They then state that 1,287 megawatt hours is enough to power 120 average US homes for a year. According to https://housegrail.com/how-many-houses-are-in-the-us/ there were more than 140 million houses in the US in 2020. Thus, the energy cost of training one LLM represents roughly 3/35000 of a percent of the energy used by all homes in the US in a year.