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 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. 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 = 3.7 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.

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42 Responses

  1. Charles Cook

    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?

    Reply
  2. ajmal

    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

    Reply
    • admin

      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.

      Reply
  3. Tim

    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.

    Reply
    • admin

      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.

      Reply
  4. webby

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

    Reply
    • admin

      It depends on a lot of factors, but in reasonably good sunlight with the panels pointed towards the sun, we would expect about 17 hours. Nothing replaces field testing though! The math is: 12*28/50 * 2.5 = 16.8

      Reply
  5. Andre M

    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.

    Reply
    • admin

      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.

      Reply
      • Andre M

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

  6. Eddie

    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.

    Reply
  7. siva

    how many hours are taken to charge 12v battery useing
    10w,12v solar panel

    Reply
  8. Mark

    Hi, I am going to install a 50 watt solar panel, control charger, 2000 watt continuous/4000 watt peak power inverter, and a group 29 deep cycle flooded battery, what will I be able to run with this set up? I would like to run a 15 amp chest freezer and a 22 inch tv, and one house fan will this be enough and if so how long will I be able to run this before battery runs out?

    Reply
  9. Brent

    I just bought a Coleman 6W solar battery charger. I am looking to use a 12V battery during the night to run 2 lights, a small fan, and some small speakers. My plan is to use a 400W inverter attached to the battery at night, then use the solar panel to trickle charge the battery at night. Most likely a car battery, possibly a marine gel battery. Does anyone know if my plan will work? Trying for 4 days/4 nights. Im guessing the solar panel will have a good 12+ hrs in sunlight to charge each day

    Reply
  10. Robin

    I have a small 4amp fan i wanted to run 2-3hrs a day. I have a 2.2 amp trickle charger..Can I put the trickle charger on the car battery while running my fan to maintain the car battery? Any info would be helpful.

    Reply
  11. Catherine

    Hello
    I am a novice here. What is the least watt/volt solar cell that is needed to charge an iPhone, etc.?
    How does the size of the solar cell fit in to the equation?

    Reply
    • Voltaic Systems

      We typically recommend 3.5 as the smallest panel for charging an iPhone. Smaller than that and you will typically be frustrated with charge times.

      Reply
  12. mathsongroup

    Pls, i want set up a solar pv phone charging centre in my house (village) to charge about 6 phones (3.7v and 2.5w each) at a time. do i need inverter and controller? pls let someone help me for the calculation of the components; panel, inverter and controller.

    Reply
    • Voltaic Systems

      You don’t need an inverter. Everything can stay in DC. For this sort of application, you could use the 9 Watt Kit which includes a 9 Watt solar panel and the V44, 12,000mAh battery and 2 USB ports. If you really wanted to reduce costs, you could just get the 9 Watt panel and the USB regulator. This would work to charge phones on sunny days, but you wouldn’t have any power storage if you skip the battery.

      See:
      http://www.voltaicsystems.com/9-watt-panel
      http://www.voltaicsystems.com/9-watt-kit

      Reply
  13. mathsongroup

    Thank You for the instruction. Pls how many female usb regulators or phones can be connected to the output of the 9w panel?

    Reply
  14. olayinka

    I have a need to set up a 30 phone capacity charging station. What kit do I need to have i.e panel, battery, inverter, charge controller

    Reply
    • Voltaic Systems

      It is certainly possible to do without an inverter / charge controller, but you could also do it that way as well. Using our equipment, you could get 7-8 x 9 Watt kits which have a 9 Watt panel and V44 battery. From each V44 you could run a USB hub so that you could charge 4 phones from each V44 battery. In this way, you skip going from DC to AC and back to DC. Or you could get a large panel, say 100 Watts and connect it to a battery over 200 Watt hours with a built in inverter and run lots of USB chargers out of that. Have a look at earthtech products for a range of products in this category.

      Reply
  15. Selina Zhou

    How long would it take to recharge a IPhone 6s, fully, using a 100 watts solar panel?

    Reply
    • Voltaic Systems

      Same time as from the Apple wall charger, assuming you have a regulated USB output of 5V, 2A. The iPhone has a maximum charge rate so you won’t be able to charge faster than from a wall.

      Reply
  16. M faisal khan

    I have 150 Watt soler panel and 105 amp battery. What time is required to charge my 12 volt battry.

    Reply
  17. Gurpreet

    I want to purchase a 13000 mah 5 V power bank but company donot mention how long it will take to charge power bank fully in a good sunny day…any idea ahow approx it will take to charge it fully?

    Reply
    • Voltaic Systems

      It depends on how efficient the battery charges from solar power and the size of the solar panel. If we assume that the battery charges efficiently from solar, a reasonable estimate is (Battery Capacity in Watt hours) / (Power of Solar Panel) * 2. You need to take the size of the solar panel, divide by 48 Watt hours (the voltage of the internal battery is actually 3.7) and multiply by 2.

      Reply
  18. raja

    i had 12v 10w panel and 12v 40w panel how much capacity battery and Amps charge controller are require please help me
    I planning for an led garden light

    Reply
    • Tony Kauffmann

      How much capacity of the battery you need depends on the power consumption of your light and how long you want the light to run.

      Reply
  19. DEEPANKAR

    how can we charge a small solar panel fast. can we use other things to charge without a Sun?

    Reply
    • Tony Kauffmann

      All of our voltaic universal batteries can also be charged from a wall power outlet, either through a USB port or through the included 18V AC charger for the V72. So you don’t need to sun to charge these batteries.

      Reply
  20. RockAndHardPlace

    I know someone living in their vehicle and they need a cpap machine while they sleep. They have an inverter, and it runs their machine 6-7 hours from the car battery before shutting off. When it shuts off, there is still enough of a charge on the battery to start the vehicle. I think the inverter won’t allow them to run it dead, but it let’s it go until that’s all that’s left. He is about to buy a solar setup from a private party that has a total of 32 watts in two solar panels, and has a charge controller with it. Where we live, it is always completely overcast or raining. Would the panels still charge the battery to full in the 8 hours or so that we have daylight?

    Reply
    • Voltaic Systems

      There are a lot of unknown variables. a) the size of the battery inside the CPAP machine and b) whether the battery inside the CPAP will charge effectively. However, it seems like you could be short on power given the conditions.

      Solar won’t work as well When it is raining, the power output of the panels will likely be about 5% of its rated output or 1.5W. However, the CPAP may not accept this low amount of power so it could be zero. The easiest way to figure it out would be to test on a fully drained CPAP machine.

      Reply

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