AP Environmental Science

## Learning Outcomes

- Distinguish between **energy conservation** and **energy efficiency**
- Explain how insulation, air sealing, and duct sealing reduce home energy demand
- Calculate electricity use in **kilowatt-hours (kWh)** and estimate electricity cost
- Identify the largest categories of home energy use and judge which changes save the most energy
- Explain how thermostats, LEDs, and efficient appliances cut residential energy use
- Understand how passive solar design lowers heating and cooling demand
- Evaluate how home energy conservation reduces fossil fuel use, air pollution, and greenhouse gas emissions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand home energy conservation, with a focus on the following syllabus points:

- Know that **energy conservation** means reducing energy use, while **energy efficiency** means getting the same service with less energy
- Identify major home conservation methods, including insulation, air sealing, duct sealing, thermostat management, efficient lighting, and efficient appliances
- Explain how reducing heat transfer and air leakage lowers heating and cooling demand
- Understand that a **kilowatt-hour (kWh)** is a unit of energy, not power
- Calculate household electricity use using watts, hours, and kWh
- Estimate electricity cost from energy use and price per kWh
- Recognise **phantom load** or standby power as energy used by plugged-in devices when idle
- Understand how passive solar design uses orientation, windows, shading, and thermal mass to reduce home energy demand

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between energy conservation and energy efficiency?
2. Why do insulation and air sealing usually reduce home energy use more than unplugging a few devices?
3. How do you calculate kWh from a device's wattage and time used?
4. What is phantom load, and why does it matter?
5. How can passive solar design reduce heating and cooling needs in a house?

## Introduction

Home energy conservation is about reducing the amount of energy a household needs for heating, cooling, lighting, water heating, and appliances. This matters in AP Environmental Science because homes are major energy users, and most energy in the United States has historically come from fossil fuels. Lower home energy demand means less coal, oil, and natural gas need to be burned, so there are lower emissions of **carbon dioxide**, **nitrogen oxides**, **sulfur oxides**, and particulates.

For exam purposes, this topic is not just a list of tips. You need to understand the house as a system. The biggest energy savings often come from reducing heating and cooling demand first, because those uses usually take the largest share of household energy. That is why weatherization, insulation, and good building design are often more effective than only focusing on a few small behaviour changes.

> **Key Term: energy conservation**
> Reducing energy use, often through behaviour changes such as turning off lights, lowering thermostat settings, or using less hot water.

### Conservation, efficiency, and why homes use so much energy

Students often mix up conservation and efficiency. Conservation means using less. Efficiency means getting the same result with less energy input. Turning off an unused light is conservation. Replacing an incandescent bulb with an LED is efficiency. Taking shorter showers is conservation. Installing a better water heater is efficiency.

> **Key Term: energy efficiency**
> Providing the same service with less energy through improved technology, design, or equipment.

This distinction matters because APES questions may ask for a demand-reduction strategy and expect you to classify it correctly. A student who says "using an LED is conservation" is partly right in effect but wrong in category.

In homes, not all end uses matter equally. A common residential pattern is that energy use is dominated by:

- **space heating**
- **space cooling**
- **water heating**
- **appliances, lighting, and electronics**

A useful APES takeaway is that heating and cooling usually make up the largest share. That means if a house leaks warm air in winter or cool air in summer, the HVAC system has to run longer. This raises electricity use or fuel use and increases pollution from energy production.

A major reason homes waste energy is heat transfer. Heat moves from warm areas to cooler areas. In winter, indoor heat tends to move outdoors. In summer, outdoor heat tends to move indoors. The three big pathways are:

- **conduction** through walls, roofs, and windows
- **convection** through moving air
- **radiation**, especially solar heat through windows and roofs

If you understand those pathways, most home conservation methods make sense. Insulation slows conduction. Air sealing slows convection and uncontrolled air leakage. Shade and reflective surfaces reduce radiant heat gain.

> **Key Term: weatherization**
> Improving a home's energy performance by reducing unwanted heat loss or gain and cutting air leakage through measures such as insulation, caulking, and sealing.
>
> **Test Tip:** On APES questions, do not just say a change "saves energy." Explain how. For example: "Adding attic insulation reduces conductive heat loss, so the furnace runs less often."

### The building envelope: insulation, air sealing, and duct sealing

The most tested idea in home conservation is the **building envelope**. This means the parts of the house that separate indoors from outdoors: walls, roof, attic, windows, doors, floors, and foundation.

> **Key Term: building envelope**
> The outer shell of a building that separates indoor space from outdoor conditions.

If the envelope is weak, energy is wasted constantly. That is why many home energy experts recommend fixing leaks before replacing major equipment. A high-efficiency furnace in a leaky house still wastes energy because conditioned air escapes.

