This allows a rough and ready calculation of the surplus costs associated with two different levels of variable renewables both in absolute values and in euros per MWh.4 One finding from this comparison is that the results differ from country to country. This is due both to the different costs for renewables (both in terms of plant-level costs and grid costs) and the different structures of existing energy systems in OECD countries. Substituting a MWh produced by a high-variable cost gas plant overall produces a smaller economic loss than substituting a MWh produced by a low variable cost nuclear plant. It was said above that grid-level system costs constitute the monetisable portion of the external effects of different power generation technologies mediated by the electricity grid. Total system costs would also include thus external effects that would be difficult to monetise.