I'm running a Raspberry Pi based GNSS receiver from a 26 Ah SLA battery and an 80W panel. Just passed 2 weeks of uptime in a cloudy period of southern hemisphere autumn.

A monte carlo simulation using historical conditions said it had a ~95% chance of no downtime over 3 winter months. A slightly larger battery would bring that up to 99%.

The Pi (3b+), GNSS reciever (u-blox ZED F9P), and Waveshare 7600G 4G modem average about 3.5W idle. The GNSS reciever is about 0.1 - 0.2 W of that. Wifi would be more energy efficient, I imagine.