Everyone knows about Tim Yeo, chair of the Commons Energy and CC committee with significant vested interests in - and plentiful income from - renewable energy schemes (google 'Tim Yeo interests' for reams of stuff). But sometimes he lets slip.
"You can't have a cap on ['green' subsidies] and guarantee meeting the carbon budgets, because no-one knows what the cost of low-carbon energy will be."
He then goes on to say that it's OK if Osborne insists on a cap in return for even more ludicrous 'carbon budgets', because the cap can later be changed, but the budgets are more binding. Needless to say this will suit his business interests well.
No-one knows. Now there's a way to run a policy.