Part IX: Tools, Calculators, and TestingChapter 15. Design Tools and Data
How Much Energy Do You Actually Use?
A household energy census: heat, hot water, and electricity, in units you can compare.
- Difficulty
- Beginner
- Cost
- Free
The next project should attack the largest piece of the household's energy, not the piece with the best photograph. Electricity, gas, oil, propane, and wood are written in different units. Convert a year of them into one unit and the order of jobs usually gets obvious: envelope and heat in a cold climate, hot water where the heat is already decent, electricity after the loads that run all night. A photovoltaic array sized before that census is how people buy modules for a bill they could have shrunk.
The bill guide is how to read a single fuel. The heat-loss guide is where the heat goes. The fuel-cost guide turns the same units into money. This page only makes them comparable. A kilowatt-hour here is site energy, the energy that crossed your meter or your tank, not the fuel a distant power plant burned to make the electricity.
Difficulty
- Level
- Beginner
- Time
- An evening with a year of bills
- Skills
- Adding fuels you usually think of as separate hobbies
Cost estimate
Free
You need twelve months, not a January bill and a July bill averaged by feel. Wood will not be on a utility statement. Estimate it with the fuel guide or leave it as a separate line you refuse to call zero.
Materials
| Item | Qty | Notes |
|---|---|---|
| A year of electricity | 12 months | Kilowatt-hours. The dollar total hides the season. |
| A year of combustion fuel | Each one you use | Therms of gas, gallons of propane, gallons of oil. Hot water is inside these if the same fuel heats it. |
| A wood or pellet note | If you burn any | Cords or tons, with the honesty the fuel guide asks for. The form below leaves wood out on purpose. |
One unit
Use kilowatt-hours as the common coin. A therm of natural gas is about 29.3 kilowatt-hours. A gallon of propane is about 26.8. A gallon of fuel oil is about 40.6. Those are approximate higher-heating-value conversions so you can add a year together. They are not your utility's billing factor if the bill already printed therms or kilowatt-hours. Prefer the bill's own unit and convert once at the end.
A heat pump's heat does not appear as gas. It appears as electricity, and the heat in the room is larger than that electricity by the coefficient of performance. Do not "correct" the electric bill upward and also add a gas bill you no longer have. Count the energy you actually bought.
The car is a different tank. Leave gasoline and diesel off this house total or you will size a solar project for a commute. Write the car down on its own line if you want to see it, but keep it out of the array math.
Hot water is usually hiding inside the gas or the electric total. You do not have to split it to see whether heat dominates. If you want to split it, a summer month of fuel, when the heat is off, is a rough floor for water and cooking, and the hot-water waste guide is what to do with that floor. Phantom electric loads are covered in the lighting guide. Neither split changes the rule: shrink the big bar before you decorate the small one.
Composite: the electric-only house that was not
Invented bills. The household adds up kilowatt-hours, feels virtuous, and prices an array. The oil tank, filled three times a year, was never converted. In kilowatt-hours the oil is the larger bar, and the envelope would have moved both the oil and the comfort before a module did. The composite lesson is to convert every fuel you pay for, including the one that arrives by truck and therefore feels unrelated to "energy." Not a real account number. The links below are other people's worksheets. Read them there.
Collect a full year
Electricity and every combustion fuel. A single month stands for the season you happened to open the drawer.
Convert to kilowatt-hours
29.3 per therm, 26.8 per gallon of propane, 40.6 per gallon of fuel oil. Electricity is already there. Wood stays a separate honest estimate.
Leave the car out of the house total
Commuting fuel will swamp a house and then tempt a meaningless array.
Aim the next project at the large bar
Heat and leaks before modules if heat is the bar. The sizing guide comes after the bill has been told the truth.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/References/Calculators/
- https://www.builditsolar.com/References/Source/TMYWeather/TMYWeather.htm
- https://www.builditsolar.com/References/Source/RadOnColProgram/RadOnCollector.htm
- https://www.builditsolar.com/References/SketchUp/SketchUpEx.htm
- https://www.builditsolar.com/References/Suppliers/SolarHeating.htm
- https://www.builditsolar.com/References/HowMuch/HowMuch.htm
- https://www.builditsolar.com/References/BestRefsRS.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.