RAVPower 24W 3-Port Solar Charger Expired

Quick electrical power primer (for DC power).


Energy (how much work you can do) is measured in Joules, but battery-driven stuff usually uses Watt-hours (Wh) instead.
Power (energy per second) is measured in Watts (Joules per second).
Volts is analogous to water pressure. You can think of it as electrical pressure.
Amps is analogous to water flowrate (volume of water moving past a point each second). You can think of it as electricity flowrate.
Power = Volts * Amps
Amp-hours (Ah) is another way to measure energy capacity, dependent on voltage. To get energy capacity, you need to multiply Ah by voltage. For 12V batteries the voltage is 12V, so Watt-hours = 12V * Amp-hours. For laptop batteries, it's typically 11-15 Volts. For alkaline batteries it's 1.5 Volts. etc.

A 12V deep cycle battery is typically about 100 Ah in capacity. If it's lead-acid, you probably don't want to discharge it much below 50%, so figure about 50 Ah of that is usable capacity. 12V * 50 Ah = 600 Wh of energy capacity.

This charger has a capacity rating of 24 Watts. That is, under ideal conditions (cloudless sunny day, summer, noon, with the panels pointed right at the sun) it will generate 24 Watts. Under those ideal conditions, 600 Watt-hours / 24 Watts = 25 hours. It would take 25 hours to charge your deep cycle battery back to 100% after fully depleting it.

Obviously the sun won't stay in its noon-time position for 25 hours straight. To get more realistic daily numbers, you need to multiply by something called capacity factor. That's the ratio of average generation vs rated capacity (usually over a year). This takes into account night, clouds, weather, and angle of the sun changing with time of day and the seasons. For the continental U.S. it averages about 0.145, closer to 0.11 at northern latitudes, and up to 0.19 in the desert southwest. You can see the capacity factor for any zip code using the PWatts site (just put in any numbers - the capacity factor for the zip code is listed in the output).

https://pvwatts.nrel.gov/

Using the 0.145 average for the U.S., that means a 24 Watt rated panel will generate an average of (24W)*(0.145) = 3.48 W on average. So to recharge your deep cycle battery with 600 Wh would require (600 Wh) / (3.48 W) = 172 hours, or a little over 7 days.

There are also seasonal variances (the sun is up longer, and is higher in the sky in summer, so sunlight has to travel through less air and more of it reaches the ground). So in summer it'll be less than 7 days, in winter it'll be more than 7 days.

So if you normally drain your deep cycle battery in a week, yes this solar panel could work to top it off during spring, summer, and fall. Although you'll also need some sort of voltage regulator to step up the 5V USB output to the ~14V the deep cycle battery needs to charge and hold it there even when the sun gets low in the sky. There are also complications with charge rate and battery maintenance which are beyond the scope of this post.

If you normally drain your deep cycle battery in a day, then you'll need a panels rated for about 7x more Wattage to recharge it in a day. Or about a 160 Watt panel. More if you plan to use it during the winter.


The efficiency is how much sunlight it converts to electricity. The 24 W is after efficiency (and under ideal conditions - noon, summer, sunny, panels tilted at the sun).

Sunlight hits the Earth's surface with about 750-800 Watts per square meter of power. A 22% efficient panel will convert 22% of that into electricity, or 165-176 Watts/m^2. From the diagram on the Amazon link, this looks like four 30 cm x 12 cm panels. So that's 4*(0.3m)*(0.12m) = 0.144 m^2. (170 W/m^2)*(0.144 m^2) = 24.5 Watts. Which checks out with the advertised 24 Watt capacity.

But for generation over the course of a day, you need to multiply by the capacity factor as I did in the above example. We calculated 3.48 Watts average output for this above. So in a day of charging it'll generate (3.48W)*(24h) = 83.5 Wh of energy (capacity factor takes into account night, so we multiply by 24 hours, rather than number of hours of daylight).

A 2000 mAh power bank is 2.0 Ah @ 3.7 V (the voltage of a single Li-ion cell). (2.0 Ah)*(3.7V) = 7.4 Wh of energy storage capacity. So this will provide plenty of power to charge multiple power banks in a sunny day. (There are some losses associated with voltage conversion and battery charging. But 83.5 Wh >> 7.4 Wh so there's plenty of overhead.

It doesn't do fast charging itself, only normal 12 watt max. You'd be better off using this to charge a battery pack that does have fast charging output

It can't charge at more than 12w per port, which is the regular USB standard. But it's important to oversize the solar panel- a 24w panel is more likely to be able to provide the full 12w in more conditions such as cloudy, early/late day, etc. A 12w panel would probably only be putting out 6w or so on average.