⚡ Stored Energy Tool

Capacitor Energy Calculator — Joules, mJ, µJ from Voltage & Farads

Enter capacitance and voltage. The calculator converts them to joules, millijoules, microjoules, nanojoules and watt-hours — live on every keystroke.

Capacitor Energy Calculator

E = ½ × C × V² — joules, millijoules, microjoules, watt-hours, live.

Capacitance (C)

Voltage (V) in volts

Quick presets

Stored energy

7.2µJ

Full scientific: 7.200e-6 J

Watt-hours: 0 Wh

Charge stored: 1.200e-6 C

E = ½ × C × V² = ½ × 1.000e-7 × 12²

E = Q² / (2C) = 1.200e-6² / (2 × 1.000e-7)

E = ½ × Q × V = ½ × 1.200e-6 × 12

⚠️ Microcopy. Energy scales with the square of voltage — doubling V gives four times the stored joules. This is why a 10 µF cap at 50 V carries 25× more energy than the same cap at 10 V. Never assume a discharged capacitor is safe — a 100 µF cap charged to 400 V (typical of tube gear) stores 8 joules, enough to give a dangerous shock.

The V² relationship

Why Doubling the Voltage Gives Four Times the Joules

If voltage is V, charge is Q = C × V. The work required to assemble that charge is energy = average voltage × total charge = (V/2) × (CV) = ½CV². The derivation is pure arithmetic — no exotic physics — but its consequences are widely misestimated in field work. A 10 µF capacitor charged to 50 volts stores 3.125 millijoules. The same capacitor charged to 25 volts stores 0.781 millijoules — exactly one quarter. Not half. Doubling V quadruples E.

Industry practice assumes a charged capacitor is safe below 50 volts. If you double that to 100 volts, the stored energy rises by a factor of four — the zap you feel is four times harder, not twice. And if you go to 400 volts, it is sixteen times harder than the 100-volt version. This is why a 100 µF cap charged to 400V stores 8 joules — enough for a dangerous shock — even though the same cap at 50V stores only 0.125 mJ.

The nonlinear response continues at scale. If you increase capacitance by a factor of 10 (say from 100 nF to 1 µF), energy also scales by 10 — linear with C, quadratic with V. If you increase both C and V by 10, energy increases by 1000 (10 × 10²). This is why energy-critical designs — defibrillators, camera flashes, pulsed power — always balance a high-voltage rail against a larger capacitance rather than chasing voltage alone.

Close-up of electrolytic capacitors with voltage and capacitance markings, illustrating the relationship between voltage rating and stored energy

Capacitor energy scales with V²Photo: Unsplash

Reference

Real-World Capacitor Energy Cheat Sheet

Six common capacitor types with their voltage ratings and the exact energy they store — side-by-side with the equivalent capacity of a 3.7V lithium battery. Use this to sanity-check any calculation before you work on a live circuit.

Capacitor Type	Typical µF	Typical Voltage (V)	Energy Stored (mJ)	Equivalent Battery (mAh @ 3.7V)
100 pF ceramic @ 50V	0.000 1	50	0.125 µJ	≈ 0.000 01 mAh
10 µF X7R @ 25V	10	25	3.125 mJ	≈ 0.000 23 mAh
1 000 µF electrolytic @ 16V	1 000	16	128 mJ	≈ 0.0096 mAh
1F supercap @ 2.5V	1 000 000	2.5	3.125 J	≈ 0.23 mAh
100 µF film @ 400V	100	400	8 J	≈ 0.59 mAh
4 700 µF electrolytic @ 35V	4 700	35	2.88 J	≈ 0.21 mAh

Every number in this table was computed from E = ½CV² — try each one in the energy calculator above to verify. For the full toolset, visit the calculators hub, or land on the home page at Capacitor Calculator to decode capacitor codes, convert units, and run the parallel-plate and HVAC under-load tools.

