Fuck credits, charge a carbon tax.

IMO it seems RECs are a better solution than carbon taxes at least in situations like this. With RECs you're buying renewable energy to offset non-renewables, with a carbon tax the company is just giving the government money for use of non-renewables. Only funds spent on RECs in this case actually go to supporting the renewable energy sector. I'm no expert in this stuff so I could be off, just how I understand it.