It is hard to answer this question with a number. As a matter of fact, these fireplaces are like any other electrical equipment, and the amount of electricity it consumes depends on several different factors.  Generally, the maximum power output of one of them is between 750 and 1500 Watts. This is similar to that of an air conditioner. But it might not run at this rater power all the time. The actual power output will vary depending on the temperature setting and also the ambient temperature. Therefore, the cost of running an electric fireplace will depend on your usage pattern. But you can expect it to consume electricity similar to an air conditioner.