How the estimate is calculated
Energy first. Each item is multiplied by a published energy figure: prompts per day times Wh per prompt, plus images times Wh per image, plus video seconds times Wh per second. Monthly totals use 365 / 12 days and yearly totals use 365 days.
Carbon and water from energy. Emissions in grams equal kWh times grid intensity in gCO2e per kWh. The default is 349.7 g/kWh, the US average from EPA eGRID2023 (770.884 lb CO2e per MWh). Water in mL equals Wh times water use effectiveness in L/kWh, because the two factors of 1,000 cancel. The default of 0.55 L/kWh is the US average on-site WUE used by Li et al. in Making AI Less Thirsty. Tick the off-site option to add 3.142 L/kWh for water evaporated at power plants.
Worked example. The default of 20 ChatGPT prompts at 0.34 Wh and 2 images at 2.907 Wh is 12.6 Wh per day, or 4.6 kWh per year. That is about 2.5 liters of on-site water, 1.6 kg CO2e on the average US grid and 242 smartphone charges at EPA's 19 Wh per charge.