Class 10 · Society and design
Is a solar panel worth it? Cost and benefit
A rooftop solar system costs ₹60,000 and cuts the electricity bill by ₹12,000 a year. Is it worth it?
What you will learn: Work out payback time and weigh costs and benefits beyond money.
Small test at the end: scroll down to the end of this page for five short questions that check what you have learnt. Tap an answer, then press Check answer. Go to the questions ↓
Jump to: Go deeper · Check yourself · For grown-ups · For teachers
Safety first
Plans only. Never go on a roof. Do not touch wires.
Keep it safe:
- Plans only: never go on a roof to look at or measure panels.
- Do not touch electrical equipment or wires.
You will need
- Paper and pencil
- A calculatorOptional.
- A cost-benefit tableColumns: item, money, non-money.
Time: About 45 minutes.
What do you think?
A ₹60,000 system saving ₹12,000 a year pays for itself in 5 years. Upkeep then cuts the saving to ₹10,000 a year. How long will payback take now?
Make a prediction and write one reason for it. Then tap the guess closest to yours to see how to test it. It is fine to change your mind after checking.
Still about 5 years
Hold that thought and write your reason. Then test it: divide the cost by the new yearly saving.
A little longer
How much longer? Write your guess in years and your reason, then work it out from the cost and the new saving.
About twice as long
Write your reason. Then check it: what yearly saving would make payback 10 years? Is that what is left?
Not sure
That is fine. Start from what payback means: how many years of saving add up to the cost?
Need a hint?
Payback time = cost ÷ yearly saving. What happens to the answer when you divide by a smaller number?
The story
Dadi is thinking about solar panels for the stall roof. The salesperson says, "It costs ₹60,000 and saves you ₹12,000 a year!" Dadi asks the twins, "Is that a good deal?" Tanmay says yes at once. Tara asks what costs the salesperson might have left out. Is it worth it?
The idea
Payback time = cost ÷ yearly saving. ₹60,000 ÷ ₹12,000 = 5 years. After that, the savings are a gain.
Compare payback with how long the system lasts. If it lasts 25 years, about 20 of those years are savings after payback.
Running costs you might forget, such as cleaning and repairs, reduce the yearly saving and so make payback longer. Replacing parts later, such as the inverter, cuts the lifetime savings.
Weigh benefits and risks that are not money too: cleaner air, the roof space the panels take up, and whether the family will stay in the home long enough.
Worked example
A family weighs solar panels (made-up numbers)
- Cost ₹60,000; saving ₹12,000 a year. Payback: 60,000 ÷ 12,000 = 5 years.
- Expected life: 25 years, so about 20 years of savings after payback.
- Forgotten costs: cleaning and repairs of about ₹2,000 a year cut the net saving to ₹10,000, so payback becomes 60,000 ÷ 10,000 = 6 years.
- Non-money: cleaner air; but they rent, and may move in 3 years.
Payback in about 6 years: good value if they stay, a poor fit if they move first. The numbers alone do not decide.
Try it
Work out payback.
Cost ÷ yearly saving.
Compare with lifetime.
How many years of savings after payback?
Add hidden costs.
Cleaning, repairs, replacement parts. Recalculate.
List non-money factors.
At least two benefits and two risks.
Decide and explain.
Would you recommend it? For whom, and why?
What you make (paper or offline)
Your cost-benefit table
Payback time with working.
A table of money and non-money costs and benefits.
A recommendation that says who it suits.
Make it on paper. Keep it with you. Nothing to upload.
What did we find out?
Read this after you have tried it. Look back at your guess and your reason first.
60,000 ÷ 10,000 = 6 years. Losing a sixth of the saving added a whole year: a fifth more time.
Which part of your reasoning held, and which part did the numbers change?
Go deeper
Comparing scenarios and justifying a recommendation
A scenario is one set of assumptions: the yearly saving, the upkeep, replacements, and how long the family stays. Change one at a time and see which changes the answer most.
A recommendation names the choice, the scenario it depends on, and what would change your mind.
Worked example: three scenarios for the same ₹60,000 system
- Base: net saving ₹10,000 a year, an inverter replaced for ₹15,000 in year 12, the family stays 25 years. Payback 6 years; lifetime gain 2,50,000 − 60,000 − 15,000 = ₹1,75,000.
- Cloudier roof: net saving ₹7,500. Payback 8 years; lifetime gain 1,87,500 − 60,000 − 15,000 = ₹1,12,500. Smaller, but still well ahead.
- The family moves after 5 years and the panels stay with the house: 5 × 10,000 = ₹50,000 saved against ₹60,000 spent. A loss of ₹10,000.
- The moving date changes the answer far more than the roof or the inverter.
