You can charge a solar light without direct sun, but only two methods move a useful amount of energy: daylight, even bad daylight, and a bulb held within about two inches of the panel. Everything else you have read online is thirty to a thousand times too dim to matter. Here are the actual numbers.
The short answer
A typical solar garden light holds a 300 mAh AA NiMH cell and carries a panel that delivers around 40 milliamps in full sun. That is why the box says “6 to 8 hours of direct sunlight.” Every other light source is just a fraction of that 40 mA, and the fraction is usually brutal.
An overcast winter day still gets you there in about two days. A 9-watt LED bulb two inches from the panel gets you there in about 40 hours. The same bulb a foot away never gets you there at all, because at that distance the panel supplies less current than the battery loses to self-discharge.
What your panel can actually use
Most garden lights carry one of two panel types, and the difference decides which artificial light works.
- Amorphous silicon is the flat, uniform, reddish-brown or grey-brown panel found on inexpensive stake lights. Its bandgap is around 1.7 eV, so it responds to light from roughly 350 to 700 nanometres. Visible light only. It is blind to infrared.
- Polycrystalline silicon is the blue, speckled, visibly crystalline panel on better pathway and spot lights. Its bandgap is 1.1 eV, so it responds out to about 1,100 nanometres and can use near-infrared as well as visible light.
This matters more than anything else in this article. Run the blackbody numbers on a 2700 K incandescent filament and only about 5 to 8 percent of what it radiates falls below 700 nanometres. The other 92 percent or more is infrared. An amorphous panel converts almost none of it. A polycrystalline panel does better, reaching roughly a quarter to a third, but the rest still arrives as heat.
So the advice repeated across every article on this topic, that incandescent bulbs are best because their spectrum resembles sunlight, is wrong in a specific and measurable way. An incandescent bulb resembles sunlight to your eye. To a solar panel it is mostly a heater.
Every light source, measured
Figures below assume a clean amorphous panel, a 300 mAh NiMH cell, and the light switched off so the LED is not draining the battery while you charge. “Usable irradiance” is the portion falling in the band the panel can convert.
| Light source | Light at the panel | Usable irradiance | Charge current | Time to fill |
|---|---|---|---|---|
| Clear midday sun, clean panel | 110,000 lux | 410 W/m² | 37 mA | 8 hours |
| South window, panel sitting in the sunbeam | 45,000–60,000 lux | 170–225 W/m² | 15–20 mA | ~17 hours |
| 1,000-lumen flashlight, 2 in, flooding the panel | ~45,000 lux | 158 W/m² | 14 mA | ~22 hours |
| 60W incandescent, 2 in (see heat warning) | 24,700 lux | 102 W/m² | 9 mA | ~34 hours |
| 9W LED bulb (800 lm), 2 in | 24,700 lux | 86 W/m² | 7.7 mA | ~40 hours |
| Bright overcast, outdoors | 15,000–20,000 lux | 55–75 W/m² | 5–7 mA | ~2 days |
| Open shade, north side, clear day | 10,000–12,000 lux | 40–45 W/m² | 4 mA | ~3 days |
| Under a deep porch roof | 3,500 lux | 13 W/m² | 1.2 mA | ~2 weeks |
| 9W LED bulb, 6 in | 2,700 lux | 10 W/m² | 0.9 mA | ~3 weeks |
| Heavy overcast, rain | 2,500 lux | 9 W/m² | 0.8 mA | ~3 weeks |
| Window, 3 ft back, out of the beam | 1,200 lux | 4.5 W/m² | 0.4 mA | ~3 months |
| Any bulb at 12 in | ~700 lux | 2.4 W/m² | 0.2 mA | Never |
| Ceiling light, panel on a table | 250 lux | 0.9 W/m² | 0.08 mA | Never |
Read the bottom three rows carefully. Those are not slow. They are below the floor, and the next section explains why.
Why dim light never fills the battery
A standard NiMH cell bleeds charge fastest in its first days off the charger and then settles down — Energizer publishes 50 to 80 percent of capacity still present after twelve months of storage, and warmth makes it worse. Quoting a flat daily percentage is how people end up doing arithmetic that says a cell is empty in two months, which is not what happens. On a 300 mAh cell that works out to about 0.25 mA of continuous leakage.
A bulb twelve inches from the panel supplies about 0.21 mA. A lit room supplies 0.08 mA. Both are less than the leak. You can leave a solar light under a lamp for a month and the cell will read exactly what it read when you started, which is why so many people conclude their light is broken when the physics simply said no.
There is a threshold, not a gradient. Below roughly 1,500 lux at the panel, a standard NiMH garden light is not charging slowly. It is not charging.
Does an incandescent bulb work, and is it safe?
