Balanced, Every Second

An electricity grid is not a reservoir. Nothing accumulates in it. Whatever is being consumed at this instant is being generated at this instant, across an entire synchronised area, and the matching has to hold continuously. Almost everything difficult about electricity follows from that single fact, including most of what is difficult about renewable generation, and it is rarely explained.

The balance has a readout. Every large synchronous generator on the system turns in lockstep, and loading them draws energy from their rotation, so they slow. The frequency therefore falls when demand exceeds generation and rises when generation exceeds demand, measurable anywhere on the network. A control room watching frequency is watching supply meet demand in real time, and the tolerance is narrow: drift far enough and protective equipment begins disconnecting, which removes generation, which makes the imbalance worse.

What buys the operator time is inertia: energy stored in the rotating mass of turbines, which resists sudden change. It comes free with any large spinning machine, and it is the reason a system dominated by coal, gas, hydro and nuclear tolerates the sudden loss of a generator. Solar panels and wind turbines connect through inverters and have no rotating mass coupled to the grid. Systems with high shares of them must either keep some synchronous plant running, install synthetic inertia in the inverter control software, or respond to deviations far faster than any inertia would have required.

This explains something otherwise puzzling about grid batteries. Their early economics were driven not by storing energy for the evening but by frequency response: injecting or absorbing power within milliseconds, orders of magnitude faster than a thermal plant can react. Bulk energy shifting needs many hours of capacity and works on entirely different economics. The two applications share hardware and are otherwise different businesses.

Then there is curtailment, which reaches the press as waste and is really geography. Wind farms are built where the wind blows, which is generally not where the demand is, and the lines between the two have finite capacity. When the constraint binds, output is cut regardless of unmet demand three hundred kilometres away. The fix is pylons, not turbines, and pylons require planning consent through populations that receive none of the benefit.

The physics of long-distance transmission is, by contrast, surprisingly accommodating. Raising the voltage cuts the current for a given power, and resistive loss goes as current squared, so high-voltage lines lose only around two to three per cent per thousand kilometres. Moving power across a continent is physically reasonable. What stops it is cost and consent.

The general lesson is that an electricity system is a control problem before it is a generation problem. Adding capacity that produces power at times not of its choosing does not simply add a number to a total; it changes what the control problem is. That is neither an argument against renewable generation nor a detail, and treating it as either produces most of the bad argument in this area.