Energy: Stores, Transfers, and Conservation

A parked bicycle and a moving bicycle both have energy. Motion is only one way energy can be stored. Energy is a quantity we track when things change; it is not a material we can pour or hold.

Choose what you are studying

A system is the object or group of objects you choose to study. Everything outside it is the surroundings. You could study just the water in a kettle. You could also study the kettle and water together. The choice changes which transfers count as energy entering or leaving your system.

Name the store

One object can be part of several stores. A hot car driving uphill has kinetic energy and thermal energy. Its arrangement with Earth also gives it gravitational potential energy. A rock resting on a cliff has no kinetic energy, but its other stores remain.

Measure an amount

The energy unit is the joule, written J. Lifting an apple about 1 meter takes roughly 1 J. Large amounts are easier to write in kilojoules or megajoules:

For example, 2.5 kJ means 2.5 groups of 1,000 J:

Changing the unit changes the number, but not the amount of energy.

Connect stores with forms of energy

Some books use forms of energy and energy transformations. “Kinetic energy” means energy in a kinetic store. “Chemical energy” and “thermal energy” match chemical and thermal stores. Light, sound, and electrical energy describe energy being transferred between stores.

Potential energy comes from the position or arrangement of things that interact. These include an object and Earth, parts of a spring, magnets, charges, and particles in substances. Kinetic energy is energy of motion. Thermal energy is often grouped with motion because particles move randomly inside materials.

A force is a push or pull, measured in newtons (N). Energy is measured in joules. Say, “The moving ball has kinetic energy,” not “The ball contains force.” A force can transfer energy as it moves something.

A store name tells you the kind of energy. To compare amounts, you must also know what changes and what stays the same.

Mass and speed

At the same speed, a heavier object has more kinetic energy. Doubling its mass doubles the kinetic energy. At the same mass, doubling speed gives four times the kinetic energy.

These cart data show the difference:

Compare the first two rows: speed doubles, and energy rises from 2 J to 8 J. Compare the first and last: mass doubles, and energy rises from 2 J to 4 J. A bicycle moving at 20 km/h has four times its kinetic energy at 10 km/h. This helps explain why higher-speed crashes can cause much more damage.

At fixed speed, a graph of kinetic energy against mass is a straight line through zero. At fixed mass, a graph against speed curves upward and gets steeper. Equal increases in speed bring larger and larger energy increases.

Height and stretch

The gravitational store depends on mass, height, and gravity strength. Lifting a heavier object or lifting it higher increases the store. Near Earth's surface, doubling the height lifted doubles the energy added. The same lift stores less energy on the Moon, where gravity is weaker.

Choose a reference level, such as the floor. A book's gravitational energy changes when its height above that level changes. The store belongs to the book and Earth together because they attract each other.

Stretching, squashing, bending, or twisting an object increases its elastic store while it can spring back. At the same stretch, a stiffer spring stores more energy than a softer spring.

Magnets and charges

Separating attracting magnets takes a pull and increases their magnetic store. Letting them come together decreases it. Pushing repelling magnets closer increases their store. The direction depends on whether the objects attract or repel.

Charged objects behave in a similar way. Pulling an attracting charged balloon away from your hair increases the electrostatic store of that arrangement.

Temperature, material, and amount

The thermal store depends on temperature, mass, and material. Warming the same object increases its thermal store. More water at the same temperature has a larger store. A warm bathtub can hold far more thermal energy than a small cup of hotter tea. Temperature alone cannot tell you the total.

Chemical energy depends on the substance and its amount. More fuel holds more energy; fat holds more per gram than sugar. A running flashlight decreases its battery's chemical store.

Nuclear energy depends on the types and numbers of nuclei. Splitting very heavy nuclei, such as uranium, releases energy. Joining very light nuclei, such as hydrogen in the Sun, also releases energy.

A store describes where energy is held. A transfer pathway describes how energy moves from one store to another. Four main pathways are mechanical, electrical, heating, and radiation.

Transfers by forces

A force transfers energy mechanically when it moves something. This is called doing work. Pushing a cart, lifting a bag, and stretching a bow all involve mechanical transfers. When a bow launches an arrow, the string pushes the arrow. The bow's elastic store decreases as the arrow's kinetic store increases.

A push alone does not tell you how much energy has moved. You also need to consider the movement it causes. A force and an amount of energy are different quantities.

