Not optimal in winter for outdoor entertainment

I have four SnuggleSafes in use for our rabbits who live all year in the garden. In winter, a maximum of 3-4 hours SnuggleSafe keeps warm. According to the manufacturer, the heating pad keeps reportedly even up to 10-12 hours warm.
Hm, but not at temperatures below 5Grad plus.
The heating in the microwave at 4 SnuggleSafes every night is this quite expensive for such a short duration heat.
As probably worth more a rabbit Accessible heating pad. #
I would not buy anymore.