Greenhouse Supplemental Lighting: How to Pick Between LED and HPS, What It Costs Per Square Meter, and What It Means for Your Irrigation Schedule

Supplemental lighting is one of those greenhouse investments where the decision tree looks simple on paper but gets tangled fast once you dig in. HPS is cheaper to buy. LED saves on electricity. Pick one. Except then you find out your irrigation schedule needs reworking, your heating bill shifts, and the “cheaper” option quietly eats its purchase-price advantage in operating costs before the first growing season ends.

I’ve talked to growers who bought LED systems expecting a straightforward swap and ended up underwatering their tomatoes for three weeks because they didn’t account for the drop in canopy temperature. Others stuck with HPS and watched their July electricity bills erase any margin they’d gained from the lower upfront cost. Neither mistake is about the technology being bad. It’s about not connecting the dots between light, heat, water, and money.

What Each Lighting Type Actually Costs Per Square Meter

Let’s get the numbers on the table first. These are 2025-2026 averages drawn from Dutch greenhouse suppliers and Midwest US distributors. Prices shift with order volume and regional variation, but the ratios hold.

For a 1,000 m² greenhouse targeting 150 µmol/m²/s supplemental light (a common target for tomato and cucumber production):

HPS (1000W double-ended fixtures): – Fixture cost: $250-350 per unit, roughly 1 fixture per 8-10 m² – Total fixture cost: $25,000-44,000 for the space – Installation (wiring, hanging, reflectors): $15-25 per m² – Bulb replacement: every 10,000-12,000 hours, $60-90 per bulb – Electricity draw: roughly 1050W per fixture (including ballast losses) – Heat output: about 350W of radiant heat per fixture that hits the crop directly

LED (top-lighting modules, 600-650W): – Fixture cost: $600-1,200 per unit, roughly 1 fixture per 6-8 m² – Total fixture cost: $75,000-200,000 for the space – Installation: $10-18 per m² (lighter fixtures, simpler mounting) – No bulb replacement (rated 50,000+ hours to 90% output) – Electricity draw: 600-650W per fixture – Heat output: roughly 100-150W of convective heat per fixture, most of it rising away from the crop

The upfront gap is real. HPS runs about half to a third of the capital cost of LED for the same light output. But the monthly numbers flip the story. At $0.12/kWh, running those HPS fixtures 16 hours a day for a 120-day supplemental lighting season costs roughly $24,000 in electricity. The equivalent LED setup costs around $15,000. That’s a $9,000 difference per season. Add bulb replacements every two seasons, and the gap widens further.

Most operations reach LED payback in 3 to 5 years. Growers in regions with electricity above $0.15/kWh hit it faster. Growers in the Pacific Northwest, where hydropower keeps rates below $0.08/kWh, sometimes never cross over. The math is local.

Why Your Irrigation Schedule Has to Change

Here’s the part most lighting guides skip. HPS fixtures pump a lot of radiant heat directly onto the crop canopy. That heat raises leaf temperature 2-4°C above air temperature, which drives transpiration. Plants pull more water through their roots to cool themselves. Your irrigation system has to keep up.

LEDs produce less radiant heat and more of what they do produce rises convectively toward the ceiling rather than hitting the leaves. Leaf temperature under LED typically runs 1-2°C below air temperature. Transpiration drops. If you keep the same irrigation schedule you had under HPS, you’re overwatering. Roots sit wet. Oxygen drops. Root pathogens get an invitation.

A Dutch trial on greenhouse tomatoes published in 2023 compared HPS and LED compartments with identical air temperature and VPD. The LED compartment used 15-20% less irrigation water to maintain the same substrate moisture levels. Growers I’ve spoken with who made the switch without adjusting their irrigation timers saw runoff EC spike as water sat in the substrate longer and salts concentrated.

The fix isn’t complicated but it needs attention during the transition. If you’re switching from HPS to LED, start by reducing irrigation volume 10-15% in the first week. Monitor substrate moisture with sensors or by weight. Expect to adjust frequency more than volume, because the plants are still transpiring, just less aggressively. If you’re going the other direction, from no supplemental light to HPS, plan for 20-30% more water volume during lit hours and watch your substrate dryback carefully in the first two weeks.

The Heating Tradeoff Nobody Mentions

HPS waste heat isn’t entirely wasted. In cold climates, that radiant heat offsets your heating system’s load. A 1000W HPS fixture dumps about 3,400 BTU/hour into the greenhouse. If you’re heating with natural gas at $0.80/therm, that HPS “waste” heat is worth roughly $0.03 per fixture-hour in avoided heating costs.

LEDs don’t give you that. When a Dutch or Canadian greenhouse switches from HPS to LED, the heating system has to pick up the slack. In a well-insulated Venlo-type greenhouse in the Netherlands, that can mean 15-25% more natural gas consumption during the lighting season. The LED electricity savings still usually win, but the net advantage shrinks by a third to a half once heating is factored in.

This matters most for growers in northern latitudes running supplemental light from October through March. If you’re in a warmer climate using lights primarily for cloudy-day insurance rather than full-season photoperiod extension, the heating penalty is negligible, and LED’s cooling advantage in summer becomes the bigger factor.

How to Decide Without Losing a Season

Run your own numbers. Start with three things: your electricity rate per kWh, your annual hours of supplemental lighting, and whether your heating system currently runs during the months you’d use lights. If electricity is cheap and winters are cold, HPS still makes sense in plenty of cases. If electricity is expensive or you’re lighting for 2,000+ hours a year, LED pulls ahead fast.

Factor the irrigation change into your transition plan either way. New lights mean new watering patterns. Budget two weeks of active monitoring after installation. The sensor data from that period is worth more than any manufacturer’s spec sheet.

And if you’re building a new greenhouse from scratch rather than retrofitting, the conversation changes entirely. LED lets you mount fixtures closer to the crop, which means lower roof heights and less structural steel. On a new build, the capital savings from a shorter greenhouse can offset a big chunk of the LED price premium before you even plug anything in.