How much of your home's electricity could one lightning bolt cover?
― The extreme part isn't the amount, it's the speed
The sky flashes white, and a moment later a sound rumbles up from the pit of your stomach. Something with that much force seems like it should power a whole house for years. But when you actually do the maths, a single lightning strike carries only about a week's worth of an ordinary household's electricity. Lightning is still dangerous, though, because it crams that amount into a ten-thousandth of a second.
It's a summer evening. Outside the window, it suddenly goes dark.
A little while later, everything flashes pure white.
A few seconds after that, a sound rattles all the way down to the ground, and you flinch without meaning to. Many people, at this point, think "if only I could turn that energy into electricity, I wouldn't need to pay for it." That instinct is half right.
There are just two points to grasp
A lightning strike's voltage is roughly 100 million volts, and its current is roughly 30,000 amps. Multiply them together and the instantaneous power comes to about 3 trillion watts. All of Japan's generating capacity put together doesn't reach that.
That power is sustained for about a thousandth of the time it takes to blink. Since energy is "power × time," the total amount turns out to be surprisingly small.
These two facts are two sides of the same phenomenon. Let's look at them one at a time.
"Power" and "amount" are completely different things
In everyday speech we don't really distinguish between "huge energy" and "huge power." In science, though, the two are kept strictly separate.
Power is how much is used per second, measured in watts. Amount is the running total, including how many seconds it lasted, measured in joules.
Think of a bathtub. Power is how hard you twist the tap open; amount is the total water that collects. Even a thin trickle left running all night will overflow the tub. Conversely, even a wide pipe opened for just 0.1 seconds and then shut again will only fill a few cups. Lightning is the case of "a wide pipe opened for a single instant." Take a look at Figure 1.
So lightning is something to dodge, not to harvest
This calculation leads to a somewhat surprising conclusion: it explains why capturing lightning to use as electricity never becomes practical, no matter how long people keep proposing it.
If a single strike only yields about a week's worth of household power, it simply doesn't pay off. On top of that, you can't choose where or when it strikes, and the electricity that arrives races through in a ten-thousandth of a second. Any device built to capture it would need to be many orders of magnitude tougher than the power grid itself.
Put the other way round, that's also where the danger lies. Even though the total amount is small, it's so concentrated in an instant that anything in its path is heated almost instantly to extreme temperatures. That's why the moisture inside a tree trunk can flash into steam so suddenly that it splits the trunk open.
Light arrives almost instantly, but sound travels only about 340 metres per second. Multiply the number of seconds you counted by 340 to get the rough distance to the lightning. Three seconds means about a kilometre.
So what should you actually do?
- The moment you hear rumbling, take shelter in a building or a carAvoid standing under a tree or out in the open in a wide space
- Stay away from tall trees, utility poles, and metal fencesIf someone nearby collapses, don't rush to them — first call 911/999 from a safe spot
- Stay indoors for about 20 minutes after the last rumbleEven if the sky clears, another strike can still follow
Lightning can strike several kilometres away from the centre of a storm cloud. Treat any area where you can hear thunder as an area where a strike could already land. Crouching low outdoors is a last resort for when shelter isn't available, not a guarantee of safety. Always defer to official weather warnings and instructions from emergency services or local authorities.
In summary
The total energy in a lightning bolt is roughly a week's worth of household electricity. The truly extreme thing was never the amount — it was the speed at which that amount gets crammed into a ten-thousandth of a second. Too little to store and use, too fast to be near. That is the true nature of lightning.
What makes lightning so astonishing isn't the amount it carries.
It's the speed at which that amount is packed into less time than a single blink.
Why lightning seems to pick out trees is covered in Why shouldn't you shelter under a tree in a thunderstorm?, and why storing electricity is so hard is covered in Why is electricity so hard to store?
- Check the labels on your appliances and write down the wattage of three of them. A hair dryer, a microwave, and a light are easy examples.
- Recall roughly how many minutes a day you use each one, and multiply the wattage by that time. The ranking by size can end up reversed.
- When thunder rolls, from a safe spot indoors, count the seconds between the flash and the sound. If the count keeps shrinking each time, the storm cloud is getting closer.
Only do step 3 from inside a building, and keep some distance from windows and walls.
For readers who want more ― terminology, formulas, and links to textbooksEach label below shows exactly which school level the content belongs to
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Physics Basics / Physics"
- High school+Advanced high-school content, or textbook sidebar material
- UniversityNot taught in high school — university-level content (discharge physics, atmospheric electricity)
- ResearchNot even settled as "textbook fact" at university — something researchers are still actively investigating
Middle schoolTerminology: this phenomenon has proper names
- Power: the amount of energy transferred per second, measured in watts. This is the "power" referred to throughout this article.
