[ngould]

In the U.S., the main reason prices go negative is because of the production tax credit. Wind producers get a subsidy per MWh produced. Since the unsubsidized marginal cost of production is zero, the result is a negative effective production cost. So if wind is the marginal energy producer, the clearing price goes negative.

I don't know how things work in Germany, but I'd be surprised if the negative prices there had much to do with the costs of starting/stopping conventional units. In most electricity markets, participants have to bid in their marginal cost. Even if you're an inflexible nuke, your marginal cost is positive. You usually need genuinely negative marginal cost units to drive the clearing price negative. That only really happens when subsidies are part of the picture.