Hydroponic Grow Lights: PPFD Targets and the Real Electricity Bill

Q: What PPFD do I need for hydroponic lettuce?

200-300 micromoles per square meter per second at canopy level for 16-18 hours a day, which lands a daily light integral of 12-17 mol per square meter per day. From a 2.2 micromole-per-joule fixture that is about 12-18 watts per square foot, or roughly 112 W over an 8 sq ft bed.

Q: How much does it cost to run a grow light for a year?

Multiply watts by hours per day by 365, divide by 1,000, then multiply by your electricity rate. A 100 W fixture running 16 hours a day uses 584 kWh, which is $99 at 17 cents per kWh and $175 at 30 cents. Per square foot of leafy canopy, budget $11-14 a year on an efficient LED and $22-44 on fluorescent, HID or a cheap panel.

Q: Are expensive LED grow lights worth it?

Only where photon efficacy differs. Paying $160 rather than $50 for 2.7 micromoles per joule instead of 1.4 saves about $88 a year on 8 sq ft of canopy and repays the price difference in roughly 14 months. Spectrum switches, apps and dimming curves rarely return anything on a small home system.

Q: How many hours a day should grow lights be on?

Leafy greens and herbs want 16-18 hours, microgreens and seedlings 14-16, strawberries about 14, and tomatoes, peppers and cucumbers 14-18 and no more, since continuous light causes leaf chlorosis in tomato. Set hours to hit the daily light integral target rather than copying a default timer setting.

Q: Can a window replace a grow light?

Not in winter. An indoor south-facing windowsill commonly supplies 2-6 mol per square meter per day through the cold months, against the 12-17 a lettuce head wants. From spring through early autumn a genuinely bright sill can carry leafy greens without any fixture.

Updated September 2026 · Editorial team · Topic: hydroponics

Hydroponic Grow Lights: PPFD Targets and the Real Electricity Bill — Hydroponics

Lighting an 8 sq ft indoor salad bed for a year costs $90 to $180, and the spread has little to do with the plants. It comes from one number budget fixtures rarely print: photon efficacy, in micromoles per joule. A $40 panel and a $160 bar can put the same light on the canopy — the cheap one bills $85 a year extra.

Short answer: Leafy greens need a daily light integral of 12–17 mol/m²/day — about 200–300 µmol/m²/s of PPFD over 16 hours, or 12–18 W per square foot from a decent LED. Fruiting crops need 20–30 mol/m²/day and 25–35 W/sq ft. At 17¢/kWh a year of light runs $11–14 per square foot on a 2.2–2.7 µmol/J fixture, $22–44 on a cheap panel. That is $0.67 per head of lettuce and 6¢ an ounce of microgreens — but $3.30–4.90 per pound of indoor tomatoes, more than the store charges.

Reviewed by the BackyardStead Lab editorial team. We publish real ROI, plain numbers and USDA/extension data so you can judge for yourself — we run the math, not a farm. Educational information only: backyard-chicken and livestock rules vary by city, home canning must follow USDA/NCHFP-tested methods (botulism risk), and mushrooms should be grown only from a known-species kit — never foraged on our word.

PPFD, DLI, and the one formula you need

PAR is the 400–700 nm band plants photosynthesize in. PPFD is how densely those photons land on the canopy now; DLI is the day's total in moles. Plants respond to the total, not the peak:

DLI = PPFD × hours × 0.0036

So 250 µmol/m²/s for 16 hours gives 14.4 mol/m²/day, inside the lettuce band. The same fixture on an 8-hour timer delivers 7.2 mol and grows pale, stretched heads that take twice as long to size up. Lengthening the photoperiod beats buying a bigger light.

Wattage on the box is not a light measurement: a panel sold as "1000W" typically pulls 100–150 W from the wall. The honest comparison is PPF — total µmol/s emitted — divided by actual draw. That ratio is photon efficacy, spanning 0.7 to nearly 3.0 µmol/J.

PPFD and DLI targets by crop

Crop	Target DLI (mol/m²/day)	PPFD needed	Photoperiod	W/sq ft (2.2 µmol/J)
Microgreens, seedlings	6–12	120–210 µmol/m²/s	14–16 h	8–12 W
Lettuce, leafy greens	12–17	200–300	16–18 h	12–18 W
Basil, soft herbs	14–20	250–350	16 h	15–22 W
Strawberries, day-neutral	17–22	330–440	14 h	20–26 W
Tomatoes, peppers, cucumbers	20–30	350–520	14–18 h	25–35 W

Lettuce tolerates very long photoperiods, though some cultivars answer with tipburn. Tomato does not: past about 18 hours the leaves yellow and mottle, so extra light for fruiting crops must arrive as intensity, not hours. No windowsill reaches these numbers in winter — a south-facing sill gives 2–6 mol/m²/day, which is what makes indoor hydroponics an electricity project.

What each fixture type costs to run

Priced per square foot of leafy canopy at 250 µmol/m²/s, 16 hours a day, year-round, 17¢/kWh, with 75% of emitted photons reaching the canopy.