Three faces of one formula

E = ½CV² — and Two Other Forms You Should Know

All three formula forms describe exactly the same physical quantity. Which one to use depends on what you know. E = ½CV² is the workhorse when you have C and V from the label or circuit. E = Q²/(2C) wins when you have a known charge Q but cannot easily measure V. E = ½QV is the simplest and most intuitive — average voltage times total charge — and appears naturally in pulsed-power designs where Q is measured in coulombs and V in volts.

💡 Calculation tip. Before trusting any online result, sanity-check with a known example from the cheat sheet above. If you plug 100 µF @ 16V into any tool and do not get 128 mJ, something is wrong with the tool — not your capacitor.

⚠️ Safety warning. A capacitor is discharged only when both V = 0 and no stored charge remains. Even after power-off, high-voltage caps can hold lethal energy for minutes or hours. Always discharge through a 20 kΩ 5 W resistor before handling — never short the terminals.

❌ Don't do this. Never derate a capacitor below its voltage rating to save cost. Exceeding the voltage permanently damages the dielectric even once — a 25V cap at 26V can develop a dead short inside minutes, releasing ½CV² across a spark with thermal consequences.

E = ½CV² ← use when you know C and V

E = Q²/(2C) ← use when you know Q but not V directly

E = ½QV ← simplest, uses Q and V directly

The derivation of E = ½CV² follows directly from the definition of capacitance C = Q/V and the work-energy theorem. The energy U stored in an electric field between two plates with charge Q and potential difference V is U = ½ Q V. Substituting Q = CV gives U = ½ CV². Substituting V = Q/C gives U = Q²/(2C). All three forms are algebraically identical — pick the one whose variables you can measure directly.

Density matters

Energy Density vs Power Density — Why Supercaps Don't Beat Batteries

The energy density of a capacitor system (J/kg) defines how much you can store per kilogram of hardware. Power density (W/kg) defines how fast you can deliver it. These two numbers never trade off cleanly — capacitors win decisively on power density (10⁷–10⁸ W/kg for supercapacitor modules) but lose badly on energy density (30 J/kg vs lithium-ion's 150–250 J/kg).

Capacitor Families — Energy Density

• Ceramic C0G / NP0: ~0.001 J/kg (very low, extremely safe)
• Film capacitors: ~0.1–1 J/kg (audio crossovers, snubbers)
• Aluminum electrolytic: ~1–10 J/kg (bulk smoothing, hold-up)
• Supercapacitor EDLC: ~10–30 J/kg (memory backup, energy harvesting)

Battery Families — Energy Density

• Lead-acid: ~30–40 J/kg (UPS, starting batteries)
• Nickel-metal hydride: ~40–60 J/kg (hybrid vehicles)
• Lithium-ion (LiCoO₂): ~150–250 J/kg (phones, laptops, EVs)
• Lithium-sulfur (Li-S): ~250–400 J/kg (next-gen research)

If you need 100 J for a camera flash burst (capacitor job) or a 3-second regenerative braking event (supercap job), a capacitor is the right choice. If you need 10 000 J to run an amplifier for an hour, a battery is non-negotiable. The dividing line sits somewhere around the 10-second horizon — anything shorter benefits from a capacitor, anything longer from a battery. Hybrid systems — supercaps peak-shaving in EVs, capacitors for flash in cameras — combine both families on purpose.

One engineering pitfall worth flagging: if your datasheet specifies a hold-up time T and an operating voltage window V₁ → V₂, the formula you need is E = ½ × C × (V₁² − V₂²). Not ½CV₁². The difference between charged and discharged voltage is what matters. A 48V supply that must ride through to 42V is not storing energy for 48V — it is storing the energy in the 6V drop, and that drop depends nonlinearly on both voltages.

Safety protocol

A Discharged Capacitor is Not What You Think

Industry shorthand calls a capacitor "discharged" when V is less than 50V. That is the definition that matters for industrial work — above 50V, even a millijoule can cause a startle reaction that results in dropped tools or tripping back into live circuits. Follow the protocol below before touching any capacitor larger than 1 000 pF or charged above 25V.