Insulation is a key defense against heat transfer.

> **Key Term: insulation**
> Material used to slow heat transfer through walls, attics, floors, and other parts of a building.

A common measure of insulation quality is **R-value**.

> **Key Term: R-value**
> A measure of resistance to heat flow; a higher R-value means better insulating ability.

Higher R-value means heat moves more slowly through the material. Students should avoid saying that insulation "stops" heat. It does not stop it completely; it reduces the rate.

Consider two attics:

- Attic A has thin insulation and many gaps.
- Attic B has thicker, higher-R-value insulation and sealed cracks.

In winter, heat escapes quickly from Attic A by conduction through the ceiling and by warm air leaking through gaps. The furnace must replace that lost heat. In Attic B, less heat escapes, so the furnace runs less. The same logic works in summer for air conditioning.

Air sealing is just as important as insulation.

> **Key Term: air sealing**
> Closing gaps and cracks where indoor air escapes or outdoor air enters.

A common student error is assuming insulation alone solves the problem. It does not. If air leaks remain, they can greatly reduce the benefit of insulation. Warm indoor air can escape through cracks around plumbing penetrations, recessed lights, attic hatches, and window frames. Outdoor air can enter through those same paths. This air movement increases heating and cooling demand.

Two common methods are:

- **caulking** for stationary cracks
- **weatherstripping** for movable joints around doors and windows
  > **Key Term: weatherstripping**
  > Material used to seal movable joints, especially around doors and windows, to reduce air leaks.

Duct leakage is another major waste pathway. If heated or cooled air travels through ducts in an attic, crawl space, or garage, leaks in those ducts mean conditioned air is lost before it reaches living spaces.

> **Key Term: duct sealing**
> Closing leaks in heating and cooling ducts so conditioned air reaches the intended rooms.

This is why duct sealing can be high impact. A home can have an efficient HVAC unit but still waste energy if ducts leak into unconditioned spaces.

A **home energy audit** helps identify these problems.

> **Key Term: home energy audit**
> A whole-house assessment that identifies where energy is being used and where it is being wasted.

Audits may include checking insulation, looking for drafts, examining ductwork, and sometimes using blower door tests or infrared thermography. For APES, you do not need technical depth on those tools, but you should know the audit is the first step because it helps prioritise the most effective upgrades.

> **Test Tip:** If an FRQ asks for the best way to reduce home energy use, start with the biggest loads: heating, cooling, and water heating. Small electronics matter, but they are usually not the largest source.

### Calculating home energy use and cutting it with everyday technologies

APES frequently tests simple energy math. The most important unit is the **kilowatt-hour**.

> **Key Term: kilowatt-hour (kWh)**
> A unit of energy equal to using 1 kilowatt of power for 1 hour.

Students often confuse power and energy. **Watts** measure power, or rate of energy use. **kWh** measures total energy used over time.

The core formula is:

$$
\text{kWh} = \frac{\text{watts} \times \text{hours used}}{1000}
$$

And electricity cost is:

$$
\text{cost} = \text{kWh} \times \text{price per kWh}
$$

Worked example:

A house replaces ten 60-watt incandescent bulbs with ten 10-watt LEDs. Each bulb is used 4 hours per day.

Step 1: Calculate daily energy use for incandescents

$$
10 \text{ bulbs} \times 60 \text{ W} = 600 \text{ W}
$$

$$
\text{kWh per day} = \frac{600 \times 4}{1000} = 2.4 \text{ kWh/day}
$$

Step 2: Calculate daily energy use for LEDs

$$
10 \times 10 \text{ W} = 100 \text{ W}
$$

$$
\text{kWh per day} = \frac{100 \times 4}{1000} = 0.4 \text{ kWh/day}
$$

Step 3: Find daily savings

$$
2.4 - 0.4 = 2.0 \text{ kWh/day}
$$

Step 4: Find annual savings

$$
2.0 \times 365 = 730 \text{ kWh/year}
$$

Step 5: If electricity costs \$0.15 per kWh, calculate money saved

$$
730 \times 0.15 = \$109.50 \text{ per year}
$$

This shows why LEDs are such a strong APES example. They use at least **75% less energy** than incandescent bulbs and can last much longer.

Common calculation errors:

- forgetting to divide watts by 1000
- treating kWh as power instead of energy
- forgetting to multiply by number of devices or days

Thermostats are another high-yield topic. Heating and cooling demand can be reduced by thermostat setbacks. The U.S. Department of Energy notes that changing the thermostat by **7°–10°F for 8 hours a day** can save as much as **10% per year** on heating and cooling.

