Power & Energy
Bulbs in series vs parallel: which shines brighter?
Two identical bulbs wired in parallel are each as bright as one bulb on its own, because each gets the full supply voltage. Wired in series they share the voltage, so each gets half, and since power goes as voltage squared, each gets only a quarter of the power. Parallel is brighter, and it costs the battery four times as much.
Power, three ways
A bulb's brightness follows the power it takes, in watts. Combine P = V × I with Ohm's law and you can work it out from whatever you know:
Two lamps on 12 V
Take two 150 Ω lamps and a 12 V battery.
In parallel, each lamp is straight across the 12 V:
Each draws 80 mA, so the battery supplies 160 mA, about 1.9 W in all.
In series, the lamps share the 12 V, 6 V each, and one 40 mA current flows through both:
Half the voltage, half the current, a quarter of the power. The battery supplies just under half a watt.
Live experiment
Read the watts
A lamp, here a resistor rated 2 W, sits across a battery with a power meter in the wire. Turn its resistance down until it takes a watt, 82 Ω on 9 V, and V × I, V² ÷ R and I² × R all agree. Then double the battery: four times the watts, and past 2 W the lamp smokes.
“Watts” is lesson 1 of 6 in Watt Harbour, Power & Energy World 1. Free with an account. It’s open as soon as you sign up.
When the bulbs are different
Mix a high-resistance bulb with a low one and the answer flips with the wiring. In series the current is shared, so P = I² × R favours the bigger resistance. In parallel the voltage is shared, so P = V² ÷ R favours the smaller one.
That's the classic puzzle: a 60 W and a 100 W bulb in series. The 60 W bulb has more resistance, so it glows brighter, roughly 23 W against 14 W for a pair of 230 V bulbs. Real filaments have less resistance when cooler, so measured numbers differ, but the 60 W bulb still wins.
Watch the rating
Every lamp and resistor has a power rating. An 82 Ω resistor on 9 V takes about 1 W, comfortable for a 2 W part. Move it to 18 V and it takes 18² ÷ 82, about 4 W: double the voltage, four times the heat, and it smokes. Check the watts whenever the voltage goes up.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Watts” is lesson 1 of 6 in Watt Harbour, Power & Energy World 1. Free with an account. It’s open as soon as you sign up.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 4 W, and the 2 W lamp smokes. Twice the volts pushes twice the current, and power is the two multiplied: four times the watts, about 3.9 W. That's past the lamp's 2 W rating. V² ÷ R says the same: 18² ÷ 82 is about 4.
- Why not “Half a watt”? A bigger battery can't make the same lamp take less power. It takes about four times as much.
- Why not “1 W”? The lamp's resistance is unchanged, so more voltage pushes more current and the power rises.
- Why not “2 W”? Doubling the voltage also doubles the current, so the power goes up four times, not two.
Show the answer
Answer: A quarter of a watt. In a chain they share the 12 V, 6 V each. 36 ÷ 150 is 0.24 W: a quarter of the brightness, and the battery works a quarter as hard.
- Why not “A watt”? A watt each is what they take straight across the battery. In a chain each sees only 6 V.
- Why not “Two watts”? Two watts is the pair's total in parallel. In series the pair takes under half a watt.
- Why not “Nothing”? The chain is still a complete loop, so 40 mA flows and each lamp takes about a quarter of a watt.
Show the answer
Answer: How fast energy is used. Watts are joules per second: a rate.
- Why not “How much energy is stored”? Stored energy is counted in joules or watt-hours. Power is how fast that energy is moved.
- Why not “The push on the charges”? The push on the charges is voltage, measured in volts.
- Why not “The flow of charge”? The flow of charge is current, in amps. Power is volts times amps.
“Watts” is lesson 1 of 6 in Watt Harbour, Power & Energy World 1. Free with an account. It’s open as soon as you sign up.
Questions people ask
Why are bulbs in parallel brighter than bulbs in series?
In parallel each bulb is connected straight across the supply, so it gets the full voltage. In series the bulbs share the voltage. Power goes as voltage squared over resistance, so two equal bulbs in series each get half the voltage and a quarter of the power.
A 60 W and a 100 W bulb are wired in series. Which is brighter?
The 60 W one. It has the higher resistance, and in series the same current flows through both, so by P = I² × R the bigger resistance takes more power. For two 230 V bulbs that's roughly 23 W against 14 W, though real filaments change resistance as they heat.
Do bulbs in parallel drain a battery faster?
Yes. Each parallel bulb draws its full current, so two draw twice what one does, and four times what the same two draw in series. They're brighter because the battery is working harder.
What happens when one bulb blows?
In series the loop is broken, so every bulb goes out. In parallel each bulb has its own path across the supply, so the others stay lit. That's why house lights are wired in parallel.
Related topics
- Series vs parallel circuitsOne path or many: what stays the same, what adds up, and how to combine resistors.
- Electrical power explained: P = V × IP = V × I: how fast energy turns into heat, and why a resistor has a power rating.
- Batteries in series vs parallelChain cells for more volts, put them side by side for more hours, and never mix a tired one with a fresh one.
- Ohm's law explainedV = I × R: set any two of voltage, current and resistance and the third follows.