You want something that people buy and use all the time so that they will have a natural feel for what's a lot or a little. For a medium-sized value, you might consider the average tank of gasoline. For a large value, you might consider the energy required to heat an average size house for the average winter in Amsterdam (or London). For a really big value, you might want to consider the energy it takes to heat all of London (or all of Europe) for the average winter. For a smaller value I have used one bag of charcoal or one rick (or cord) of wood. Another example would be that a 1000 MW coal-fired plant consumes an entire train load of coal per day and can power a medium-sized city. Of course, a 1000 MW gas-fired plant consumes about the same mass of fuel, only you don't see it silently flowing through a pipe. As they say, "out of sight, out of mind."