Q: "My Cozy Legs radiant heater's instructions state to plug into a 115 VAC outlet. Is that a standard American household outlet?" ..Kay.
Answer: Yes, that is a standard USA outlet. Your question sent me to investigating alternating current and its voltages. (Somehow I don't recall learning this in high school physics. Or maybe I forgot?)
For residential alternating current (AC), the voltages 110V AC, 115V AC, 117V AC, 120V AC, and 125V AC are all functionally the same. Electric companies have slowly increased the line voltage over the last 100 years to cope with increased loads. The tolerance is generally +/-10%. That's +/- 11.5 volts in this case. So, a 115VAC appliance will run between 103.5 and 126.5VAC. Interestingly enough, AC power in Mexico is about 127 VAC with occasional power surges causing appliances to blow fuses.
For years, Safe Home Products has sold Kill a Watt meters and Watts Up meters that give you electrical information such as power quality of an AC outlet and what voltage, watts or amps an appliance draws, etc.
Thanks again for your interesting question.
Sincerely,
Barbara