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Grow light cost calculator.

Estimate how much electricity an indoor grow light uses — in kWh per day and per month — and what it costs to run, in any currency.

  • Results update as you type
  • No account needed
  • The formula is always shown

Your grow light

Pre-filled with an illustrative example — not my office's actual usage. Change any value.

W

The real power draw from the spec sheet or a plug-in meter — not an “equivalent” wattage.

h/day

How long a timer or you actually run the light.

days

Usually 30.

per kWh

From your bill. Any currency.

Symbol or code, e.g. €, $, £, SEK.

Estimated running cost

Updates as you type

€12.00per month

Energy per day
1.60 kWh
Energy per month
48.00 kWh
Yearly projection
€144.00

12 × this month — not a forecast of a full year's bill.

How it's calculated

100 W ÷ 1000 × 16 h × 30 days × €0.25/kWh = €12.00

An estimate for the light only, from the numbers above. Real bills can differ.

The formula

How the calculator works

Monthly cost = (watts ÷ 1000) × hours per day × days per month × price per kWh

Watts (W) and kilowatts (kW) measure power — how fast the light draws electricity. 1000 W = 1 kW.

Kilowatt-hours (kWh) measure energy — power × time. It's the unit your electricity bill charges for.

Full guideHow much does a grow light cost to run?Worked example, step-by-step method and common mistakes
  1. W ÷ 1000KilowattsThe light's real power draw, converted from watts.
  2. × hoursHours per dayHow long it actually runs each day.
  3. × daysDaysThe period you're costing — usually a month.
  4. × pricePrice per kWhWhat you pay for each kilowatt-hour.

Before you trust the estimate

Six things that make real costs differ from the calculation.

Use the real power draw

“Equivalent” wattage on the box describes brightness, not electricity use. Use the actual watts from the spec sheet.

Measure if you can

A plug-in energy meter between socket and light shows the true draw — and can total kWh over a week.

Dimming changes the draw

A dimmed light usually uses less power, but not necessarily in proportion to brightness. Measure at your setting.

Prices and tariffs vary

Use your own per-kWh price. On a time-of-use tariff, cost the hours in each price band separately.

Other equipment adds up

Fans, pumps, heaters and dehumidifiers use electricity too. Cost them separately and add them.

Bills include more

A per-kWh calculation doesn't include fixed standing or connection charges, or every part of a bill.

More detail in the grow-light cost guide, or browse all guides.