1Remove power. Open the main disconnect, lock and tag it, then verify zero voltage across the supply rails before opening the panel.
2Discharge through a 20 kΩ 5 W resistor. Hold the leads across the terminals for 30 seconds. Never short with a screwdriver blade — the arc pits lugs and can weld the blade to the can.
3Verify V = 0 with a multimeter. Set to DC mode, probe across the terminals. The reading must be 0.00V. If not, go back to step 2 and wait longer.
4Use one hand. Keep your other hand out of the panel so fault current cannot cross your chest and through your heart.
5Wear PPE. Safety glasses at minimum — energy released from an internal failure can propel electrolytic fluid at high velocity. Insulated gloves are not overkill for caps above 100 joules.

Questions About Capacitor Energy Calculations

How do I calculate energy stored in a capacitor?▼

The energy stored in any capacitor follows E = ½CV² exactly. Enter the capacitance C in farads and the voltage V in volts, multiply C by V², then divide by two. The result is in joules. This calculator does the arithmetic live — pick any unit for C (pF through F) and enter V directly in volts.

Why does energy scale with the square of voltage?▼

Because you need twice as much charge per volt at higher voltage to maintain the same capacitance. Energy is work done assembling a charge Q against an opposing voltage V — and since Q grows linearly with V, the work grows quadratically. Doubling the voltage on the same capacitor quadruples the stored energy, which is why high-voltage caps pack surprising punch.

Is a charged capacitor dangerous even at low capacitance?▼

Yes. A 100 µF cap charged to 400 V (typical of tube gear and old motor start circuits) stores 8 joules — enough to give a painful shock and startle reaction, and potentially enough to trigger the 'can't let go' response on small children. Treat any capacitor above 1 000 pF charged above 50 V as potentially hazardous. Always discharge through a 20 kΩ 5 W resistor before handling.

How many joules does a typical capacitor store?▼

The range spans nine orders of magnitude. A 100 pF ceramic @ 50V stores 0.125 microjoules — barely enough to feel. A 1F supercap @ 2.5V stores 3.125 joules — enough to light an LED for several seconds. A 100 µF cap @ 400V stores 8 joules — enough for a painful zap. The cheat sheet table above maps common parts to their stored energy.

What is capacitor energy density?▼

Energy density is energy per unit mass, usually J/kg. Film capacitors achieve about 0.1–1 J/kg, electrolytics 1–10 J/kg, and supercapacitors up to 30 J/kg — about one-tenth of a lithium-ion battery's 150–250 J/kg. Capacitors win on power density (how fast they deliver energy), not energy density (how much they store).

How do I convert capacitor energy to watt-hours or mAh?▼

1 Wh = 3 600 J exactly. Divide your joules by 3 600 to get Wh. To convert to mAh at a specific battery voltage, divide joules by (3.6 × V_battery) × 1 000. At 3.7V, 1 Wh ≈ 270 mAh. This calculator shows Wh directly, and the cheat sheet adds an 'equivalent battery' column at 3.7V for quick comparison.

Why are there three formulas for capacitor energy?▼

All three are equivalent — they just express the same physical quantity through different variables. E = ½CV² is what you use when you know capacitance and voltage. E = Q²/(2C) works when you know stored charge but not voltage directly. E = ½QV is the simplest and most intuitive form — average voltage (V/2) times total charge (Q). The calculator shows all three forms with live numbers.

Can I store enough energy in a capacitor to power something useful?▼

Supercapacitors routinely back up SRAM memory chips for years (microwatts), and larger banks can supply burst power for camera flashes, defibrillators (tens of joules), and even regenerative braking in hybrid trucks (kilojoules). For continuous power beyond a few seconds, batteries still win — but for high-power pulses, nothing beats a capacitor.

Keep calculating

Related Capacitor Calculators

The energy formula E = ½CV² sits right next to charge (Q = CV), time constant (τ = RC), reactance (X꜀ = 1/(2πfC)), and color code decoding in the capacitor knowledge graph. Open the tools below to answer the rest of the questions you will encounter alongside energy storage.