It works only at contact range, and at contact range it is a genuine hazard.
Two inches from a 60-watt incandescent bulb, the panel receives about 24,700 lux of visible light and roughly 2,000 W/m² of total radiant energy. That is double the thermal load of noon sunlight in July, arriving almost entirely as infrared, onto a small dark plastic housing with no airflow.
NiMH cells are specified for charging between 32 °F and 113 °F. Above that they lose capacity permanently and can vent. ABS and polycarbonate housings soften somewhere around 190 to 220 °F. A bulb at two inches will cross both limits inside an hour.
There is also a supply problem. The US Department of Energy’s 45 lumens-per-watt standard took general-service incandescent bulbs off American retail shelves in August 2023. The articles telling you to use a 60 to 100-watt incandescent are recommending a product you would have to dig out of a garage drawer.
Per lumen delivered, an LED and an incandescent charge an amorphous panel at about the same rate. The incandescent simply adds roughly twenty times more radiant heat and burns about seven times more electricity to produce those lumens. There is no version of this where the incandescent is the better choice.
LED bulbs and flashlights
An LED bulb is the safer artificial option and performs about the same, but the distance rule is unforgiving. Light from a bulb falls off with the square of distance: move from 2 inches to 6 inches and you lose 89 percent of it. Move to 12 inches and you have lost 97 percent.
Flashlights look better on paper because a focused beam concentrates output. A 1,000-lumen light held two inches away, aimed to cover the whole panel, delivers around 45,000 lux and charges in roughly 22 hours. The problem is the flashlight’s own battery, which holds around 11 Wh and runs about two hours at that output. You would drain it eleven times over to move 0.36 Wh into the garden light.
The honest accounting on artificial charging: running a 9W LED for 40 hours consumes about 364 Wh of grid electricity to store 0.36 Wh in the cell. That is a round-trip efficiency near 0.1 percent. It costs about twelve cents in California, so it is cheap. It is just not a strategy, it is an emergency measure.
Indirect daylight is the method that actually works
This is the answer the rest of the internet buries under the bulb tricks. Even a grey, raining day outdoors beats every household lamp, because the sky is enormous and a bulb is not.
If you cannot get direct sun, do this in order of effectiveness:
- Put the light outdoors in open sky, not under cover. Open shade with a clear view of the sky gives you 10,000 to 12,000 lux. The same spot under a porch roof gives you 3,500. The roof costs you a factor of three.
- Face the panel up, not sideways. Under overcast, light comes from the whole sky dome rather than one direction, so a flat upward face collects the most. Aiming at where the sun would be helps only on clear days.
- Use a windowsill, and put the panel physically in the sunbeam. Clear double glazing passes about 70 percent of visible light; low-E glass passes 40 to 60 percent and deliberately blocks infrared, which penalises polycrystalline panels more than amorphous ones. In the beam you still get 45,000 to 60,000 lux and a full charge in under a day. Three feet back from the same window, out of the beam, you drop to 1,200 lux and three months.
- Clean the panel first. A film of dust and pollen can cost 10 to 25 percent, and on a cloudy day you have no margin to give away. See how to clean the solar panel on garden lights.
We cover the broader question in do solar lights need direct sunlight to charge.
Do mirrors help?
On a clear day, yes, in one narrow case. On a cloudy day, no.
A mirror redirects light, it cannot create it. If your panel sits in shade while direct sun falls a few feet away, a mirror angled to throw that beam onto the panel is a real gain, potentially doubling what the panel receives. A household back-silvered mirror returns 85 to 90 percent of what hits it, and aluminium foil considerably less because it scatters.
Under overcast there is no beam to redirect. Light arrives diffusely from the entire sky, so a mirror placed near the panel blocks roughly as much sky as it reflects. The net gain is close to zero, and can be negative.
The practical catch nobody mentions: the sun moves 15 degrees per hour. A fixed mirror holds useful aim for perhaps 30 to 60 minutes before the reflected patch walks off the panel.
What to do instead
If your lights are not charging, the light source is usually not the problem.
- Check for a USB port. A growing number of units, particularly solar spot lights and shed lights, include a USB-C input. If yours has one, use it. It charges in two to four hours at full current instead of forty, and it is the only method here that is genuinely efficient.
- Replace the cell. Factory cells are often 200 to 300 mAh and degrade after two or three seasons. A fresh low-self-discharge NiMH AA costs a couple of dollars, holds charge for months instead of weeks, and fixes far more “won’t charge” complaints than any bulb will. Read can I use a higher mAh battery in solar lights before sizing up.
- Switch the light off while charging. The LED draws 10 to 30 mA when lit. Under any artificial source that is 50 to 100 times the charging current, so a light left switched on in a dim room drains faster than it fills.
