Giles Parkinson:

Last week, for the first time in memory, the wholesale price of electricity in Queensland fell into negative territory – in the middle of the day.
 For several days the price, normally around $40-$50 a megawatt hour, hovered in and around zero. Prices were deflated throughout the week, largely because of the influence of one of the newest, biggest power stations in the state – rooftop solar.
 “Negative pricing” moves, as they are known, are not uncommon. But they are only supposed to happen at night, when most of the population is mostly asleep, demand is down, and operators of coal fired generators are reluctant to switch off. So they pay others to pick up their output.
 That’s not supposed to happen at lunchtime. Daytime prices are supposed to reflect higher demand, when people are awake, office building are in use, factories are in production. That’s when fossil fuel generators would normally be making most of their money.