Grow light running costs: calculate the real UK cost

The short version

Multiply the light’s measured watts by hours used and your electricity unit rate. Standing charges are not added because you pay them whether or not the light is on. The useful test is not whether a method can work, but whether it keeps the roots supplied with water, dissolved nutrients and oxygen while the leaves receive enough light.

• Use actual wall draw where possible.
• Divide watts by 1,000 to get kilowatts.
• Your bill’s unit rate is better than a national example.

A practical way to do it

1. 1

   Find the power draw in watts.

   Make this change deliberately, then observe the plant and system before changing the next variable. That separation makes the result useful.

2. 2

   Enter the daily lighting hours.

   Make this change deliberately, then observe the plant and system before changing the next variable. That separation makes the result useful.

3. 3

   Use pence per kWh from your tariff.

   Make this change deliberately, then observe the plant and system before changing the next variable. That separation makes the result useful.

4. 4

   Compare weekly, monthly and yearly totals.

   Make this change deliberately, then observe the plant and system before changing the next variable. That separation makes the result useful.

What to watch

Look at the whole system before diagnosing a single leaf. Check new and old growth separately, lift the lid to inspect root colour and texture, confirm that water or air is moving, and compare today’s readings with the last entry in your log.

Make the setup easier to live with

Put inspection points where you can reach them without moving plants. Label cables, nutrient bottles and measuring tools. A waterproof tray, a marked full-water line and a simple notebook prevent more trouble than an elaborate controller. When a crop finishes, remove roots promptly and clean the parts you can now reach.

Questions people ask