Outdoor solar lighting is a wonderful idea. No wires, so you can stick one anywhere. ‘Solar’ is still cool, and this seems like a perfect application – charge during the day, light-up when it’s dark.
We grew tired of $5 outdoor solar lights breaking after two weeks, and of $15 ones staying on for a couple of hours after dark, which in the winter means about 9pm.
So, let’s find out what the higher end has to offer. Malibu Solar Spotlight is not cheap, at $50 at Home Depot. It promises 10 hours of light, at 54 Lumens. That’s about one twentieth of the light that a 100W incandescent (old style) bulb puts out. But LED light shines in one direction and it looks really bright. Remember that time you shone a new LED flashlight into your eye, despite all the warnings from the manufacturer, your mom, and the priest.
The Malibu device does come with a separate solar panel unit, connected to the light fixture by a black wire of about 3′. The angle of the panel adjusts with a screw, so you can place and aim the panel where the sun does shine. The panel is relatively sizable at about 20″sq. and labeled on the back at 2W.
Alibaba lowball prices for bare 2W panels are around $3. Or they quote $0.70/W, making the 2W panels about $1.50. Now, Amazon has ones at that rating for $10, and packaged as a product, in a case, 2W panels are $30. I may be able to say more about price and power output, after reading up on monocrystalline, polycrystalline, and amorphous silicone types used in solar panels.
The light comes from an array of nine LEDs, making them 6 Lumens each. These are old school, the ones with a rounded top. Nowadays, a single 3V LED emitter, the one that’s square and has a light yellow color, will put out way more that 60 Lumens, and cost under $0.50 on Alibaba.
The sturdy cast metal outer housing feels good in the hand. The plastic standing pole and spike do not. The electronics subsystem has an ON switch on the back of the unit. A maze of wires towards the front does not inspire confidence in this aspect of the design from Asia.
It also houses two Fullriver 900mAh 18500 LiFePO4 3.2V batteries, in parallel. Fullriver is a serious industrial battery brand from China. The LiFePO4 chemistry is known for fast charge and for constant voltage, and is used for rechargeable tools and even some electric cars. Being specialty items, these batteries could cost $5-10 each, retail. Off brand lowball prices on Alibaba are around $1 each.
As to the 10 hours at 54 Lumens claim? It would take some 3Wh to charge each battery, at 100% charger efficiency. After allowing for some inefficiency, we’d need some equivalent of 2W over 4 hours from the panel. Should be doable on a bright sunny day. Now, even imagining that we can get the full 900mAh out of each battery over 10 hours, it would give us some 0.6W to work with. So, could this setup, with these old-fashioned LEDs produce 90 Lumens per W? This is known as Luminous efficacy, and state-of-the-art LEDs are pushing the 300 Lumens per Watt output performance (perfect conditions in the lab, of course). The ones inside Malibu are going to be significantly below 100.
Oh, the first Malibu unit we bought at Home Depot went right back – DOA. But the second one’s been working for a year now, and is still impressively bright well after midnight. As to ten hours – not really, more like 4-5.