The first difference is the way that offsets and Renewable Energy Credits (RECs) are measured. Carbon offsets are measured in metric tons of C02 or C02 Equivalent. Renewable Energy Credits are measured in kilowatt-hours as a standard electrical measurement metric. A kilowatt hour is the amount of work that can be performed by one kilowatt of energy in one hour.

Picture a lonely, dim bulb hanging from the ceiling that turns on one hour every day, you feverishly darn socks in a carbon constrained world and it is a watt, and for the privilege of its use, you will be charged for 1 / 1000 kWh of electricity every day. These days, you probably use a more kWh per day.

The second difference between carbon offsets and renewable energy credits is that renewable energy credits only from renewable energy projects (solar, wind geothermal, biofuels, etc.), while carbon offsets can get from all sorts of different projects, including renewable energy, reducing levels of greenhouse gases entering the atmosphere.

Put another way, REC principally engaged in promoting the production of clean energy, while carbon offsets primarily involved in the prevention of emissions entering the atmosphere.

Both systems have evolved to cope with global warming systematic, but they have different approaches. REC is forward looking, focused on building a clean energy economy and provide an extra incentive for the establishment of renewable energy, while carbon offsets are oriented in the present, engaged in preventing greenhouse gases from entering the atmosphere right now.

Because of these different measurement systems and the different foci of the two programs, RECs and carbon offsets have different precision rates when it comes to carbon. Carbon offsets is about accuracy, and many of the discussions on the impact of offsets center around the degree of security a buyer has that the exact amount of carbon he / she has paid for it actually is prevented or captured. REC, on the other hand, is measured in kilowatt hours and carbon content of the “hidden” kwh varies depending on the location of the project and the quality of local power.

The dirty local electricity, the more carbon a REC ‘saves. ‘Different utilities around the country use different combinations of energy sources from coal to natural gas to renewable energy to generate electricity. These sources vary widely in their carbon content. To make things even more confusing, maybe a utility even alter mix it uses depending on the time of day when peak demand set in, they may have to rely on dirtier power sources than they would otherwise.

So it is impossible to say exactly how much carbon a pure kWh of renewable energy ‘offsets. “The closest we can come is to use the emissions factor for energy from the local utility, which is the average emissions for a mixture of sources, the tool uses to generate power, and multiplying it by the number of kilowatt-hours to produce an estimate of carbon saved per kilowatt. But it will always be an estimate.

This is not to say that the REC is not good. They are a very effective way to promote clean energy because they allow suppliers and extra incentive to keep creating clean energy and we need all the incentives we may be towards a clean energy economy. REC just not the most accurate way to offset carbon. I highly recommend using RECs to offset electricity consumption because your electricity bills will have a record of the exact number of kWh you’ve used and you can buy RECs to account for all the dirty emissions your plugged-in Macbook caused. You can then buy carbon offsets to cover all your driving and flying.

“But what about renewable carbon offsets? ‘Says you. ‘These seem like the best of both worlds! I’m getting there. These are good, and if you really appreciate the promotion of clean energy in spite of the accuracy issues, you can buy renewable energy offsets. Many times offsets will actually come from exactly the same projects as REC, but the good thing about buying the offset version instead of buying RECs and do the calculations themselves are that another, hopefully a third party verifier determines how much carbon per kWh of clean energy replaced. So you do not have to! Rest easy and lay off the carbon debt.