- Energy (electrical work): power multiplied by time. Measured in joules, or in the kilowatt-hours familiar from household bills.
- Electrical discharge: when accumulated electricity breaks through a poor conductor such as air and surges through all at once. Lightning is the most extreme example of this.
Middle schoolHigh schoolCheck the maths: how many days' worth of power is that?
The energy released by a single lightning strike varies enormously depending on conditions. Here we'll calculate using the figures commonly cited as representative values.
| Voltage between storm cloud and ground | Roughly 100,000,000 volts (100 million volts) |
| Size of the current that flows | Roughly 30,000 amps |
| Duration of that current | Roughly 0.0001 seconds |
| Household electricity use per day | Roughly 10 kilowatt-hours as a benchmark |
| Converting 1 kilowatt-hour to joules | 3,600,000 joules |
| Instantaneous power (watts) | 100,000,000 × 30,000 = 3,000,000,000,000 |
| Energy released (joules) | 3,000,000,000,000 × 0.0001 = 300,000,000 |
| Converted to kilowatt-hours | 300,000,000 ÷ 3,600,000 ≈ 83 |
| How many days of household use | 83 ÷ 10 ≈ 8 |
A power of 3 trillion watts is more than ten times the combined output of all of Japan's power stations. Even so, the total energy comes to only 83 kilowatt-hours — roughly 8 days' worth of household electricity. In terms of an electricity bill, that's on the order of a few thousand yen.
| Volt | Unit of voltage — represents the strength of the force pushing electricity along |
| Amp | Unit of current — represents how much electricity flows per second |
| Watt | Unit of power — found by multiplying volts by amps |
| Joule | Unit of energy — found by multiplying watts by seconds |
High schoolHigh school+Why does air end up conducting electricity?
High schoolAir normally does not conduct electricity. But once the voltage becomes extremely high, electrons are stripped from air molecules, creating a path for electricity to flow. This state is called ionisation.
High school+In an actual lightning strike, a faint current first branches its way toward the ground in steps of tens of metres at a time. A rising current also builds up from the ground side, and the instant the two connect, that bright flash we see races through. The visible flash is actually the current racing upward from the ground.
UniversityWhere does the energy actually go?
Of the energy released, less than a tenth is estimated to emerge as light. Most of it goes into heating the air along the strike's path to tens of thousands of degrees in an instant, and that sudden expansion becomes a shock wave — which is the sound we hear. In other words, thunder is the sound of that spent energy escaping. Whatever flows into the ground dissipates as heat within the soil.
ResearchWhat still isn't fully understood
- How does it even get started? The voltages actually measured inside clouds are often an order of magnitude smaller than what should be needed to break through air, so exactly how that first step happens remains unresolved.
- How charge separates inside a cloud. It's known that collisions between ice crystals and graupel (soft hail) separate charge, but why the direction of that separation changes with temperature and water content still isn't fully explained.
- High-energy light emitted from storm clouds. Observations have found storm clouds emitting extremely intense bursts of light, and research into what this means for the discharge mechanism itself is ongoing.
In other words, everything in this article is "the best explanation we have for now." Treat the figures given here, too, as representative values that vary enormously from one strike to the next.
Links to the school curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science — electric current and its effects | The difference between power and energy; how to read Figure 1 |
| High school | Physics Basics — electricity and energy | Calculating power and total amount separately |
| High school+ | Physics — ionisation of gases | How air comes to conduct electricity |
| University | Discharge physics / atmospheric electricity | How energy converts into heat and a shock wave |
| Research | Open questions in atmospheric electricity | How discharge begins, and how charge separates inside clouds |
| ― | Everyday relevance | Deciding to take shelter as soon as you hear thunder; reading appliance wattage |
- Japan Meteorological Agency (気象庁), "On severe phenomena accompanied by lightning"
- Japan Meteorological Agency (気象庁), "Historical weather data search"
- The Society of Atmospheric Electricity of Japan (日本大気電気学会), ed., 『大気電気学概論』 (Introduction to Atmospheric Electricity) (on the current waveform of lightning discharges and estimates of released energy)
- Uman, M. A., The Lightning Discharge (a standard textbook covering how discharges progress and how the energy is distributed)
※This article is a general-audience science explainer. The figures given are approximations meant to help illustrate the mechanism, and they vary enormously from one lightning strike to the next. For lightning safety, follow announcements from the Japan Meteorological Agency and instructions from emergency services and local authorities.