A worked thermostat idea:

- If a house spends \$1,200 per year on heating and cooling
- and a setback saves 10%

$$
1200 \times 0.10 = \$120 \text{ saved per year}
$$

Programmable or smart thermostats help automate this. But there is an exception: some heat pumps in heating mode may trigger inefficient backup resistance heat if setbacks are too aggressive. So "lowering the thermostat always saves more" is not universally true.

Home conservation also includes reducing hot-water demand:

- lower the water heater setting to about **120°F**
- use low-flow showerheads
- use faucet aerators
- wash clothes in cold water where possible

These changes matter because water heating is often a large share of home energy use. Shorter showers reduce both water consumption and the energy needed to heat that water.

Another concept is **phantom load**.

> **Key Term: phantom load**
> Electricity used by a device when it is plugged in but not actively operating.

Examples include televisions, chargers, game consoles, and microwave clocks. This standby use can add up, but students should keep it in perspective. It matters, yet it usually saves less than fixing major heating or cooling losses. Smart power strips and unplugging unused electronics can reduce this waste.

**ENERGY STAR** appliances are also important examples of efficiency. These appliances use less energy, and often less water, than standard models. For APES, the key idea is that improved appliance efficiency lowers household demand, which lowers emissions from energy generation.

### Passive solar design and evaluating the best home conservation strategies

Home energy conservation is not only about equipment. Good building design can reduce energy demand before the HVAC system even turns on. This is the idea behind **passive solar design**.

> **Key Term: passive solar design**
> Designing a building to use sunlight, thermal mass, insulation, and shading to reduce heating and cooling needs without mechanical equipment.

In the Northern Hemisphere, south-facing windows are usually best for winter solar gain. Sunlight enters during winter, warming interior materials such as stone, brick, or concrete. These materials act as **thermal mass**, absorbing heat and releasing it slowly.

> **Key Term: thermal mass**
> Material such as brick, concrete, or stone that absorbs heat and releases it slowly.

Good passive solar homes also use:

- roof overhangs or awnings to block high summer sun
- deciduous trees for seasonal shading
- insulation and airtight construction to keep collected heat indoors
- careful window placement to avoid too much west-facing glass, which increases afternoon summer heat gain

A common misconception is that passive solar means "more windows everywhere." That is wrong. Window placement matters. Too much poorly placed glass can increase unwanted summer heating and also increase winter heat loss at night.

So how should you evaluate conservation options on an exam?

The best answer usually depends on:

- the biggest energy use in the house
- the climate
- the condition of the building envelope
- the type of heating system
- cost and practicality

For example:

- In a cold climate, attic insulation and air sealing may produce bigger savings than replacing a few bulbs.
- In a hot sunny climate, shading, reflective roofing, and reducing west-facing solar gain may be especially useful.
- In a home with old incandescent bulbs, LED replacement gives fast and reliable savings.
- In a leaky house, equipment replacement alone may disappoint unless the envelope is fixed too.

A strong APES evaluation links household actions to environmental effects:

- lower electricity demand can reduce coal or natural gas combustion
- lower fuel use reduces **carbon dioxide** emissions and climate change pressure
- lower combustion also reduces **sulfur dioxide**, **nitrogen oxides**, and particulates
- reduced demand can lessen strain on energy infrastructure and lower household costs

The central logic is simple: the cleanest energy is often the energy that never had to be produced.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Energy conservation means reducing energy use, while energy efficiency means providing the same service with less energy
- Home heating and cooling usually make up the largest share of household energy use
- The building envelope includes walls, roof, windows, doors, and other parts separating indoors from outdoors
- Insulation reduces heat transfer, and higher R-value means better resistance to heat flow
- Air sealing, weatherstripping, and caulking reduce unwanted air leakage and lower heating and cooling demand
- Duct sealing prevents conditioned air from being lost before it reaches living spaces
- A home energy audit identifies where energy is wasted and helps prioritise improvements
- A kilowatt-hour is a unit of energy, not power, and is calculated from watts and time used
- LEDs use far less energy than incandescent bulbs and are a strong example of home energy efficiency
- Thermostat setbacks can reduce annual heating and cooling costs, though some heat pumps need special caution
- Phantom load is electricity used by plugged-in devices when idle
- Passive solar design reduces home energy demand through orientation, glazing, shading, and thermal mass

## Key Terms and Concepts

- energy conservation
- energy efficiency
- weatherization
- building envelope
- insulation
- R-value
- air sealing
- weatherstripping
- duct sealing
- home energy audit
- kilowatt-hour (kWh)
- phantom load
- passive solar design
- thermal mass


Energy Conservation - Home energy conservation