Transfers in circuits

An electric current transfers energy electrically. A battery can supply energy to a lamp, motor, or heater through a circuit. The battery's chemical store decreases. The receiving stores depend on what the circuit does: a turning motor can increase a load's kinetic store.

In circuit talk, electricity usually means flowing electric charge. Electrical energy is the energy being transferred. Charge goes around a complete circuit and is not used up in the lamp. As much charge returns to the battery as leaves it. A battery runs down because its chemical energy store decreases.

Transfers from hotter to cooler

Heating transfers energy from a hotter object to a cooler one. A hot pan heats a cooler hand. Warmer room air transfers energy to a cold drink. Without further heating or cooling from elsewhere, the temperatures move toward the same value.

Heating can happen through touching materials, called conduction, or through moving liquids and gases, called convection. The temperature difference sets the direction. When ice cools your hand, energy leaves the warmer hand and enters the colder ice. “Cold” is not flowing into your skin.

Keep these three ideas separate:

Transfers by waves

Light and infrared are electromagnetic radiation. They carry energy and can cross empty space. Sunlight reaches Earth this way. Light absorbed by a wall can increase the wall's thermal store.

Sound also carries energy, but it needs matter, such as air, water, or a solid. It cannot cross empty space. Sound involves particles vibrating and pushing on nearby particles, so it is linked to mechanical transfer. A fading sound spreads energy through its surroundings, warming them slightly.

Light, sound, and current describe ways energy travels. They are not extra stores to add to an object alongside its chemical or thermal store.

Start by comparing the beginning and end of an event. Look for a store that decreases and one that increases. Then explain the physical process linking them.

Build the account

1. Choose your system.
2. Describe what changes: speed, height, stretch, temperature, or chemical state.
3. Name the stores that decrease and increase.
4. Name the transfer pathways.
5. Include every transfer so the energy totals balance.

A ball thrown upward

After the ball leaves the hand, it rises and slows. Its kinetic store decreases while the gravitational store of the ball and Earth increases. Gravity transfers energy mechanically.

At the highest point, the ball stops for an instant. Its kinetic energy is zero, but its gravitational energy is not. As it falls, the gravitational store decreases and the kinetic store increases. Air resistance also transfers some energy to thermal stores of the ball and air.

A flashlight diagram

A stores diagram uses boxes for stores and arrows for pathways. For a filament flashlight, the chain is:

Chemical store of battery → electrically → thermal store of filament → by radiation and by heating → thermal store of surroundings.

The filament becomes hot and gives out light. The room absorbs the light and warms slightly. “Lamp” alone is a device name, so a box should name the store in that lamp. The arrows show how energy reaches the next store. This diagram is also called an energy chain or energy flow diagram.

Other devices have different destinations. A charging phone increases its battery's chemical store. A kettle increases the water's thermal store electrically and also warms the kettle and air. A plant absorbs sunlight and increases its chemical store. A solar panel receives radiation and transfers energy electrically. Burning wax decreases a candle's chemical store, with transfers by heating and radiation.

Explain what causes the change

When a car brakes, friction slows it down. Its kinetic store decreases and the brakes' thermal store increases. The hot brakes then transfer energy to cooler surroundings by heating.

For a car speeding up, the engine turns the wheels. The tires push backward on the road, and the road pushes forward on the tires. That forward force speeds up the car as energy from the fuel's chemical store reaches its kinetic store. Naming energy alone does not explain the force or process involved.

Use evidence

Suppose a rolling ball hits a box. The ball slows and the box starts sliding. A clear argument has three parts:

• Claim: Energy was transferred from the ball to the box.
• Evidence: The ball slowed while the box began moving.
• Reasoning: The ball's kinetic store decreased and the box's increased. The box's new kinetic energy came from a transfer during the collision.

Sound or warming can show that some energy reached other places too. The moving box need not receive all the energy the ball lost.

Energy cannot be created or destroyed. This is the law of conservation of energy. An object can lose energy, but that energy must reach another store or leave the chosen system.

Choose the boundary

When no energy enters or leaves a system, its total energy stays the same. A cooling drink alone loses energy. Include the drink and everything it warms, and the decrease in one store matches increases elsewhere.

