Have a look here, you are sure to find what you are looking for. https://www.akku-handel.ch/

The Bosch Smart Home outdoor siren can be installed without a mains power supply thanks to its integrated solar panel with battery backup, allowing completely wireless operation. It can also operate from a low voltage supply (5-28 V) or 230 V if available. However, in order for the siren to trigger an alarm and send notifications, it must be connected to a Bosch Smart Home Controller, as without this controller, no alarm signal will be triggered in the event of tampering or break-in. Wi-Fi alone is not enough, as communication is via ZigBee with the controller. So you can mount it in a location with Wi-Fi but without mains power, using the solar panel, but you need the Smart Home Controller for the system to work fully.