Renewable Energy

You’ll be able to

Compare the major renewable energy sources and their trade-offs
Distinguish intermittent from continuously available renewables
Calculate the power output of a solar array from area, intensity, and efficiency

The renewable sources

Renewable energy is replenished on a human timescale. Solar uses photovoltaic (PV) cells to turn sunlight directly into electricity, or mirrors to concentrate heat. Wind turbines convert moving air into electricity. Hydroelectric power spins turbines with falling or flowing water, usually behind a dam. Geothermal taps heat from Earth’s interior for electricity or direct heating. Biomass burns organic matter (wood, crop waste, biofuels like ethanol). Tidal and wave power capture the ocean’s motion. Together they emit little or no CO₂ during operation.

Trade-offs of each source

No source is impact-free. Solar and wind are clean but intermittent — they generate only when the sun shines or wind blows, so they need energy storage or backup. Hydroelectric dams provide steady power but flood land, block fish migration, disrupt sediment flow, and displace people. Geothermal is reliable but limited to geologically active regions. Biomass is renewable but releases CO₂ and air pollutants when burned and can compete with food crops for land. Manufacturing solar panels and wind turbines also requires mining and energy.

Intermittent vs. continuous

A key distinction for grid planning is whether a source is intermittent or continuous. Intermittent sources — solar and wind — vary with weather and time of day, so they must be paired with energy storage (batteries, pumped-storage hydro) or backup generation to provide power on demand. Continuous (baseload-capable) renewables — geothermal, hydroelectric, and biomass — can run steadily around the clock. Matching supply to demand is one of the central challenges of expanding renewable energy.

Solar array power output

power output = area × solar intensity × efficiency

Area in m², solar intensity in W/m², efficiency as a decimal. The result is the electrical power in watts. Real panels convert only ~15–22% of incoming sunlight.

Worked example

A rooftop solar array covers 20 m². The sunlight striking it delivers 1,000 W/m², and the panels are 20% efficient. What is the electrical power output?

1.Write the relationship: power = area × solar intensity × efficiency.
2.Convert efficiency to a decimal: 20% = 0.20.
3.Substitute: power = 20 m² × 1,000 W/m² × 0.20.
4.Multiply: 20 × 1,000 = 20,000; then 20,000 × 0.20 = 4,000 W = 4 kW.

Answer: 4,000 W (4 kW). The 80% not converted to electricity is mostly lost as heat — a reminder that even "clean" sources are far from 100% efficient.

Checkpoint

Which of the following is a major drawback specifically of relying on solar and wind power?

Tip

Sort renewables into intermittent (solar, wind — weather-dependent) and continuous (geothermal, hydro, biomass — run steadily). Exam questions about grid reliability or the need for battery storage are usually pointing at the intermittency of solar and wind.

Checkpoint

A large hydroelectric dam produces clean, steady electricity but also creates environmental problems. Which is a genuine drawback of such dams?

On the exam

For solar-output problems, keep the three factors straight — area, intensity (W/m²), and efficiency as a decimal — and multiply. A frequent follow-up asks how much energy (kWh) the array makes over several hours: multiply the power (in kW) by the number of hours.

Answer the 2 checkpoints as you read.