A device can also keep some incoming energy in its own stores. A charging battery does this. For a device whose own stores do not change overall, the account is:

Find a missing amount

A kettle receives 240,000 J. The water gains 216,000 J. The remaining energy warms the kettle and air:

Check by adding the two gains. They total 240,000 J. For a motor receiving 500 J and lifting a load with 350 J, the wasted part is:

The 150 J still exists, mainly in thermal stores after heating and sound have spread it out.

Follow a swing

At the highest point, a pendulum bob stops briefly. Its kinetic store is empty and its gravitational store is largest for that swing. At the lowest point, the bob moves fastest and its kinetic store is largest.

Air resistance and friction at the pivot transfer energy to the surroundings. Each swing rises a little less. The pendulum's moving-and-raised stores shrink, while thermal stores elsewhere grow.

Compare bounce heights

A ball is dropped from rest at 2.0 m and returns to 1.5 m. For the same ball near Earth's surface, gravitational energy follows height. Compare its energy at the two points where it is briefly at rest:

The rebound has 75% of the original gravitational energy. The remaining 25% reached thermal stores and traveled away as sound during the fall, impact, and rebound. The ball's squashing also temporarily stored energy elastically during impact.

Explain a height limit

A roller-coaster car released from rest cannot rise above its starting height without more energy input. Reaching a greater height would need more gravitational energy than it started with. Friction and air resistance reduce the energy left for climbing, so a real car reaches less than the ideal height.

An engine or another push can add energy and change this limit. Always include that input when following a powered ride.

People say energy is “used up” when a battery runs down. Its chemical store has decreased; energy has spread elsewhere. “Saving energy” means needing less from fuel stores or paid electrical supplies. Real machines dissipate energy as they operate, so a machine cannot keep delivering work forever without a fresh supply.

A speaker is meant to make sound. A toaster is meant to warm bread. Whether a transfer is useful depends on the job you want done.

Decide what serves the purpose

Light from a lamp is useful. Warming the lamp and room is usually wasted for that lighting job. Heating from a room heater is useful because warming the room is its purpose. Sound is useful from a speaker but usually wasted from a vacuum cleaner.

Wasted energy has not vanished. It has been transferred in a way that does not serve the intended purpose. It often ends up warming the surroundings.

What dissipation means

Dissipation is energy spreading out into the surroundings. Each part of a large room may warm by a tiny amount. The energy is still present, but gathering it back for another useful job is very difficult.

For example, light from a lamp is absorbed by walls and objects. Their thermal stores increase. Sound spreads and warms its surroundings slightly. A bicycle's motion eventually warms brakes, tires, road, and air. Conservation describes the unchanged total; dissipation describes the spreading out.

Match the improvement to the waste

An explanation should connect the change to its effect. “Oil makes the machine better” gives little detail. “Oil reduces friction between the moving parts, so less energy warms them” identifies the wasted transfer.

Keep useful friction

Friction has useful jobs too. Tires need grip to turn and stop. Brake friction deliberately removes a vehicle's kinetic energy. Reducing all friction would make the car harder to control.

Choose the place where the unwanted transfer happens. Lubricating an engine's moving parts can reduce waste. Making tire-road grip vanish would remove a force the driver needs.

A stores diagram shows where energy goes. A Sankey diagram adds amounts: the width of each arrow represents energy or power. Wider arrows stand for larger amounts.

Read the branches

The input arrow enters from the left. The useful branch continues straight ahead. Wasted branches turn away, usually downward. Labels name each transfer and give its amount and unit.

The output widths must add to the input width. This shows conservation of energy. Arrow length does not measure energy. A long, thin arrow can represent less energy than a short, wide one.

Draw a motor's energy account

A motor receives 200 J. It transfers 140 J usefully to a load's kinetic store, 50 J by heating, and 10 J by sound. Use squared paper with a scale of one square of width for 10 J.

1. Draw the input shaft 20 squares wide.
2. Continue a useful shaft 14 squares wide straight ahead.
3. Branch off a heating shaft 5 squares wide.
4. Add a sound branch 1 square wide.
5. Label the input and every output with their amounts and pathways.

Check the values:

Check the widths:

The widths are measured across each arrow shaft, at right angles to its direction. Keep the branch widths the same as they turn.

Find a missing branch

Another diagram has 400 J input, 100 J useful output, and 20 J sound. The heating branch must show the rest:

At 20 J per square, the widths would be 20 squares input, 5 useful, 1 sound, and 14 heating. The heating branch is the largest output, even if it is drawn shorter than the useful branch.

