How far the wind and solar forecast misses

Why a forecast miss matters to the grid

Most electricity in Germany is sold the day before it is used, in an auction that closes at noon, with each trader bidding on its own forecast of the weather. By 18:00 the four grid operators publish theirs: the forecast of the next day's wind and solar output this page measures. Wind and sun then arrive as the weather allows. The gap between the forecast and what the turbines and panels produce has to be closed on the day itself, by trading in the intraday market and, in the last minutes, by the grid operators' balancing power.

The miss shown here adds up the gaps quarter-hour by quarter-hour, ignoring their sign, and divides by what was generated, so it reads as a share of the output: a miss of a tenth means the forecast was off by a tenth of what was generated. Bias keeps the signs: positive when the forecast ran high, negative when it ran low. A forecast can have a large miss and almost no bias, when it errs high and low in turn.

Regional figures run higher than the national one. A forecast that is too high in one area and too low in another partly cancels in the national total, and a small fleet, such as onshore wind in the south-west, shows its misses as a larger share of a smaller output. Offshore wind is forecast for fewer, larger parks, so a single change in wind speed moves a large part of the output at once.