Most people who buy a new Greenbuilt or Energy Star home are thrilled with how low their utility bills are. But occasionally we get calls from homeowners who suspect that their energy bills for their new house are too high. Using your HERS rating, there are a few reality checks that you can perform to figure out if the bills are actually higher than we would expect, and if so try to pinpoint the likely cause.
Your HERS rating includes estimates of annual energy use that are broken into 4 categories: heating, cooling, water heating, and lights/appliances. If you’re looking at the cost estimates, you’ll also see service charges. I think it’s easier to analyze the actual energy use: kWh for electricity and therms for gas.
The first thing I do is look at the HERS fuel summary report (which includes kWh usage for each category) and calculate several things. These are approximate, but they seem to work for most people in Western NC. In general, I find that the annual heating and cooling estimates for Asheville are a bit low in the HERS rating (due to how the model works and how people set their thermostats), hot water is usually about right, and lights/appliances is often over-predicted by up to 30%. So with that in mind, I calculate:
- Expected peak heating month: Annual heating kWh (and/or therms) divided by 3
- Expected peak cooling month: Annual cooling kWh divided by 2
- Monthly water heating: Annual water heating kWh (and/or therms) divided by 12
- Monthly lighting and appliances:
- High estimate: Annual lighting and appliances divided by 12.
- Low estimate: Annual light and appliances divided by 12, then multiplied by 0.70
- You can use either of these estimates based on how you think your usage stacks up to the typical American.
- PV generation: For PV, I find that it’s best to the PV Watts web site and get a month by month estimate of your production. You can use their estimate for each month of the year.
These numbers set my expectations for how your house will perform.
The next step is to compare the bills to the predictions above. To do this, you can use a simple method to separate the heating and cooling from the rest of the bills.
- Estimate the hot water plus lighting and appliances part of your usage: Find as many months as you have available of very mild weather when you were unlikely to have used any heating or cooling. These should be your lowest bill months. I usually find that May and October work best. If you have a couple years of bills, you can average the low months. If not, just use the lowest month you have. This kWh (and/or therms) usage should be about equal to your lighting/appliances plus hot water usage from above. If you’re using significantly more, then you have a high bill problem that is not related to heating or cooling.
- Estimate your cooling energy usage: Find the cooling month with the largest bill (probably July or August). Subtract your actual lighting/appliance/hot water energy use (from the step above) from the total energy use in this month to get your estimated peak month cooling energy use. Compare this to my estimated peak cooling month in the section above. If it’s significantly higher, you have a high bill problem related to cooling.
- Estimate your heating energy usage: Find the heating month with the highest bill (probably January or February). Subtract your actual lighting/appliance/hot water energy use from the total energy use in this month to get your peak month heating energy use. Compare this to my estimated peak heating month in the section above. If it’s significantly higher, you have a high bill problem related to heating.
So if you did find a problem, what do you do next?
If you have a high bill problem related to lighting/appliances/hot water, think about ways you may be using additional energy on these things. Also, it’s VERY helpful to understand that if you have one of these problems and it’s located inside the house (most are), then it is likely to reduce your need for heating (you may see that your heating bill was lower than expected) but your cooling energy use is likely to be higher than expected. That’s because this wasted energy becomes heat inside your house. The most common culprits are:
- Plumbing recirculation loops: most of these are big energy wasters. They are not accounted for in the HERS index right now, but they will be soon. Until then, if you have one of these loops I would suspect it. You can make this better by putting this on push-button or occupancy sensor control.
- Dehumidifiers: Energy Star models that have a humidity sensor are the best option here.
- Large loads that are not included in the HERS index: spas, hot tubs, kilns, driveway melters, electric radiant floors left on all the time in bathrooms, AV equipment, wine coolers, etc.
- An excessive amount of refrigerators or freezers. Most people don’t realize that a mini-refrigerator or wine cooler can use almost as much energy as a full-size refrigerator.
- Landscape or well pumps running all the time.
High bill problems related to cooling aren’t that common. Things to check if this happens to you:
- Are you keeping it overly cold inside? Anything less than about 72 degrees is not a good idea, and 75 would be better.
- Are you opening windows/doors while the AC is on?
- Are you trying to run the AC only during the day and open windows at night? This doesn’t work in WNC– it actually creates more humidity to remove and costs more than just running your AC all the time.
- Do you also have high lighting/appliance use? If so, that’s adding to your AC load and may be the whole problem.
- If none of this seems to apply, it’s worth a call to your HVAC installer, just to get things checked out.
High bill problems related to heating happen more frequently, but are often just a perception problem. One of the biggest perception problems is when homeowners tell me “I wasn’t even there and turned my thermostat down”. While it seems like you should use less heating energy, keep in mind that you also used less lighting/appliance energy, which heats your house (as does your living in it), so there isn’t as much savings as you might think. If this is happening to you, here are some things to look into.
- Are all your windows locked? If not, your house is 10% leakier than it could be. Lock them.
- Do you have a heat pump with electric backup? If so, it might be that your peak in January is just that much higher. Electric backup tends to use a lot more energy all in the coldest month. But if it’s using a lot more in warmer months too (like November and March) you could have a legitimate problem. I’ve had a couple clients whose heat pumps were malfunctioning and they were using backup heat all the time. If that’s happening, your thermostat should say “emergency heat” somewhere. So, a service check from your installer may be in order there.
- Are your lighting/appliance/hot water bills significantly lower than my estimate? In that case, your heating use will be a bit more to make up for it.
- Are you using a realistic thermostat setpoint? 68-70 is ideal, more than 72 isn’t a great idea.
- Are you using thermostat setbacks on a heat pump? If so, make sure you’re doing this only with an adaptive recovery thermostat and never setting it back manually.
Do you have solar PV and net metering? If so, that makes your bills a bit harder to read. You have to subtract the PV from each month that you analyze. Keep in mind that the “generation” kWh that you get from the utility is just the amount of energy that was sold to them – much of the time when you’re producing power you are also using it and not selling it to them. I find it’s best with PV to have a system like the SiteSage or a Sunpower monitor to track how much energy you are generating. If that isn’t matching up reasonably well with the monthly production from PVWatts (or the calculation your installer gave you) it’s worth checking in with the installer for troubleshooting help.
Finally, remember that every house is different, and they are different in ways that are not obvious. So comparing to your last house or your neighbor’s house is not as relevant as you might think. The HERS rating isn’t perfect, but it provides the best reality check that I know of for new houses.
Copyright 2016. Amy Musser.