Use watts or find a fraction

A Sankey diagram can show power in watts. A 60 W filament lamp giving 3 W of light has 57 W of wasted output. All labels on that diagram must use power, not a mixture of W and J.

The useful fraction can be found from amounts or widths. For the first motor:

So 70% of the input is useful. The diagram's scale cancels because all widths use the same scale.

When amounts are unknown, use a simple stores-and-pathways diagram. Drawing equal-width Sankey branches without numbers would suggest equal amounts, even though you have no evidence for that claim.

Efficiency tells you what share of a device's input is useful. A device that sends three quarters of its input to the intended job has efficiency 0.75, or 75%.

Calculate efficiency

Use matching units before dividing. Multiply the decimal by 100 for a percentage. Efficiency is a ratio, so it has no unit.

A kettle receives 240,000 J and transfers 216,000 J to the water:

A filament lamp receives 60 J each second and wastes 57 J by heating. First find useful light output:

Then divide useful by input:

Dividing 57 by 60 would give the wasted fraction. It would not give efficiency.

Use power instead

You can compare useful output power with input power:

A motor takes in 1,200 W and provides 900 W usefully. Its efficiency is 900 divided by 1,200: 0.75, or 75%. Power works because it compares energy transferred in the same amount of time.

Find a useful output

Multiply the input by the decimal efficiency:

An engine receives 400 kJ at 25% efficiency. Change 25% to 0.25 first:

It transfers 100 kJ usefully and wastes 300 kJ. Multiplying by 25 would treat 25% as twenty-five whole inputs.

Find the input needed

Divide the useful output by decimal efficiency:

A device must deliver 180 J usefully at 60% efficiency:

The needed input is larger than the useful output. Of the 300 J, 120 J is wasted.

Check the units and result

For 0.5 kJ useful output from 2,000 J input, first convert 0.5 kJ to 500 J. The ratio is 500 divided by 2,000, giving 0.25 or 25%. Using 0.5 directly would mix units.

Efficiency cannot exceed 100% when all inputs are counted and the device is not drawing down its own stores. A result above 100% often comes from reversing the division. Also distinguish 0.7 from 0.7%: 0.7 is 70%. A room heater can be close to 100% efficient for room warming because nearly all its electrical input ends up warming that room.

A fair efficiency comparison starts with a clear job. For lamps, that might be giving the same amount of light. For motors, it might be lifting the same load through the same height.

Compare equal light outputs

Suppose a 9 W LED lamp and a 60 W filament lamp each give 3 W as light. Their efficiencies are:

The LED wastes 6 W, while the filament lamp wastes 57 W.

Both lamps do the same lighting job. The LED needs much less input for it. Over the same operating time, it uses less electrical energy and costs less to run at the same unit price.

Separate power from efficiency

A 1,500 W room heater has high power and can be close to 100% efficient for warming the room. The 9 W LED above has low power and 33% efficiency for light output. A low power rating alone cannot tell you the useful fraction.

A large motor can be powerful and efficient. A small motor can have low power and waste a large share. You need useful output and total input to judge efficiency.

Improve the largest wasted transfer

Electric motors usually transfer a larger share of their input usefully than gasoline engines. Engines transfer much of their fuel energy to hot engine parts, exhaust, and air. This helps explain why a similar electric car needs less energy to cover the same distance.

An improvement should address a real unwanted transfer. Lubrication cuts heating from friction in moving parts. Streamlining reduces drag. Insulation slows heating from a hot oven or water tank into the room. A better lamp design can give the required light with less wasted heating.

Compare energy labels

Annual-use labels estimate electrical energy in kWh per year. Compare appliances doing similar jobs: for example, refrigerators of the same size. A larger refrigerator may use more energy simply because it has more space to cool.

A model with lower annual use costs less to run at the same electricity price. Its purchase price may be higher, so compare both purchase and running costs when choosing one. If the electricity supply burns fuel, needing less electrical energy also reduces the associated fuel use and pollution.

A lamp marked 60 W transfers 60 J each second. Power measures the rate of energy transfer. It tells you how quickly an energy amount is being transferred.

Watts and kilowatts

One watt (W) means one joule per second. One kilowatt (kW) is 1,000 W. Typical electrical inputs include a 9 W LED lamp, a television around 100 W, a 1,000 W toaster, and a 1,500 W hair dryer or space heater. Appliances that heat things often have high power ratings.

