What is the formula for the energy stored in a capacitor?

The energy is E = ½ · C · V², with capacitance C in farads, voltage V in volts and energy E in joules. A 1 F capacitor charged to 10 V stores ½ × 1 × 10² = 50 J. The factor of one half comes from integrating the charge as the capacitor fills, because the early charge goes on at a lower voltage than the last charge. If you forget the ½ you double every answer, which is the single most common slip.

How do I find the charge on a capacitor?

The charge is Q = C · V, in coulombs, where C is in farads and V in volts. A 470 µF capacitor at 16 V holds Q = 470e-6 × 16 ≈ 7.52 mC. Charge scales linearly with voltage while energy scales with voltage squared, so the two answers behave differently when you change the volts. This tool shows Q next to the energy on every solve so you do not have to run a second calculation.

Why does doubling the voltage quadruple the energy?

Because voltage appears squared in E = ½CV². Holding capacitance fixed, going from V to 2V multiplies the energy by (2V)² / V² = 4. A 1 mF cap at 50 V stores 1.25 J, and at 100 V it stores 5 J, four times as much, not twice. This is why high-voltage energy banks pack so much punch and why doubling a supply voltage is a real safety jump, not a small one.

How do I back-solve the voltage from a target energy?

Rearrange E = ½CV² to V = √(2E / C). To store 50 J in a 1 F supercapacitor you need V = √(2 × 50 / 1) = 10 V. Tick the energy and capacitance boxes, type both, and the tool returns the required voltage. It guards the math: a zero or negative capacitance has no real solution, so you get a clear message instead of a NaN.

What units does the calculator accept and return?

Capacitance takes pF, nF, µF, mF or F to match what is printed on the part, voltage is plain volts, and energy can be entered or read in µJ, mJ, J or kJ. Internally every value is converted to SI base units (farads, volts, joules) before the formula runs, then the result is shown on a readable prefix. A 100 nF cap at 5 V comes back as 1.25 µJ rather than a long string of zeros.

Does this handle supercapacitors and energy storage banks?

Yes. Pick the F unit and a 1 F supercapacitor at 5 V reads 12.5 J directly. For a bank of identical caps the series and parallel reference under the result shows the equivalent capacitance: two 1 F caps in parallel give 2 F (double the energy at the same voltage), in series give 0.5 F. That lets you trade capacitance against voltage rating when sizing a hold-up or burst-energy bank.

Size a camera flash or strobe capacitor

A xenon flash dumps a known energy into the tube per pop. You have a 400 µF photoflash cap and a 330 V boost rail and want to know the joules behind the flash. Tick capacitance and voltage, read about 21.8 J, and compare it to the tube rating. Drop the rail to 165 V and the energy falls to 5.4 J, a four-fold cut from a two-fold voltage change, exactly the dimmer behaviour you expected.

Check a power-supply bulk or hold-up capacitor

A switching supply has to ride through a brief input dropout on stored energy alone. Enter the bulk cap and its rail voltage to see the joules available, then compare against the energy your load draws during the hold-up window. If 470 µF at 16 V (about 60 mJ) is not enough, the tool makes it obvious you need more capacitance or a higher pre-dropout voltage before the rail sags.

Rate a supercapacitor energy bank

Sizing a supercap to bridge a power gap or fire a burst load? Pick the F unit, enter the capacitance and working voltage, and read the stored joules directly. Use the series and parallel reference to trade a 2 F parallel pair against a 0.5 F series pair at higher voltage, then re-read the energy each way to land on the bank that fits both your joule budget and the cell voltage limit.

Teach or check homework on E = ½CV²

Working through a circuits problem set, you want to verify a by-hand answer for the energy or back-solve the voltage that stores a target number of joules. Type the two knowns, watch the third appear, and confirm the charge Q = C·V on the side. The shareable URL lets a student send the exact figures to a tutor, who reopens the identical calculation rather than retyping it.

Capacitor Energy Calculator — E = ½CV² + Charge Q = CV

E = ½CV² stored energy + charge Q = CV — solve any one of C, V, E from the other two — browser-only

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Enter any two of capacitance, voltage, or stored energy. The third value plus the plate charge Q = C·V are solved instantly with E = ½CV².

