Grow Lights for Houseplants: What the Specs Actually Mean
Grow lights for houseplants work or fail on how high you hang them, not on price. How to read PPF, PPFD and hours, and what running one costs.
Grow Lights for Houseplants: What the Specs Actually Mean
Whether a grow light rescues your plant depends mostly on how high you hang it. The big number on the box is what the lamp emits in total. What reaches the leaves is a much smaller number, and it falls away fast with distance.
That gap is where most disappointment with grow lights for houseplants comes from. People buy a well-reviewed lamp, hang it at a height that looks right in the room, and get a plant that carries on exactly as before.
The short version
- Hang it close. University of Minnesota Extension puts foliage houseplants at 12 to 24 inches from the lamp. Six feet up is decoration.
- PPF is not PPFD. The first is what the lamp emits, the second is what lands on the plant. Only the second matters.
- A manufacturer that publishes output at several distances is telling you something the others are hiding.
- 12 to 14 hours a day, and never more than 16. Plants need the dark half.
- Running cost is small, and you can work out yours exactly rather than guessing.
How far should a grow light be from the plant?
Closer than feels natural. University of Minnesota Extension puts foliage houseplants at 12 to 24 inches below the lamp, flowering plants at 6 to 12, and seedlings at 4 to 6.
That range is not a suggestion, and the manufacturer's own data shows why. Soltech publishes output at several distances for its Aspect Gen 2, a 36 watt lamp at $200: roughly 50 µmol/m²/s at one meter, about 34.9 at four feet, and about 15.5 at six feet.
Now put that against what the plant needs. UMN's own figure for the least demanding category is Low light (PPF: 50-150 umol m-2s-1 / 50-250 foot-candles / 10-15 watts).
At one meter, that $200 lamp is delivering roughly the bottom of the low light band. At four feet it is under it. At six feet it is delivering about a third of the minimum, which is to say the plant is being lit rather than grown.
None of that is a fault in the lamp. It is the inverse square law doing what it always does, and it applies to every lamp ever sold. The distance is the setting you control, and it is worth more than the difference between an $85 lamp and a $200 one.
PPF, PPFD, and the number that actually matters
Two acronyms, one letter apart, and the difference decides whether a purchase works.
PPF measures everything the lamp emits, in micromoles per second. It is a property of the lamp alone.
PPFD measures what arrives on a given surface, in micromoles per square meter per second. It depends on the lamp, the distance, and the beam angle. PPFD is the figure that has to clear the plant's requirement.
The trap is that the two are quoted in similar-looking units and PPF is always the larger, friendlier number. Soltech's Vita bulb, at $85, lists PPF 25.95 μmol/sec with no distance figures at all. Its Aspect Gen 2 lists both PPF and PPFD at three distances. Same manufacturer, two levels of disclosure.
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The confusion is industry-wide rather than a marketing trick. Even UMN's own table labels its column PPF while giving the units of PPFD. When you compare a lamp's specification against a plant's requirement, check that both sides are talking about the same quantity, and if the lamp only publishes total output, you have no way of knowing what your plant will receive.
A manufacturer that publishes output at several distances is doing you a favor. Treat its absence as a signal.
How much light does the plant need?
UMN gives three bands, each with a photon figure, a foot-candle equivalent, and a rough wattage:
| Category | Photon figure | Foot-candles | Watts |
|---|---|---|---|
| Low | 50 to 150 | 50 to 250 | 10 to 15 |
| Medium | 150 to 250 | 250 to 1,000 | 15 to 20 |
| High | 250 to 450 | over 1,000 | over 20 |
The foot-candle column is the useful bridge, because it connects to what your window already does. If you have run the light estimator you have a foot-candle range for your spot, and you can see how far a lamp has to carry you.
For most people in a dim room the honest target is the low band, and the low band is reachable. Lifting a plant from below the floor of low light up into it is the difference between a plant that holds and a plant that grows. Chasing the high band in a living room is a different project.
How many hours a day?
UMN puts foliage houseplants at 12 to 14 hours a day and flowering plants at 14 to 16. Illinois Extension sets the ceiling: Duration of 14 - 16 hours is sufficient. Do not expose plants to more than 16 hours of light.
Leaving a lamp on around the clock does not speed anything up. Plants run processes in the dark that they cannot run in the light, and a continuous photoperiod is a stress rather than a bonus. For a few common plants the night is more than that, since it is the signal that starts flowering, which is why a lamp near a Christmas cactus in autumn prevents the thing you want: do plants need darkness.
A timer settles that, and it is worth checking whether one is included. The Aspect Gen 2 ships with an outlet timer for US and Canada orders. The Vita bulb does not include one, so budget for a plug timer if you go that route, because "I will remember" is not a lighting schedule.
What do grow lights for houseplants cost to run?
Less than most people fear, and you can calculate yours exactly instead of trusting a stranger's average.
The formula is:
watts × hours per day × 30 ÷ 1000 = kilowatt-hours per month
Then multiply by your own rate, which is printed on your electricity bill.
