You have a 2 Watt, 6 Volt panel and a 1,000 mAh, 3.7V battery, how long does it take to completely charge? The quick and very wrong answer would be to figure out the Watt hours of the battery (3.7 * 1Ah = 3.7 Watt hours) and divide. The reality is about 2.5 times longer.

There are three main reasons for the difference, even in ideal conditions. First, the Wattage rating on the panel is the open circuit Voltage multiplied by the peak current. When you connect a panel to a battery, the Voltage drops down to that of the load, about 4.5V. Finally, all the power that enters the battery does not get converted into storage energy. Some percentage is lost as heat as the process to convert the incoming power into stored power takes energy.

In field tests, we’re seeing that the combined loss factor is about 2.5. So Divide the Watt hours of your battery by the Wattage of your panel and multiply by 2.5. In our example above you would get 3.7 Watt hours / 2.0 Watts * 2.5 = 4.6 hours to fully charge. If you have cloudy conditions, your panel is not pointed at the sun or your panel’s Voltage is not well matched to your battery, this could increase.

With large-scale systems, maximum power point tracking is used to increase production efficiency. We haven’t seen any cost-effective examples in small-scale systems yet. If you know of any, please drop us a line.

14 Responses to Estimating Battery Charge Time from Solar

  1. Eddie says:

    I have the 13 watt battery charger from harbor freight charges at 12 and 24 volts. I got a 40 amp charge controler and a 1.2 amp inverter that plugs into a cigaret lighter socket, the panel kit came with a cigaret lighter socket that I can this plug into. The question is ….will this supply suffecent power for a power tool charger to and charge an 18 volt battery? Specs of the charger:120 VAC, 60 watt. Or… Can I connect the 18 volt battery direct to the battery charge port on the charge .

    And… What is the average amps hours on 12 volt battery?

    Thanks for your kind response.

  2. Andre M says:

    I am looking to mount one or two solar panels on my cargo bike (long john) for bicycle touring. As such I wonder if I can charge my Thinkpad (it has a 20V input, but the battery itself is a mere 10.8V, 7.8Ah) with one or two of your panels and how I would go about setting it up.

    I doubt it could take 36V input if I took two of your 16.8 and wired them in series, and I wonder how it would do with two of your 16.8 panels wired in parallel?

    I will probably buy the V60 battery, but in order to save losses I would prefer to be able to charge the laptop directly.

    • admin says:

      Hi Andre,

      It might work directly from the panels if wired in parallel, it is hard to tell without testing. In general laptops/tablets have fairly sophisticated charge circuits and are looking for something pretty close to spec. It will definitely work from the battery.

      • Andre M says:

        Thank you, that was quick. I might just give it a go and see what happens. Now, to find the money :D

  3. webby says:

    I had a 50 watts solar panel and 12 volts battery 28 amp. how many hrs will take the battery to get fullcharge?

  4. Tim says:

    I want to charge a laptop battery (about 11V, 4400 mAH) and a 9V, 2100mAH battery for a smaller device. Do I need to get two kits, one 18V and one 12V? Or can one kit charge both? I’m thinking a 12V charger will probably be okay with a 9V battery, but I don’t know about an 18V charger. Thank you.

    • admin says:

      Hi Tim,

      In general, we recommend using the panels to charge our V60 battery and then set the output on the battery to charge your devices. The laptop would most likely charge from the 16V setting on the V60 and the 9V battery from the 12V source.

      In some cases, it will be possible to charge the devices directly from the panels. Its hard to know without knowing the specs of the devices, but you would want to put 12V into the 9V battery and 18V into the 11V battery as you suggest. The key here is to make sure there is some protection to make sure you are charging safely: no over charge, no short circuit, etc.

  5. ajmal says:

    i want to use an ac fan for 6 hours daily by a car battry and i want to charge this battry by a 40w panal. what will be the time required to charge it by this panal? plz help mn

    • admin says:

      It depends on the Voltage of your panel, the size of the car battery, your charge controller that you’re using and the weather conditions. You should be able to do it in about a day of full sun though. Please consider using a deep-cycle rather than a standard car battery, it will work better.

  6. Charles Cook says:

    Could the 16.8 Watt Panel be used to charge a 12 Volt vehicle battery?

    My primary use will be a laptop, cell phone, and a 12 volt battery for my hand held Ham Radio.

    Will one panel be enough?

  7. Pingback: Estimating battery charge time from solar « adafruit industries blog

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