Quick examples

Known values (pick two)

Capacitance (C)

Voltage (V)

Energy (E)

Result

Capacitance (C)

400 µF

Voltage (V)

330 V

Energy (E)

21.78 J

Charge Q = C·V

132 mC

Formula

E = ½ · C · V² · Q = C · V

Series / parallel equivalent

Parallel

800 µF

Series

200 µF

Two identical capacitors of the value above. Parallel adds (C+C), series halves (C/2). Use it to sanity-check an energy bank.

What this tool does

Free capacitor energy calculator built on the textbook relation E = ½ · C · V². Give it any two of capacitance, voltage and stored energy and it solves the third, plus the charge held on the plates Q = C · V. Capacitance is typed in the units printed on real parts, pF, nF, µF, mF or F, voltage in volts, and the energy comes back in µJ, mJ, J or kJ on an auto-picked human scale. The classic gotcha lives here too: energy rises with the square of voltage, so a part charged to twice the volts stores four times the joules, which is why a 400 µF camera-flash capacitor at 330 V holds about 21.8 J while the same part at 165 V holds only 5.4 J. A small series and parallel reference sits below the result so an energy bank of two equal caps can be sanity checked at a glance, since parallel adds and series halves for matched parts. Everything runs in your browser with one-click copy and a shareable URL that reproduces the exact figures. 100% client-side, nothing is uploaded.

Tool details

Input: Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy + Preview; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 9 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Developer; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Capacitor Energy Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Size a camera flash or strobe capacitor
A xenon flash dumps a known energy into the tube per pop. You have a 400 µF photoflash cap and a 330 V boost rail and want to know the joules behind the flash. Tick capacitance and voltage, read about 21.8 J, and compare it to the tube rating. Drop the rail to 165 V and the energy falls to 5.4 J, a four-fold cut from a two-fold voltage change, exactly the dimmer behaviour you expected.
Check a power-supply bulk or hold-up capacitor
A switching supply has to ride through a brief input dropout on stored energy alone. Enter the bulk cap and its rail voltage to see the joules available, then compare against the energy your load draws during the hold-up window. If 470 µF at 16 V (about 60 mJ) is not enough, the tool makes it obvious you need more capacitance or a higher pre-dropout voltage before the rail sags.
Rate a supercapacitor energy bank
Sizing a supercap to bridge a power gap or fire a burst load? Pick the F unit, enter the capacitance and working voltage, and read the stored joules directly. Use the series and parallel reference to trade a 2 F parallel pair against a 0.5 F series pair at higher voltage, then re-read the energy each way to land on the bank that fits both your joule budget and the cell voltage limit.
Teach or check homework on E = ½CV²
Working through a circuits problem set, you want to verify a by-hand answer for the energy or back-solve the voltage that stores a target number of joules. Type the two knowns, watch the third appear, and confirm the charge Q = C·V on the side. The shareable URL lets a student send the exact figures to a tutor, who reopens the identical calculation rather than retyping it.

Common pitfalls

Dropping the factor of one half. The energy is ½CV², not CV². Forgetting the ½ doubles every answer — a 1 F cap at 10 V is 50 J, not 100 J. The half comes from integrating charge against a rising voltage as the capacitor fills.
Treating energy as linear in voltage. Charge Q = CV is linear, but energy E = ½CV² is quadratic. Doubling the voltage quadruples the joules, not doubles them, so a small voltage bump on a big bank is a large energy jump.
Mixing up the capacitance unit. A "100" typed as F instead of µF is off by a million. Always match the field unit to the marking on the part — 100 µF and 100 pF differ by a factor of a million in both energy and charge.

Privacy

Every calculation — the ½CV² energy, the charge Q = C·V, the unit conversions and the series-parallel reference — is plain JavaScript that runs in your browser tab. No capacitance, voltage or energy figure ever leaves the page, and there is no logging of what you computed. The one caveat: the shareable URL encodes your values and units in the query string, so a "share link" pasted into chat will record those numbers in the recipient server's access log. For sensitive design figures, use the copy button and paste the text rather than sharing the URL.

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Capacitor Energy Calculator — E = ½CV² + Charge Q = CV

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Capacitor Energy Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Size a camera flash or strobe capacitor

Check a power-supply bulk or hold-up capacitor

Rate a supercapacitor energy bank

Teach or check homework on E = ½CV²

Common pitfalls

Privacy

FAQ

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