Pay as produced
You take whatever the asset generates, whenever it generates it. If the wind blows at three in the morning, you have bought power at three in the morning, whether or not you were using any.
Who carries the risk: the buyer. You carry volume risk and shape risk, and you deal with the surplus and the shortfall.
What it costs: the lowest headline price of the three, because the seller has handed you the uncertainty. That headline is misleading unless you have modelled the top-up and spill.
Suits: buyers with flexible load, or with an experienced supplier already handling their shape, or generators who need the volume risk off their own books to finance.
Pay as forecast
Volumes are contracted against a forecast, and the generator bears the cost of deviating from it.
Who carries the risk: mostly the seller, within tolerance bands. Outside those bands it comes back to you, and the width of the bands is the negotiation.
What it costs: more than pay as produced, because you are buying predictability.
Suits: buyers who need a number they can budget against, and sellers confident in their forecasting.
Baseload
A constant volume in every period regardless of what the underlying asset does. The seller shapes and firms the output, filling the gaps from their own portfolio or the market.
Who carries the risk: the seller, entirely, which is why only counterparties with a portfolio can offer it.
What it costs: the highest of the three, and the premium is the price of the firming service.
Suits: buyers who want a PPA to behave like a supply contract, and who value simplicity over the last few pounds per megawatt hour.
How to compare them properly
Never on headline price. Take your own half-hourly consumption, apply each structure to the last two or three years of actual generation and market data, and see what each would have cost in total, including top-up, spill and imbalance.
The ranking often reverses. A pay as produced deal that looked five per cent cheaper can cost more once the shortfall is bought at winter evening prices and the surplus is sold at summer midday prices.