Wattage is one of the most misunderstood specs in indoor growing. Growers obsess over it, manufacturers inflate it, and the result is a lot of people buying the wrong light. This guide explains what wattage actually means, why it's not the whole story, and how to work out exactly what you need for your setup.
What Does Wattage Actually Mean?
Wattage measures how much electricity a grow light consumes — not how much light it produces. Two lights can draw the same wattage and deliver very different amounts of usable light to your plants, depending on the quality of the LEDs, the driver, and the optics.
This is why PPFD (Photosynthetic Photon Flux Density) is a more useful number than wattage when comparing lights. PPFD tells you how much photosynthetically active light is actually hitting your canopy per second. Wattage tells you what it costs to run.
That said, wattage is still a useful starting point — as long as you know what you're comparing.
The 40–50W Per Square Foot Rule
A commonly used guideline for quality LEDs is to aim for 40–50 true watts per square foot of growing space. This applies to fruiting and high-demand crops. For leafy greens, herbs, and seedlings, you can get away with less — around 20–30W per square foot.
Here's how that translates to common tent sizes:
- 60x60cm (0.36m²) — 150–200W
- 80x80cm (0.64m²) — 200–300W
- 1.0x1.0m (1.0m²) — 300–400W
- 1.2x1.2m (1.44m²) — 400–600W
- 1.5x1.5m (2.25m²) — 600–900W
- 2.4x1.2m (2.88m²) — 800W+
These figures assume a quality LED with an efficiency rating of 2.5 µmol/J or above. If you're buying a budget light with a lower efficiency rating, you'll need more wattage to achieve the same result — which often means more heat, higher running costs, and worse results.
Why “Equivalent Wattage” Claims Are Misleading
Many cheaper LED lights are marketed with claims like “600W equivalent” or “replaces a 600W HPS.” This refers to the light output of an older HID lamp — not the actual power draw of the LED. The LED itself might only draw 100–150W. Whether that's actually equivalent depends entirely on the quality of the diodes and the PPFD produced.
Always look for the true (actual) wattage draw — usually listed as “actual power consumption” in the specs — rather than the “equivalent” figure.
What About Running Costs?
This is where wattage does matter directly. A 400W LED running 18 hours a day uses 7.2 kWh per day. At the UK average electricity rate of around 25p per kWh, that's approximately £1.80 per day, or around £54 per month. A 600W light on the same schedule would cost around £81 per month to run.
Higher efficiency lights (those with better µmol/J ratings) produce more light per watt — meaning you can achieve the same PPFD with a lower-wattage, lower-cost-to-run light. This is why investing in a quality LED upfront often pays back quickly in lower electricity bills.
Matching Wattage to Plant Type
Not all plants need the same light intensity. Here's a general guide:
- Seedlings & cuttings — low intensity, 100–200 µmol/m²/s PPFD. The SunBlaster LED Propagation Light is ideal at this stage.
- Leafy greens, herbs, microgreens — moderate intensity, 200–400 µmol/m²/s. A compact LED like the Garland Grow Light Garden Kit works well here.
- Tomatoes, peppers, fruiting crops (vegetative stage) — 400–600 µmol/m²/s. Step up to a mid-range panel like the Elite Lighting Quantum LED Panels.
- Fruiting crops (peak production) — 600–900+ µmol/m²/s. High-output lights like the Omega 720W Plus or the Photon 720W LED are built for this.
Dimmable Lights: The Flexible Option
One of the biggest advantages of modern LED grow lights is dimmability. A dimmable light like the Maxibright Varidrive 500W/1000W lets you dial back intensity during propagation and early growth, then ramp it up as plants mature and demand more light. This gives you flexibility across the full growth cycle without needing to buy multiple lights.
The Bottom Line
Wattage is a useful shorthand, but it's not the whole picture. Focus on true power draw, PPFD output, and efficiency rating when comparing lights — and match your choice to the size of your space and the demands of your crop. Buy once, buy well, and your light will pay for itself in better results and lower running costs over time.
Browse our full range of LED grow lights — from compact propagation strips to high-output full-tent panels.