# Offsetting with Solar, over what period do you measure the carbon offset

I'm looking to offset the carbon footprint of my wedding.

I'd like to do it by helping out some local community solar projects.

I've calculated that the wedding's footprint will be about 23,000 kg of CO2, and that this would be offset by about 37,000 kWh of solar generation (if 1kg of CO2 = 1.64 kWh).

But over what period do I measure those kWh to calculate how much of a contribution to community solar is meaningful? Over 1 year? Over 5 years? And how do I estimate what would be generated over that time period - I know peak will be far from actual, and how many sunny days do you estimate?

What's an appropriate way to calculate the appropriate contribution?

There isn't an easy answer on offsetting time: in theory, the sooner the better, because every year that passes with higher greenhouse gas levels. increases the heat content of the Earth.

If you wanted it just to come out net zero addition of greenhouse gases to the atmosphere, over the PV's whole lifetime, then reckon on 20 years if it's monocrystalline. Maybe 15 years if it's thin film. We don't have a lot of experience of the latest thin-film tech, so lifetimes are a bit uncertain, but 15-20 years won't be too far from the end result.

As for average production, there are sites like PVGIS to help you calculate that; here in Britain, and in Germany, average capacity factors are 9-10%, so you might reckon on say 800 full-load hours equivalent, per year. So if a system was 1kW peak, that would be 800 kWh each year. Accumulated over 20 years, that's 16,000 kWh. You might want to reduce that by 10-15% to account for the greenhouse-gas footprint of manufacture of the PV system itself: specifics would vary, depending on the system.

• Brilliant. That's exactly what I wanted. Thanks! Commented Oct 17, 2013 at 5:28

I see this question was asked 6 years ago ... I hope things have gone well since then. As the issue is still relevant, here is my solution:

(1) Investing in projects in developing countries has the advantage that your money is going to generate something new, and there will be a development bonus as well. The disadvantage is that it is hard to get full details about what is happening with your money. Going through a reliable intermediary (e.g. Gold Standard [https://www.goldstandard.org/]) can help. This approach is also also cheap, at \$10-15 per tonne CO2.

(2) Investing in something local will help to move your community towards greater sustainability, so it is a good option if possible.

Since sustainability is a long term issue, it would be best to put the money in a project which you can follow over time.

• This doesn't seem to answer the question. I think the OP wants to know how to calculate when he has offsetted his footprint with energy from solar panels. Commented Nov 4, 2019 at 12:58
• That is the question asked, but is it well posed in strict scientific sense? His wedding would, no doubt, have involved conversion of mineral carbon into atmospheric CO2, and solar panels don't reverse that. If he was asking for equivalence based on UNFCC conversion factors, you could arrive at a mathematical answer, but if, on the other hand, it is a more general question about compensation for the damage done, then I think my answer is relevant. Commented Dec 9, 2019 at 22:00

As an investor, I have a similar problem. For example, I want to fund the production of wind turbines. If my lifestyle requires 5 kW (peak) of wind turbines, should my wind turbine production assets be capable of producing wind turbines for the 5 kW peak production in 5 years, 10 years, or 20 years?

I also want to fund the production of electrolysis cells as an environmentally friendly way of storing energy for longer terms than few hours. Let's say my lifestyle requires 2 kilowatts (peak) of electrolysis cells. Should my electrolysis cell production assets be capable of manufacturing the electrolysis cells in 5 years, 10 years or 20 years?

Similarly, I want to fund the production of solar cells, too. Let's say I need 2.3 kW (peak) of solar cells for my lifestyle. Should my solar cell manufacturing assets be capable of manufacturing the solar cells in 5 years, 10 years or 20 years?

There's one timeframe you cannot exceed and it's the lifetime of a particular component. For example, the lifespan of solar cells is around 25 years and approximately the same is true for wind turbines too. The lifespan of electrolysis cells is measured as a function of hours of operation, so the lifetime will depend on how often these components are used.

I generally use a timeframe that is somewhat lower than lifetime of the components. So, for example I currently own enough of Vestas Wind Systems to create 540 watts (peak) of wind turbines per year. Thus, the required 5 kW will be created in bit more than 9 years.

I think this value (less than 10 years) is good, because it is faster than the need to take action on climate change. If the whole energy sector would shift entirely to renewables and clean energy storage within less than 10 years, climate change would not be a problem.

I would recommend using 10 years as the timeframe. It's less than the lifetime of the components such as solar cells, wind turbines or electrolysis cells. It's also fast enough to make a meaningful impact on climate change.

• Keep in mind that "lifetime" means different things for different systems. Solar PV degrades slowly over time, so the "lifetime" number is just a convention for how much degradation is considered "acceptable" -- usually 10 to 15%. The panels keep producing power, but if some other system component fails after that (inverter, batteries, racking), it's probably not worthwhile to replace compared to upgrading the complete system.
– LShaver
Commented Dec 28, 2019 at 16:39