How much should we spend to save a tonne of carbon emissions? That’s one question raised in this article for Building magazine, prompted by a report commissioned by the British Council for Offices (BCO), which attempts to attach some cold, hard numbers to the disparate costs and benefits of small-scale installations of technologies such as photovoltaic panels and biomass boilers. Their startling conclusion is that commercial developers are paying an average of £380 per tonne of emissions reduced and, in the most extreme cases, as much as £2,800. The authors argue that the economics of renewables is strikingly similar to any other method of power generation – and that you wouldn’t ask a developer to pay for a mini nuclear power station on the roof of an office block. But does such a narrow focus on costs give the full picture? Others argue that no other property investment is ever subjected to such close analysis, and that if green technologies don’t stack up, perhaps the problem is not the technologies themselves but the way the calculations are done. They pose a different question: given the urgency of cutting our carbon emissions and the cost of replacing our ageing power stations, can we afford not to seize every opportunity to reduce the bill?