Estimate the cost to cook rice including energy.
Rice is one of the few foods where the energy to cook it is a meaningful fraction of what it costs. Three hundred grams of dry rice at $2.20 a kilogram is sixty-six cents of food, while twenty-five minutes in a 700 W cooker is about 0.29 kWh — eight cents, or eleven percent of the total. The other figure worth knowing is yield: rice absorbs water and roughly triples in weight, so 300 g dry becomes about 900 g cooked, which at a 180 g serving is five portions. That tripling is why dry rice looks so cheap per kilogram and why cost per cooked serving, at roughly fifteen cents, is the number that makes it the cheapest staple on the shelf.
Rice batch cost including energy and yield
Rice cost = cost per kg x dry grams / 1000; energy kWh = watts x minutes / 60 / 1000; batch cost = rice cost + energy kWh x rate; cooked weight = dry grams x 3; servings = cooked weight / 180
Close to it. White long grain takes up about twice its weight in water, giving a factor near three; brown rice absorbs slightly more and takes longer to cook, pushing both yield and energy cost up.
Usually, because it is insulated, lidded and switches to a low keep-warm draw once the water is absorbed. A hob burner heats the room as well as the pan, so more of its energy is wasted.
Because rice is extremely cheap per gram, so a fixed energy cost is a large share of a small total. For an expensive protein the same energy is a rounding error.