- Vita, 20 watts, 12 hours a day: 20 × 12 × 30 ÷ 1000 = 7.2 kWh a month
- Aspect Gen 2, 36 watts, 12 hours a day: 36 × 12 × 30 ÷ 1000 = 12.96 kWh a month
At a hypothetical 15 cents per kilowatt-hour, purely to show the arithmetic, those come to about $1.08 and $1.94 a month. Your rate is probably not 15 cents, which is the point of giving you the formula rather than a headline number.
On the technology choice, UMN's comparison comes down to LEDs being more efficient, longer lived, and cooler running, while fluorescents cost less to buy but draw more power and need replacing sooner. The running cost of either is small in absolute terms. The purchase price is the real decision.
Reading a spec sheet without being sold to
Every figure below is taken from the manufacturer's own product page, and both lamps are made by the same company, chosen because it publishes more detail than most.
| Vita | Aspect Gen 2 | |
|---|---|---|
| Price | $85 | $200 |
| Power | 20 W | 36 W |
| Total output (PPF) | 25.95 μmol/sec | 50 μmol/sec |
| Output at distance (PPFD) | not published | ~50 @ 1 m, ~34.9 @ 4 ft, ~15.5 @ 6 ft |
| Color temperature | 3000K | 3000K |
| CRI | 98 | 98 |
| Rated life | 30,000 hours | 50,000 hours |
| Dimmable | yes | yes, 10 to 100 percent |
| Timer included | no | yes, US and Canada |
| Beam angle | 36° or 60° | 60° |
Four things to take from a table like this, whichever brand you are looking at:
Distance data or nothing. Without it you cannot check the lamp against a requirement.
Beam angle changes everything. A narrow beam concentrates the same output onto a smaller area, which raises PPFD in the middle and drops it at the edges. Two lamps with identical PPF can behave completely differently over a shelf.
Color temperature and CRI are about your room, not the plant. A 3000K lamp at CRI 98 renders a living room normally, which is why these exist in the first place. UMN notes that White lights or mixed/balanced light bulbs are suitable for most plants at any stage of growth, so a warm white lamp is not a compromise on plant performance. The purple lamps are not more effective for foliage houseplants, they are just harder to live with.
Rated life is in hours, so convert it. At 12 hours a day, 30,000 hours is around seven years and 50,000 is around eleven. Both outlast the lease.
What has and has not been tested here
Nothing on this page has been tested by me.
I do not own either of these lamps, I have not measured their output, and I have not run them for a season to see how they hold up. Everything above comes from the manufacturer's published specifications and from extension service guidance, both linked where they are used, so you can check every figure yourself.
I have chosen not to score or rank anything for that reason. A ranking implies a comparison I have not earned, and there are enough grow light articles with confident verdicts behind them and no evidence underneath. What this page offers instead is the arithmetic: how to check any lamp's numbers against what your plant needs, at the height you can actually hang it.
Common questions
How far should a grow light be from plants? For foliage houseplants, UMN Extension recommends 12 to 24 inches. Flowering plants want 6 to 12 inches and seedlings 4 to 6. Output falls off steeply with distance, so a lamp mounted near the ceiling delivers a small fraction of what the same lamp gives at two feet.
How many hours a day should a grow light be on? Twelve to 14 hours for foliage plants, 14 to 16 for flowering ones, and Illinois Extension advises not going beyond 16. Continuous light is not better, because plants need a dark period.
What is PPFD on a grow light? PPFD is the number of photons landing on a square meter of surface each second, so it accounts for distance and beam angle. PPF, by contrast, is the lamp's total emission regardless of where it goes. Requirements for plants are given in PPFD terms, so a lamp advertised only by PPF cannot be checked against them.
Do grow lights use a lot of electricity? No. A 36 watt lamp running 12 hours a day uses about 12.96 kWh a month. Multiply by the rate on your bill for the exact figure. The purchase price matters far more than the running cost.
Is a grow light for a north facing window worth it? For light, yes, and that is the one thing about a dim room you can genuinely fix. A lamp lets you choose a light level instead of accepting the one your window happens to give, which is what makes it the standard answer for a spot below the low light floor, as covered in what a north-facing window actually gives.
Grow lights for houseplants: one thing to do before buying
Get a tape measure and check the height you were planning to hang it at.
If the answer is more than two feet above the leaves, the lamp you were about to buy will underperform its own specification sheet by a wide margin, and no amount of spending fixes that. Sort out the mounting first. The lamp is the easy part.
For the size, the mounting height, and what the lamp will add to your electricity bill each month, the grow light calculator does the arithmetic and starts from the window estimate if you have made one.
On timing, buy grow lights for houseplants earlier in the year than feels necessary. Daylight falls fastest around the September equinox and has almost stopped changing by December, so a lamp ordered in midwinter arrives after the plant has already spent two months of reserves. The week-by-week figures are in houseplant winter care.