Learn what electric energy density means. Compare how much energy different sources and devices can store per volume or mass.
Energy density tells you how much energy fits in a given space. Think of packing a suitcase. High density means you can fit more clothes in the same bag. Energy density works the same way. High density means more energy in the same volume.
Electric energy density compares different ways to store electric energy. Batteries, capacitors, and fuel all have different densities. Understanding these numbers helps you choose the right technology for the job.
Energy density is the amount of energy stored per unit volume. The basic formula is simple.
D = E / V
D is energy density. E is total energy. V is volume.
For electric fields, we have a more specific formula. The energy density of an electric field is:
Ue = 1/2 x e0 x E^2
e0 is the permittivity of free space. E is the electric field strength. The field strength is squared. Double the field and you get four times the energy density.
For magnetic fields, the formula is:
Ub = 1/2 x B^2 / u0
B is the magnetic field strength. u0 is the permeability of free space.
Think about two backpacks. One is packed tight with books. The other has the same books spread out loose. Which one fits more? The tight one. That is higher density.
Energy density is the same idea. Some batteries pack energy tight. Others are looser. A phone battery is tight. It fits a lot of energy in a small space. That is why your phone can be thin and still run all day.
A big D-cell battery takes up more space. But it holds less energy than a phone battery of the same size. So the phone battery has higher energy density.
Let us compare real numbers.
Lithium-ion battery. About 250 Wh per kilogram. That is 900,000 joules per kilogram. This is why your laptop runs for hours on a small battery.
Alkaline battery. About 100 Wh per kilogram. Lower than lithium. That is why AA batteries do not last as long as lithium ones.
Capacitor. About 5 Wh per kilogram. Much lower than batteries. But capacitors can release all their energy in a fraction of a second. Batteries release it slowly.
Supercapacitor. About 15 Wh per kilogram. Better than regular capacitors but still far below batteries. Supercapacitors bridge the gap. They store more energy than capacitors and charge faster than batteries.
Gasoline. About 12,000 Wh per kilogram. Gasoline has 50 times the energy density of a lithium battery. That is why we still use gas in cars. But most of that energy is wasted as heat. Electric motors use energy more efficiently.
Here is a table comparing energy densities.
| Technology | Wh/kg | Good For |
|---|---|---|
| Gasoline | 12,000 | Cars, planes |
| Lithium battery | 250 | Phones, laptops |
| Alkaline battery | 100 | Remote controls |
| Supercapacitor | 15 | Regenerative braking |
| Capacitor | 5 | Camera flashes |
Electric cars. Battery energy density is the biggest challenge for EVs. A Tesla battery pack weighs about 500 kg. It holds 60 to 100 kWh. That gives a range of 250 to 350 miles. Gasoline cars can go farther on less weight.
Phones. Engineers fight for every bit of energy density. A phone battery is about the size of a stick of gum. It holds about 10 Wh. That runs your phone for a day. Better density would mean longer battery life in the same thin phone.
Grid storage. For storing energy from solar and wind, we need large batteries. Energy density matters less here because weight and size are not big problems. Cost matters more. That is why grid batteries use cheaper chemistries.
Camera flashes. Capacitors have low energy density but high power density. They release energy very fast. A camera flash needs a bright burst of light. A capacitor is perfect. It charges slowly and discharges in milliseconds.
Last updated: June 15, 2026
What is the formula for energy density?
Which can store more energy per kilogram?
What does high energy density mean?
What advantage do capacitors have over batteries?
The energy density of an electric field depends on what?
Answers: B: D = E / V, B: A lithium-ion battery, A: More energy in a smaller space, B: Faster charge and discharge, B: The electric field strength squared
What is electric energy density?
Electric energy density is the amount of electric energy stored in a given volume. It tells you how much energy you can fit in a certain space. Think of it like packing. High density means more energy in a smaller package.
How is energy density calculated?
Energy density equals total energy divided by volume. D = E / V. For electric fields, we also have specific formulas. One is Ue = 1/2 x e0 x E^2. e0 is a constant. E is the electric field strength.
Why does energy density matter for batteries?
Higher energy density means a battery can store more energy in the same amount of space. That is why phone batteries keep getting smaller but last longer. Chemists work hard to improve energy density.
How does electric field energy density compare to battery density?
Batteries have much higher energy density than electric fields in capacitors. A lithium-ion battery stores about 250 watt-hours per kilogram. A capacitor stores about 5 watt-hours per kilogram. But capacitors charge and discharge much faster.
What is the energy density of air as a capacitor?
Plain air has very low energy density as a capacitor. The electric field breaks down at about 3 million volts per meter. Before that, the stored energy density is tiny compared to any battery.
