How wind, solar, and other renewable generation is forecast
Intermittent renewable generation is modelled using historical load factors to project forward generation profiles.
What is a load factor? The ratio of actual energy generated to the total installed capacity over a given period. For example, a wind farm with a 35% load factor generates 35% of its maximum theoretical output on average.
Average daily load factors for wind and solar are shown for Germany, Spain and Portugal.
Data sources
| Technology | Load factor source |
|---|---|
| Wind and solar | Weather-driven profiles from Renewables Ninja, with solar calibrated for Spain (ESIOS day-ahead profile), Italy (Terna full-fleet capacity factors) and GB (Sheffield Solar outturn) |
| Run-of-river hydro | Historical availability data from ENTSO-E |
| Biomass | Historical availability data from ENTSO-E |
The model uses a 2018 weather year, with 15-minute interval availability data to construct representative load factor profiles. These are applied to projected installed capacity for each 15-minute interval across the forecast horizon.
Filling gaps where historical data is unavailable
For technology/country pairs where historical data is unavailable, or for technologies which don’t currently exist, substitution load factors are used based on countries with similar weather patterns that have available data.
Substitution applies per technology. A country can use its own data for one technology and a neighbour’s profile for another.
| Country | Technology | Substituted with |
|---|---|---|
| Netherlands | Solar | Belgium |
| Netherlands | Offshore wind | Great Britain |
| Poland | Offshore wind | Denmark |
| Portugal | Solar | Spain |
| Ireland | Biomass | Germany |
Renewable subsidies can result in negative bidding prices
Renewable generators’ short-run marginal costs (SRMCs) are adjusted to reflect operational subsidies and renewable certificates. This can result in negative marginal costs - meaning generators are willing to pay to stay online.
Subsidised generators may receive payments for each MWh produced through:
- ROCs - Renewable Obligation Certificates
- CfDs - Contracts for Difference
- FiTs - Feed-in Tariffs
- REGOs / GOOs - Renewable Energy Guarantees of Origin / Guarantees of Origin
These subsidies are factored into bidding behaviour, which is why renewable generation sometimes bids at or below zero.
Guarantees of Origin outside GB
Outside GB, renewable generators can sell a Guarantee of Origin (GoO) for each MWh:
- GoO price – Prices from a trusted third-party source (July 2025) for 2024–2029, rising to €1.58/MWh in 2029. We then assume the price falls linearly to zero by 2035.
- Technologies – Solar, onshore and offshore wind, run-of-river and reservoir hydro, and biomass.
- Share of the fleet – Only 60% of each fleet bids with the GoO value. The other 40% bids without it.