Energy, power, and time are linked:

Here, power is in W, energy in J, and time in seconds. At steady power, twice the running time transfers twice the energy.

Work out an energy amount

A toaster takes in 1,000 W for 90 s. Multiply the energy per second by the number of seconds:

For a 1,500 W hair dryer used for 5 minutes, change minutes to seconds first:

Using 5 seconds in that multiplication would make the answer 60 times too small.

To find power, divide energy by time. A motor transferring 6,000 J in 20 s has power 6,000 divided by 20, or 300 W. To find a time, divide the energy needed by the power available. Each formula follows the same energy-per-second relationship.

Include how long it runs

High power does not always mean the greatest total energy use. A 9 W lamp running for 10 hours operates for 36,000 s:

A 1,500 W hair dryer used for 3 minutes operates for 180 s:

The lamp transfers more energy because it runs much longer.

Read the right rating

Use the appliance's electrical input to calculate energy taken from an outlet. A microwave sold as “1,000 W” may be describing cooking output. Its electrical input is higher, often around 1,500 W. Look for the input value on its label plate.

A refrigerator or oven can switch on and off as its thermostat controls temperature. Its maximum running power multiplied by 24 hours overstates daily use when it cycles off. Use the time actually running, a suitable average power, or an annual energy-use label. A charger's rating may give maximum output; its input also changes as the battery fills.

Household energy amounts would be very large numbers in joules. A kilowatt-hour (kWh) is a larger energy unit, useful for appliances and electricity meters.

What one kWh means

A 1 kW appliance running for one hour transfers 1 kWh. Since 1 kW is 1,000 W and an hour is 3,600 seconds:

A kilowatt-hour is an amount of energy. A kilowatt is a power, or rate. An appliance does not have a “kWh power rating.” Its energy use also depends on how long it runs.

Multiply kilowatts by hours

First divide watts by 1,000 and minutes by 60. A 1,500 W space heater used for 3 hours has power 1.5 kW:

A 9 W lamp used for 5 hours has power 0.009 kW:

The small decimal is sensible for a low-power lamp used for only a few hours.

Handle minutes as part of an hour

A 1.5 kW hair dryer runs for 20 minutes. This is one third of an hour, not 0.20 hours:

Using the fraction keeps the calculation exact. You can round the final answer when needed. “Kilowatts per hour” is not the energy unit; multiplying power by time gives kilowatt-hours.

Read a meter

An electricity meter records a running total of energy supplied. Subtract the earlier reading from the later one:

For readings of 45,210 kWh and 45,830 kWh:

The home used 620 kWh between readings. The later reading alone includes energy from before that period.

Convert an appliance amount to joules

A charger takes an average electrical input of 10 W for 2 hours. Use the average input because charging power changes:

Convert the resulting energy to joules:

To go from joules to kWh, divide by 3,600,000. The conversion describes the same amount in different units, just as meters and centimeters can describe the same length.

Electricity costs depend on energy used and the price charged for each kilowatt-hour. Use 18 cents per kWh for these examples.

Find the running cost

A 1.5 kW hair dryer runs for 10 minutes a day over 30 days. First combine the times:

Then find the energy:

Finally find the cost:

That is $1.35. If you multiply by 0.18 dollars instead, the answer is already in dollars. Do not turn 135 cents into 135 dollars.

Read an energy charge

For a home using 620 kWh, the energy charge is:

If the bill also lists a fixed monthly charge of $12, add it once. The total is $123.60. A fixed charge does not depend on how many kWh were used. Fuel bills follow the same amount-times-unit-price idea, using the unit listed on the bill.

Compare yearly use

At 4 hours daily for 365 days, a 60 W lamp runs for 1,460 hours. A 9 W LED providing similar light saves 51 W, or 0.051 kW:

Rounded at the end, that saves about $13.40 per year.

An annual energy label can shorten this calculation. A refrigerator labeled 400 kWh per year costs $72 at the example price. A similar model using 300 kWh costs $54. The yearly saving is $18.

Include standby

A television drawing 1 W on standby for 20 hours daily uses:

At 18 cents per kWh, that costs about $1.31. Each standby input is small, but many devices over long times can add up.

Make a home energy audit

List appliances, their electrical inputs, and daily running times. Convert inputs to kW and times to hours. Multiply each pair to find kWh per day, then multiply by the unit price. Add the energy amounts for a daily total.