Recommendation: install if the family expects to stay at least 7 years. If they may move sooner, wait.
Challenge 1
In the base scenario, the inverter replacement costs ₹30,000 instead of ₹15,000. Does the payback time change? Does the recommendation?
Work it out on paper and write your reason before you look.
See a worked answer
- Payback stays 6 years: the replacement comes in year 12, after payback.
- Lifetime gain falls to 2,50,000 − 60,000 − 30,000 = ₹1,60,000.
- The recommendation stands. A replacement cost changes the lifetime gain, not the payback.
Challenge 2
Family P has a shaded roof (net saving ₹6,000 a year) and will stay 20 years. Family Q has a sunny roof (₹12,000) but may move in 4 years. Who should install? Justify it.
Work it out on paper and write your reason before you look.
See a worked answer
- P: payback 10 years; over 20 years, 1,20,000 − 60,000 = ₹60,000 ahead, before any replacement.
- Q: 4 × 12,000 = ₹48,000 saved before moving, ₹12,000 short of the cost.
- P, despite the worse roof. How long you stay matters more here than how much you save each year.
Two of the questions in "Check yourself" use this part.
Check yourself
Five questions about what this page teaches. Choose an answer and press Check answer, or open "See the answer and why". Nothing is saved or sent: your choices stay on this screen and disappear when you leave or reload the page.
How do you want to answer? You choose, and you can change at any time.
See the answer and why
- A. 5 years Correct. 60,000 ÷ 12,000 = 5.
- B. 6 years Not this one. After 6 years the savings (₹72,000) are already past the cost.
- C. 12 years Not this one. 12 is the saving in thousands, not the years.
- D. 1 year Not this one. One year saves only ₹12,000.
See the answer and why
- A. A loss of ₹10,000 Correct. 5 × 10,000 = ₹50,000 saved, against ₹60,000 spent.
- B. They break even Not this one. Payback takes 6 years, and they leave after 5.
- C. A gain of ₹50,000 Not this one. That counts the savings but forgets the ₹60,000 cost.
- D. A gain of ₹1,75,000 Not this one. That is the 25-year gain for a family that stays.
See the answer and why
- A. The yearly saving on the bill Not this one. The saving is counted in rupees.
- B. The payback time Not this one. That is worked out from money.
- C. Cleaner air from burning less coal Correct. It matters, but it is not counted in rupees here.
- D. The price of the panels Not this one. The price is counted in rupees.
See the answer and why
- A. 5 years Not this one. That ignores the running costs.
- B. 30 years Not this one. That divides by the ₹2,000 running cost instead of the net saving.
- C. 4 years Not this one. Extra costs cannot make payback shorter.
- D. 6 years Correct. 60,000 ÷ 10,000 = 6, a year longer than before.
See the answer and why
- A. Electricity prices going up Not this one. Higher prices make each unit saved worth more.
- B. The panels lasting 30 years instead of 25 Not this one. That makes the choice better, not worse.
- C. The year-12 inverter costing ₹5,000 more Not this one. That trims the lifetime gain a little. The family is still well ahead.
- D. The family moving after 4 years Correct. They would leave before payback and lose money.
Think back
Which non-money factor mattered most to you?
Who would solar panels suit best, and who least?
Investigate further
- Do a payback calculation for an energy-saving LED bulb.
- Compare two water filters by cost per litre over five years.
Can you use it here?
A ₹3,000 water filter saves a family ₹500 a month on bottled water. When does it pay back? What could make that longer?
Ask someone at home
Ask someone at home about something they bought to save money later, such as LED bulbs or a pressure cooker. Did it pay back?
No one free to join in? That's fine. You can do it by yourself with drawings, and ask a grown-up to help with the materials.
For grown-upsTap a heading to open it. The safety notes are at the top of the page.
Before you start
- If you have an electricity bill, look at the yearly total together (private figures can stay private).
Questions that help
- "What costs might we forget?"
- "What matters that is not money?"
What's really going on
Payback time is a simple tool. Real decisions also weigh lifetime, hidden costs, risks and values.
Children should never go on roofs or touch electrical systems; this activity is calculation and discussion only.
Other ways to do it
- Use a pressure cooker or a bicycle instead of panels.
For teachersLearning aims, classroom ideas and what to look for.
Learning aims
- Calculate payback time and lifetime benefit.
- Weigh money and non-money costs and benefits.
Subject links
- Mathematics: division and rates.
- Science: energy.
- Social science: decision-making.
These links are general. They have not been checked against a specific board syllabus.
In the classroom
- Groups argue for different families with different circumstances.
What to look for
- Correct payback calculation.
- Recommendation conditional on circumstances.