Energy: worksheet

Lesson 21: Watts, watt-hours and the electric bill

Open the lesson

By the end you can:

• Tell the difference between power (watts) and energy (watt-hours).
• Work out the daily energy use of one appliance.
• Read the energy line on an electric bill.

1. A 60 W laptop charger runs 8 hours a day. A 1,500 W space heater runs 1 hour a day. Which uses more energy each day?

   Key: The space heater: 1,500 Wh against 480 Wh

2. Name one trade-off from this lesson. What do you gain, and what does it cost (money, time, space, upkeep or risk)?

Lesson 22: Load list: what you actually use

Open the lesson

By the end you can:

• Build a load list with watts, hours and daily watt-hours.
• Spot the big loads that drive system size.
• Account for surge, standby and seasonal loads.

1. Your load list totals 6 kWh a day in winter. A 1,500 W space heater running 2 hours makes up half of that. What is the best first step?

   Key: Look for another way to heat that space, or cut its hours

2. Name one trade-off from this lesson. What do you gain, and what does it cost (money, time, space, upkeep or risk)?

Lesson 23: Efficiency first: the cheapest kilowatt-hour is the one you do not use

Open the lesson

By the end you can:

• Explain why efficiency shrinks the whole energy system.
• Find standby loads and work out what they cost.
• Compare common efficiency swaps for the big loads.

1. An off-grid cabin uses 5 kWh a day. An electric water heater uses 2 kWh of that. Which change will shrink the solar and battery the most?

   Key: Replacing the water heater with a heat pump water heater, or another water-heating option

2. Name one trade-off from this lesson. What do you gain, and what does it cost (money, time, space, upkeep or risk)?

Lesson 24: Solar basics: panels, sun hours, tilt and shading

Open the lesson

By the end you can:

• Turn an array size and peak sun hours into daily energy.
• Explain why winter output is lower, and how tilt helps.
• Spot shading problems and know who installs an array.

1. A 3 kW array gets 5 peak sun hours in June and 2 in December. With an 80 percent derate, about how much does it make per day in December?

   Key: About 4.8 kWh

2. Name one trade-off from this lesson. What do you gain, and what does it cost (money, time, space, upkeep or risk)?

3. Which parts of this topic need a licensed professional or a permit, and what would you ask them?

Lesson 25: Storing energy: batteries, depth of discharge and days of autonomy

Open the lesson

By the end you can:

• Explain depth of discharge, cycle life and days of autonomy.
• Compare lead-acid and lithium iron phosphate batteries.
• Diagnose why a battery bank is flat on winter mornings.

1. Two banks both say 10 kWh. One is lead-acid kept above 50 percent charge. The other is LFP used to 80 percent DoD. Which gives more usable energy?

   Key: The LFP bank: about 8 kWh against about 5 kWh

2. Diagnose it: “The batteries are flat every morning in January.” What is the most likely cause, and what clue tells you?

   Key: Winter output is too low for winter loads, and the cold battery holds less

3. Name one trade-off from this lesson. What do you gain, and what does it cost (money, time, space, upkeep or risk)?

4. Which parts of this topic need a licensed professional or a permit, and what would you ask them?

Lesson 26: Off-grid, grid-tied and hybrid systems compared

Open the lesson

By the end you can:

• Name the main parts of an off-grid solar system and what each does.
• Compare off-grid, grid-tied and hybrid systems.
• Explain why a grid-tied system without batteries goes dark in an outage.

1. A family on the grid installs solar panels with no batteries so they will have power in storms. What happens in the next outage?

   Key: The system shuts off until the grid comes back

2. Name one trade-off from this lesson. What do you gain, and what does it cost (money, time, space, upkeep or risk)?

3. Which parts of this topic need a licensed professional or a permit, and what would you ask them?

Lesson 27: Backup power: generators, load shedding and CO safety

Open the lesson

By the end you can:

• Explain when backup power is needed and how generators fill the gap.
• Choose critical loads and plan load shedding.
• State the carbon monoxide and backfeed rules for generators.

1. It is snowing hard during an outage. Where is it safe to run a portable generator?

   Key: Outside, at least 20 feet from the house, exhaust pointed away from doors, windows and vents

2. Name one trade-off from this lesson. What do you gain, and what does it cost (money, time, space, upkeep or risk)?

3. Which parts of this topic need a licensed professional or a permit, and what would you ask them?

Lesson 28: Other sources: small wind and micro-hydro

Open the lesson

By the end you can:

• Explain what makes a site good for small wind or micro-hydro.
• Estimate micro-hydro power from head and flow.
• Decide when wind or hydro is worth a closer look.

1. A stream gives 50 feet of net head and 40 gpm in the dry season. Using the DOE rule of thumb, about how much power could a micro-hydro system make?

   Key: About 200 W

2. Name one trade-off from this lesson. What do you gain, and what does it cost (money, time, space, upkeep or risk)?

3. Which parts of this topic need a licensed professional or a permit, and what would you ask them?

Lesson 29: Project: my load list and solar sizing for winter and summer

Open the lesson

By the end you can:

• Build a winter and a summer load list.
• Size a solar array and battery for the worst month, and compare it with summer.
• Prepare what an installer needs to give you a real design and quote.

1. An off-grid plan uses 4 kWh a day all year. Sun hours are 5.5 in June and 1.8 in December. Which month should set the array size?

   Key: December, the worst month, or a plan for a generator to cover it

2. Name one trade-off from this lesson. What do you gain, and what does it cost (money, time, space, upkeep or risk)?

3. Which parts of this topic need a licensed professional or a permit, and what would you ask them?

Homestead Lab activity