Use annual labels or measured averages for cycling appliances. Compare the calculated daily amounts before choosing changes: heating and cooling appliances, or devices used for many hours, often offer the largest savings. A short use of a powerful appliance can cost less than a smaller input left on all day.

Food holds energy in a chemical store. Cells release energy through respiration. In aerobic respiration, oxygen takes part in the reactions. Plants first transfer energy from sunlight into the chemical store of the substances they make.

Calories and kilojoules

A food Calorie, with a capital C, is one kilocalorie (kcal). It is a unit of energy, not an ingredient or a material. One kcal equals 1,000 small calories.

To convert Calories to kJ, multiply by 4.184. A 190 Calorie snack bar contains:

To go the other way, divide. A food amount containing 1,000 kJ holds about 239 kcal.

Compare the same amount of food

Use per 100 g values to compare how energy-rich foods are. Use per serving values for the energy in the portion eaten. Different serving sizes can change which number looks larger.

A cereal contains 1,600 kJ per 100 g. A 30 g serving is 30 hundredths of that amount:

Doubling the serving doubles its energy. A label's per-serving number does not describe the whole package unless the package is one serving.

Fat gives about 37 kJ per gram, or 9 kcal. Carbohydrate and protein each give about 17 kJ per gram, or 4 kcal. So 10 g of fat supplies about 370 kJ; 10 g of sugar supplies about 170 kJ. Water, vitamins, and minerals supply no energy.

Where the energy goes

Energy from food supports movement, body warmth, the heart, lungs, brain, growth, and repair. Muscles transfer energy mechanically when you lift or push. Energy taken in beyond what the body transfers is added to its chemical stores, mostly as fat. Drawing more energy from stores than is taken in reduces them.

US food labels use 2,000 Calories per day as a general reference. Actual needs vary with age, body size, sex, and activity. The label reference is not one target for every person.

Connect food with power and kWh

An energy amount of 2,000 Calories is about 8,368 kJ. Spread over 86,400 seconds in a day, this is an average rate of about 97 W. Activity changes the rate: running transfers energy faster than sitting.

A 250 Calorie snack holds about 1,046 kJ. That equals about 0.29 kWh, enough energy for a 1,500 W hair dryer to run for roughly 12 minutes. Comparing the units lets you compare the amounts in food and electrical supplies. In the body, most transferred energy eventually reaches thermal stores of the body and surroundings.

Burning food releases energy from its chemical store. Some of that energy can warm water. Measuring the water's temperature rise gives an estimate of the energy the water received.

Set up the comparison

In a supervised school experiment, place a measured mass of water in a clamped boiling tube. Weigh the food sample. Mount it on a needle, light it, and hold it below the tube. Measure the water temperature before and after, and record any food left unburned.

Wear eye protection and work over a heatproof mat. Keep hair and loose clothing away from flames. Do not burn nuts or nut products where anyone may have a nut allergy.

For fair comparisons, use equal starting food masses, equal water masses, and the same flame-to-tube distance. Keep the setup and starting conditions alike. Burn each sample completely where possible. Repeat tests and calculate a mean to reduce the effect of chance variation.

Calculate the temperature rise

Subtract the starting temperature from the final temperature. If 20 g of water warms from 20 °C to 50 °C:

The final temperature alone is not the rise. About 4.2 J warms one gram of water by one degree Celsius:

For this water:

Compare per gram

If 1.0 g of food burned, divide the captured energy by 1.0 g. The measured result is 2.52 kJ per gram. If residue remains, use the mass actually burned when reporting energy released per gram burned.

A label showing 1,840 kJ per 100 g means 18.4 kJ per gram. The classroom result captures about 14% of that value:

A larger temperature rise per gram indicates more captured energy only when water masses and capture conditions match. Twice as much water needs twice as much energy for the same rise.

Explain a low result

The flame also warms the glass, clamp, and surrounding air. Radiation escapes. Some food may burn incompletely or remain as residue. Energy still held chemically in unburned food was not released.

The released energy is shared between the water and everything else it warms. Conservation still holds; the water measurement captures only part of that account. Shielding drafts and keeping a suitable, consistent flame distance can increase the share reaching the water. Laboratory equipment captures energy far more carefully. The same classroom method can compare fuels by using the mass of fuel burned.