15V

If you run your lights, blower motor and anything else electrical you will lower the voltage because you are using more amperage.
I agree with RRR 15volts is not good but I will add it will not blow anything up and Chinese voltage regulators well... Napa has better as a suggestion. Search the inter web and you will find a lot of solid state units made in USA some with adjustable voltage selection.
13.7 to 14.2-14.5 volts is standard on most alternators as high voltage.