system considered to be a remote GSM mains monitor that has been designed for transmitting text messages to stored phone numbers when a mains failure is detected. The unit is capable of storing up to three numbers, which are set by transmitting commands through text message to the phone number linked with the inserted SIM card. Detection of a mains failure sends up to four messages, once every hour, thus warning of a mains failure to each of the stored numbers. The unit will enter status mode after this time and will come back online only when power is restored. The unit also comprises of an auxiliary input to which an external switch could be connected.
A valid, full sized SIM card will have to be inserted before using the unit. Registration of the SIM card will have to be done earlier with the SIM card provider and will also need to have credit on it if users can pay as they go. It is also essential to unlock the SIM, and it need not be PIN protected, which most SIM cards are by default. It has been suggested for the SIM card to be first tested in a regular mobile handset in order to ensure that it works properly, since not all providers can promise consistent operation. Disabling voice message notifications could also be beneficial.