Library / Tools / Physics equation calculator / Transformer power

Transformer power calculator

GCSEA-level Electricity

For a transformer that is 100% efficient, the power in the primary coil equals the power in the secondary coil.

V_p I_p = V_s I_s

This free calculator is for GCSE and A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.

Quantity to find

V_pPrimary voltage

I_pPrimary current

V_sSecondary voltage

I_sSecondary current to find 8.00 A

Significant figures

Working

Write the equation
V_p I_p = V_s I_s
Rearrange to make I_s the subject
I_s = V_p I_p / V_s
Substitute the values
I_s = 230 × 0.4 ÷ 11.5
Calculate
I_s = 8 A
Give the answer to 3 significant figures
I_s = 8.00 A

Worked example

Find Is, the secondary current. Given: primary voltage Vp = 230 V; primary current Ip = 0.400 A; secondary voltage Vs = 11.5 V.

Write the equation
V_p I_p = V_s I_s
Rearrange to make I_s the subject
I_s = V_p I_p / V_s
Substitute the values
I_s = 230 × 0.4 ÷ 11.5
Calculate
I_s = 8 A
Give the answer to 3 significant figures
I_s = 8.00 A

Symbols and units

Symbol	Quantity	SI unit
V_p	Primary voltage	V
I_p	Primary current	A
V_s	Secondary voltage	V
I_s	Secondary current	A

Transformer power: rearranged

To find primary voltage: V_p = V_s I_s / I_p
To find primary current: I_p = V_s I_s / V_p
To find secondary voltage: V_s = V_p I_p / I_s
To find secondary current: I_s = V_p I_p / V_s

Exam tip

Stepping the voltage up steps the current down by the same factor. Real transformers lose a little energy as heat.

Common questions

What is the formula for transformer power?

The equation is Vp Ip = Vs Is, where Vp is the primary voltage (V); Ip is the primary current (A); Vs is the secondary voltage (V); Is is the secondary current (A).

What units should I use for transformer power?

In SI units: primary voltage in V, primary current in A, secondary voltage in V, secondary current in A. The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.

How do I rearrange Vp Ip = Vs Is?

Rearranging gives these forms: to find primary voltage: Vp = Vs Is / Ip. to find primary current: Ip = Vs Is / Vp. to find secondary voltage: Vs = Vp Ip / Is. to find secondary current: Is = Vp Ip / Vs.

Practise transformer power questions

Include unit conversions (km, g, minutes…)

Press the button for a question with random numbers. Work it out on paper first, then type your answer.

More electricity equations: Charge, current and time, Ohm’s law, Electrical power, Electrical power, Energy transferred, Resistors in series, Resistors in parallel, Resistivity. See the full list on the physics equation calculator page.