There are calculators like this one made by @upnorthandpersonal which help you calculate PV array voltage and power for low temperatures based on the specific specifications of your panels.
These are great tools and will give more precise results.
However, sometimes a quick estimate that doesn't require looking up and inputting a bunch of specs and coefficients is preferable. This table (from the 2017 NEC) is a good quick reference for those situations.
With a quick glance you can see for instance that at 0*C you should assume 110% of your arrays rated open circuit voltage, and at -25*C you should assume 120% of your arrays rated Voc.
These are great tools and will give more precise results.
However, sometimes a quick estimate that doesn't require looking up and inputting a bunch of specs and coefficients is preferable. This table (from the 2017 NEC) is a good quick reference for those situations.
With a quick glance you can see for instance that at 0*C you should assume 110% of your arrays rated open circuit voltage, and at -25*C you should assume 120% of your arrays rated Voc.