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My electric utility company sent me a graph of my home's electricity usage over time as shown in the picture below. My home beats what the company considers as efficient homes in the neighborhood when the average temperature is below ~50F. As the temperature increases above ~50F my home becomes increasingly inefficient, relatively speaking.

I'm puzzled by this. Can anyone help me understand what part of my house I need to improve? Does this mean that my home's insulation is OK, but A/C system is not? Or, does it mean that heat enters my house easily but does not escape easily, meaning insulation is good in one direction but not the other?

As reference, my house has centralized HVAC. Heating in my home is natural gas. The A/C is traditional, refrigerant-based, with the compressor located in an external unit. Most components of the HVAC system are about 20yrs old.

Thanks.

the attached graph

Fijoy Vadakkumpadan
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7 Answers7

68

The heating in your home is natural gas. You are not using any electricity to heat your home in the winter.

Therefore, from your electric company’s perspective, you are very energy efficient in the winter by not using much electricity as someone with an electric-based heating system.

statueuphemism
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Don't over think this graph. It looks like everyone in the data base has the same heating as you, gas, because if they had electric heat, the points would be much higher than 500 to 650 KWH per month. What this graph really tells you is that you're using more AC than the other people. It could be due to the position of your house in relation to the sun. You could be getting more exposure on your roof and walls due to direct sunlight and require more AC to cool it down. In the winter, you'd still have the direct sunlight that would warm up the house a bit and require less heat because there's not a direct source of cold blowing on your house. If it's sunny and 90 degrees out, the temp on the hood of your car will be much hotter due to the direct source of heat from the sun.If the sun's not out, the hood will be 90 degrees. If it's 0 degrees out and sunny, the hood of your car will be warmer due to the sun, if the sun's not out, the hood will be 0 degrees. You could check your insulation in your attic and walls to see if it's adequate and get your AC checked. We run into the exposure issue all the time down here in South Florida

JACK
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As heat can travel through your walls and roof in either direction, what you should really take from what the electricity company is telling you is that your house is inefficient all the time except when the ambient temperature outside is close to the desired internal house temperature.

If you improve the insulation to reduce the cooling energy needed then that will also help reduce the heating needed.

However, if you employ shading to reduce the incoming solar radiation in summer then that ne3eds to be removed in winter as that is useful heat addition that is free.

Solar Mike
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If your a/c system is 20 years old it is going to be much less efficient than a new a/c. This is due both to the age of the unit as well as the fact that newer units are more efficient to begin with.

It might be time to replace your a/c. I used to have the same issue with my 15 year old unit (much higher usage in the summer compared to similar homes). It died in the middle of the summer and when I replaced the a/c the problem went away (I went with the most efficient a/c model available).

On another note. I highly recommend you replace your a/c before it fails. It will fail at the most inconvenient time and you may have to wait along time to get it replaced. In my case we had to wait 2 weeks and I was forced to get 3 portable ac units just to get by. With pandemic supply chain, who knows how long you would have to wait.

At 20 years, you are past borrowed time on that unit. You should replace both the a/c and central heating at the same time.

Anonymous Coward
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Most likely, the difference is because you have AC, and many of your neighbors don't.

I'm guessing you live in the Bay Area, or somewhere close - that's a PG&E graph, if I don't miss my guess, and those temperatures are nearly identical to those in many places in the Bay Area. Most houses in the Bay Area don't have AC - where I live I can't think of one person I know who does (and the houses in my area are not cheap!). Temps that top out in the low 80s, and the low on those days is likely in the 50s, means that there's just no need for it.

If 1/3 of the folks in your area don't have AC, then - guess what - those numbers make sense.

1/3 * (500) + 2/3 * (2500) = ~1800

So maybe you're a bit inefficient, or maybe you set the AC a degree lower than the others in your area - or the numbers are more like 1/2 and 1/2 (in which case you're better than average).

In an area like the Bay Area (or really, any area with a climate like that describes - temps above freezing and never unbearably hot), there's not a lot you need to do in terms of insulation. Particularly in the summer, look at your differentials: if the house temp is 72, and the high is 84, that's a very small difference - even with terrible insulation you'll mostly not see a big difference. In the winter it's a bit more, maybe a 30 degree difference, so there the insulation matters more.

Joe
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To answer one part of question: what can you do to reduce your home's relative inefficiency in summer:

Considering the other good answers

  1. you have A/C, many of the group of compared homes do not
  2. your inside/outside temperature differential in cooling degree days is not very high. You have cool evenings and not terribly hot days.

So:

  1. Improving your insulation won't help much
  2. Using less A/C is the key.

How to use less A/C: Minimize heating from the sun and maximize natural cooling

  1. Close shades and blinds, installing better ones if needed, where the sun shines in strongly
  2. Install through-fans, ideally a strong attic fan (if you have an attic) that sucks air in through open downstairs windows and pushes it out the attic vents. Depending on the arrangement of your home, you generally want to create air flow from lower windows through the house to upper windows.
  3. Use the through-fans to cool the house at night to the coldest temperature you are comfortable with.
  4. Continue using them through the course of the day until the air being sucked in is warmer than the air inside. Then close the windows and blinds and do nothing til the house reaches the highest temperature you are comfortable with.
  5. THEN AND ONLY THEN use your A/C ... but as soon as the sun goes down, switch back to the fans.

I suspect the above will move you closer to average. If you want to get closer to "efficient" you'll just have to go colder at night and warmer in the day.

Then you can take things a step further: Upgrade your A/C. It's 20 years old. Sooner or later it will fail. When it does, install a 2-zone system (upstairs/downstairs) with timers so you only cool where it's needed, or better yet, install mini-splits in the rooms that need them most and install occupancy sensors in those rooms.

jay613
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your house is not energy efficient during the winter. it has the same insulation in winter and in summer. your electric consumption goes high during summer because you use electricity for your A/C to cool your house and switch it off during cold months.

to use less electricity during summer: Better heat insulation and more efficient A/C (e.g. DC invertor type)

dražen g
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