Fixture	Efficacy (µmol/J)	Real draw	Price	Canopy covered	Cost per sq ft / year
CFL "grow bulb"	0.6–0.8	15 W	$8–15	0.3 sq ft	$44
T5 fluorescent, 4-tube	0.9–1.0	216 W	$80–130	6.6 sq ft	$32
150 W HPS / metal halide	0.9–1.1	165 W (ballast)	$60–100	5.3 sq ft	$31
LED shop light, 4 ft	1.3–1.6	40 W	$20–30	1.8 sq ft	$22
Budget "1000 W" panel	1.2–1.6	110 W	$35–60	5.0 sq ft	$22
White LED board	2.0–2.4	100 W	$70–120	7.1 sq ft	$14
Quality LED bar	2.5–2.9	240 W	$140–280	21 sq ft	$11

Running cost is inversely proportional to efficacy, so the ranking never changes with electricity rates — only the size of the gap does. Moving 8 sq ft of greens from a 1.4 µmol/J panel to a 2.7 µmol/J bar saves about $88 a year and repays a $160 fixture in 14 months — the highest-leverage purchase indoors, well ahead of anything on the nutrient shelf.

Data note: DLI and PPFD targets consolidate published controlled-environment ranges from university extension and CEA programs, not a trial of our own. Efficacy figures are manufacturer-reported PPF-per-watt ranges typical of each class in 2026 retail listings; we metered nothing, and a given product can land outside its band. Electricity uses the ~17¢/kWh U.S. average, which runs 11¢ to 33¢ by state.

Does the light pay for itself? Crop by crop

Crop and area	Draw	Cycle	Electricity	Yield	Light cost per unit	Grocery price
Microgreens, 4 sq ft	40 W, 14 h	12 days	$1.14	~18 oz	$0.06 / oz	$2–4 / oz
Basil, 4 sq ft	68 W, 16 h	1 month	$5.55	6–10 oz	$0.55–0.92 / oz	~$3.30 / oz
Lettuce, 8 sq ft	112 W, 16 h	35 days	$10.66	16 heads	$0.67 / head	$2.00–2.50
Strawberries, 6 sq ft	138 W, 14 h	120 days	$39.40	6–9 lb	$4.40–6.60 / lb	$3–6 / lb
Tomatoes, 4 sq ft	120 W, 16 h	150 days	$49.00	10–15 lb	$3.30–4.90 / lb	$2.50–4 / lb

Value per watt falls as the crop gets bigger and slower. Microgreens return roughly 40 times their electricity, basil about four, lettuce three, and those margins survive seed and net-pot costs. Fruiting crops don't: an indoor tomato carries more electricity than a supermarket tomato costs, which is why the case for indoor tomatoes is flavor and season, never savings, and indoor strawberries tip the wrong way in high-rate states.

A greenhouse or a long outdoor season rewrites all of it: summer sun delivers 20–50 mol/m²/day free. That is the dividing line in hydroponics versus soil — indoors you buy photons, outdoors you don't.

Three ways the bill beats the spec sheet

Hanging height. Intensity drops to roughly a quarter at double the distance: a panel at 24 inches that belongs at 12 draws every watt and delivers a fraction of its rated PPFD.
Heat you pay for twice. Every watt becomes 3.41 BTU/h, so a 200 W fixture adds 680 BTU/h and a cooled room burns 40–60 W removing it.
Peak-rate hours. On time-of-use billing, running the photoperiod overnight cuts the cost of identical kilowatt-hours 20–40%.

Common mistakes, in numbers

Buying by "equivalent watts." A "600 W equivalent" panel drawing 80 W at 1.3 µmol/J emits 104 µmol/s — 3.4 sq ft of lettuce, not the 2×4 ft on the box.
Running 12 hours because the timer came that way. At 250 µmol/m²/s that is 10.8 mol/m²/day against a 12–17 target, stretching a 35-day lettuce cycle toward 50.
Lighting a fruiting crop at leafy-green intensity. A tomato at 200 µmol/m²/s sets fruit slowly and thinly; it wants 350–520, two to three times the light.
Leaving tomatoes and peppers lit 24/7. Past roughly 18 hours the foliage yellows, and the extra 6 hours adds $18 a year per 100 W with nothing behind it.
Guessing instead of metering. A quantum PAR meter costs $200–500; a phone lux meter lands within 20–30% on white light divided by 74 for 4000 K LED, 54 for daylight — but fails on red/blue panels, where lux is nearly blind to deep red.

FAQ

What PPFD do I need for hydroponic lettuce?

200–300 µmol/m²/s at canopy level for 16–18 hours, landing 12–17 mol/m²/day. From a 2.2 µmol/J fixture that is 12–18 W per square foot, or 112 W over 8 sq ft.

How much does it cost to run a grow light for a year?

Watts × hours × 365 ÷ 1,000 × your rate. A 100 W fixture at 16 hours uses 584 kWh: $99 at 17¢/kWh, $175 at 30¢. Per square foot of canopy, budget $11–14 on an efficient LED, $22–44 on fluorescent or a cheap panel.

Are expensive LED grow lights worth it?

Only where efficacy differs. Paying $160 rather than $50 for 2.7 µmol/J instead of 1.4 saves about $88 a year on 8 sq ft and repays the gap in 14 months. Spectrum switches and apps rarely return anything.

How many hours a day should grow lights be on?

Leafy greens and herbs 16–18 hours, microgreens 14–16, strawberries 14, fruiting crops 14–18 and no more, since continuous light causes leaf chlorosis in tomato. Set hours to hit the DLI target, not a default timer.

Can a window replace a grow light?

Not in winter — a south-facing sill gives 2–6 mol/m²/day against the 12–17 lettuce wants. Spring through early autumn a bright sill carries leafy greens, which is why passive setups work seasonally.

Related:

Educational information only, not professional horticultural advice. BackyardStead Lab does not operate a commercial farm or laboratory; figures here are compiled from USDA, university extension publications and published manufacturer data. Prices, energy costs, yields and fixture performance vary by retailer, state electricity rate, cultivar and room conditions — always check the specifications on the product you buy.