# Determining carbon reduction vs cost of various home upgrades

I've done some amount of upgrades to my house to reduce my overall carbon emissions, and reading online there are all kinds of suggestions for doing even more:

• Replace my natural gas water heater with electric
• Put solar panels on the roof
• Other kinds of carbon offsets
• Geothermal
• Don't replace my electric oven with gas (which I had been thinking about, since I hear how great they are for cooking)

And of course I only have a limited amount of money to spend on this, so I'm trying to determine which action gives me the best bang (most carbon reduced) for my buck.

But I almost immediately run into a problem. Even just looking at that first one - "Replace my natural gas water heater with electric" - I can easily determine what the cost would be, but figuring out the carbon impact of that is tricky. I live in Colorado which is moving towards wind power, but still has some coal plants. How much carbon is my natural gas water heater emitting now? How much carbon would an electric water heater emit (when you take into account the carbon emitted to generate the electricity)?

Another example - my house was built in the 1970s so it doesn't have a lot of the energy-saving features built in to more recent houses, and I had an energy audit done that identified a lot of small items (nothing big). For example, where the tops of my walls meet the ceiling joists there are lots of tiny air gaps. Sealing those would be quite expensive, but might be worth it depending on how much energy I would save.

Is there a resource (or resources) that would help me figure out how much carbon would be reduced by each possible action? Please note that I'm not looking for common knowledge or general lists, I'm really trying to drill down and determine for my specific house, what are the exact (or approximate) impacts of each possible action? I realize that answering that question for each action will require specific information about my house and situation, I'm hoping that somewhere online there are calculators that would help me understand the impact of each action.

### BEopt for cost/benefit analysis

BEopt is a physics-based building energy modeling software designed for single-family homes intended to do exactly this. From the description:

BEopt provides detailed simulation-based analysis based on specific house characteristics, such as size, architecture, occupancy, vintage, location, and utility rates. Discrete envelope and equipment options, reflecting realistic construction materials and practices, are evaluated.

The "engine" that drives BEopt is EnergyPlus, the open-source building energy modeling program funded by the U.S. Department of Energy, and currently the gold-standard for detailed building energy modeling.

BEopt is tailored to single family homes, and simplifies the process of modeling geometry and selecting materials. It will also model weather and utility rates based on location, and includes cost estimates for most proposed upgrades. While it will not model carbon credits, I did confirm that all of the other measures you propose are included as parametrics.

To do what you're proposing, you would model your home as-is, then include all of the measures you're considering as optimization options. The output (screenshot below) will allow you to compare the first cost, energy cost and use, and carbon emissions impact of each change, and tell you which one will result in the greatest cost or emissions savings.

This output shows annual cost vs emissions for each iteration, then detail about energy per end use, and the specifics of what's modeled in each iteration.

Some BEopt caveats:

• It is a Win32 application, so be prepared for a slow, clunky interface.
• Garbage in, garbage out -- the results are only as accurate as the inputs you provide, especially for anything in your home that may diverge significantly from national averages (thermostat set points, schedule, plug loads, etc).
• Simulation is SLOOOOOW. Your PC will be doing the heavy lifting for sub-hourly modeling across a year at a spatial resolution of about a cubic meter, for as many options as you propose. Depending on your technical prowess and how extensively you plan to use BEopt, you could extract the `*.idf` files that define the project and run them in the cloud, but for a single optimization run this is probably overkill.

### eGRID for the carbon factor

BEopt will let you input a carbon emissions factor, but this does not update based on location -- you can get your state's emissions factor from EPA's eGRID database. The natural gas emissions factor is essentially identical around the U.S. so no need to adjust this.

### PVWatts to estimate solar array size

BEopt lets you input the size of a proposed solar array, but you can use PVWatts to estimate the size that would fit on your roof, based on satellite images:

When switching to electric, you should switch to an air source heat pump. They give you 2-5 times the energy you put in as heat, depending on circumstances.

So depending on the source of electricity, you might achieve break-even or 0 carbon.