- Move the fixture, or move the panel. Many pathway lights and most wall and shed lights have a remote panel on a 10 to 16 foot cable. Putting the panel in sun and the light in shade solves the problem permanently.
- Rule out a fault first. If a unit is brand new, check for the pull tab under the battery cover. Our guides on brand new solar lights not working and solar lights not working cover the rest.
That “deep charge for 72 hours once a month” advice you will see repeated everywhere is not doing what those articles claim. There is no NiMH conditioning ritual involved. What actually happens is that switching the light off for three days stops the nightly drain, so the cell finally reaches full for the first time in months. The fix is the switch, not the 72 hours. For how long a full cell should last you, see how long solar lights stay on at night.
Safety, briefly
Do not leave any incandescent or halogen bulb within six inches of a solar light unattended. Do not charge a light whose battery compartment has been wet. If the housing is warm to the touch, you are past 113 °F and damaging the cell. And if a NiMH battery is swollen, leaking or smells sharp, it is finished, and no amount of light will bring it back.
Sources
- NOAA NCEI - National Solar Radiation Database ncei.noaa.gov The 1,000 W/m2 one-sun reference, and overcast at 10-25% of clear sky
- PVEducation (UNSW/ASU) — Band Gap pveducation.org Amorphous silicon 1.7 eV / 700 nm cutoff vs crystalline 1.1 eV / 1,100 nm
- Michael, Iqbal & Chianelli — Solar irradiance and lux illuminance extrica.com The 1 sun = ~120,000 lux conversion used throughout the table
- Apostolou, Reinders & Verwaal — Energies 7(3) mdpi.com Measured a-Si vs crystalline output under incandescent, CFL and LED sources
- Battery University BU-410 — Charging at High and Low Temperatures batteryuniversity.com NiMH charge window of 41-113F, and capacity loss and venting above it
- US DOE — General Service Lamps final rule govinfo.gov Incandescent general-service bulbs off US retail shelves from August 2023
- NFRC window performance ratings glewengineering.com Clear double pane VT ~0.70, low-E 0.40-0.60, and low-E blocking near-infrared
Questions people ask
Can I charge my solar lights with a flashlight?
Yes, but only at close range and it is a poor trade. A 1,000-lumen flashlight held two inches away, aimed to cover the whole panel, delivers about 45,000 lux and would fill a 300 mAh cell in roughly 22 hours. Your flashlight only runs about two hours at that brightness, so you would flatten its battery eleven times over to store a third of a watt-hour. Use it to get one evening of light in an emergency, not as a routine.
Can you charge solar lights in the shade?
Yes, provided the shade is open to the sky. Open shade on the north side of a building on a clear day still measures 10,000 to 12,000 lux, which fills a typical cell in about three days. Shade under a deep porch roof drops to around 3,500 lux and stretches that to two weeks. The roof matters far more than the shade. Keep the panel facing straight up with a clear view of sky.
Do you charge solar lights in the on or off position?
Off, always, when you are trying to recover a flat battery. The panel charges the cell either way, but the switch cuts the LED load. That LED draws 10 to 30 milliamps when lit, which is 50 to 100 times what any indoor light source can supply. Leave a light switched on in a dim room and it drains faster than it charges. Switch it off, charge it, then switch it back on.
What is the best artificial light to use for charging solar panels?
An LED bulb of 800 lumens or more, held two inches from the panel. Per lumen it charges an amorphous panel at about the same rate as an incandescent, without the fire risk and using roughly one seventh the electricity. Judge bulbs by lumens, not watts. The wattage figures repeated online exist because they were written when 60 watts meant 800 lumens, which it has not since 2023.
How do I know if my solar light is fully charged?
There is no indicator on most units, so test it directly. Cover the panel with your hand or a cloth for about ten seconds. A healthy, charged light comes on at full brightness almost immediately. If it comes on dim, flickers, or takes several seconds, the cell is low or worn out. For a real measurement, put a multimeter across the battery terminals: a charged NiMH AA reads about 1.35 to 1.4 volts at rest.
Can you charge solar lights with indoor room lighting?
No. A lit living room measures 150 to 300 lux at table height, which produces around 0.08 milliamps from a typical panel. A standard NiMH cell leaks about 0.25 milliamps per day to self-discharge, so the battery loses ground faster than the room light replaces it. This is not slow charging, it is no charging. You need roughly 1,500 lux at the panel before anything accumulates at all.
How do I manually charge my solar lights?
Check the housing for a USB-C or micro-USB port first, since many newer spot, wall and shed lights have one and will charge fully in two to four hours. If there is no port, the fastest legitimate route is to remove the NiMH cell and charge it in a standard AA battery charger, which takes about four hours. Do not try to wire a phone charger to the panel terminals.
