BU-413: Charging with Solar, Turbine

Learn about charging your batteries from renewable resources and what it costs.

Can I charge cellular phones, laptops or power tools with solar power or a wind turbine? Absolutely. This is an exciting way to step outside the four walls and use renewable energy. The sun provides peak power of about 1,000 watts per square meter (93W/sq.ft.) and a solar panel transforms this power into roughly 130W per square meter (12W/sq.ft.). These conditions correspond to a clear day with the solar panel facing the sun that is 42° or higher above the horizon. Surface dust on the solar panels and high heat reduce the overall efficiency.

Commercial photovoltaic systems are 10 to 20 percent efficient. Of these, the flexible panels are only in the 10 percent range and the solid panels are about 20 percent efficient. Multi-junction cell technologies are being tested that achieves efficiencies of 40 percent and higher.

A solar cell produces an open circuit voltage of 0.50–0.65V. Like batteries, solar cells can be connected in series and parallel to achieve higher voltages and currents (See BU-302: Series and Parallel Battery Configurations)

At 25°C (77°F), a high quality monocrystalline silicon solar panel produces about 0.60V open-circuit (VOC). The surface temperature in full sunlight will likely rise to 45°C (113°F), reducing the open-circuit voltage to 0.55 V per cell due to lower efficiency. Solar cells become more efficient at low temperatures and attention is needed when charging batteries below freezing temperature. (See BU-410: Charging at High and Low Temperatures)

A solar charging system is not complete without the inverter and charge controller. Most units include the inverter and charge controller to charge 12-volt lead acid batteries. Charge controllers are also available for lithium-ion to charge 10.8V packs (3 sells in series). When acquiring a charge controller for lithium-ion, observe the voltage requirements. The standard Li-ion family has a nominal voltage of 3.6V/cell; lithium iron phosphate is 3.30V/cell nominal. Connect the correct batteries for which the charge controller is designed. Do not connect a lead acid battery to a charge controller designed for Li-ion and vise-versa. Mismatch could compromise the safety and longevity of the batteries as the charge algorithm between lead and lithium-based batteries differs.

A lower-cost charge controller only produces an output voltage when sufficient light is available. With a diminishing light source, the charge controller simply turns off and resumes when sufficient levels of light are restored. These devices cannot utilize fringe power present at dawn and dusk and this limits the low-cost charge controller to applications with ideal lighting conditions.

An advanced charge controller tracks the power by continually measuring the voltage to dynamically adjust the current. This enables maximum power transfer with available light conditions that is made possible with maximum power point tracking (MPPT). Figure 1 illustrates the voltage and current source from a solar cell with varying sunlight. The optimal power is available at the voltage knee where the dropping voltage line meets the vertical power line. MPPT find this point.

It should be noted that not all charge controllers with MPPT function equally well. Some systems are course and do not respond immediately to light changes, causing the output to fall if a shadow falls on the panel. Other systems drop off too early and do not fully utilize low light conditions.

Solar Cell Voltage and Current
Figure 1: Voltage and current from source a solar cell at varying sunlight.
MPPT finds the best power point where the vertical power line meets the dropping voltage curves. (VxA=W)

You may ask: “Why can I not simply plug a 12V solar panel directly into my laptop or mobile phone?” This should work in principle, but it is not recommended. The inverter is responsible to transform the incoming DC voltage from the solar panel or wind turbine to the correct voltage range. In bright sunlight, the voltage of a 12V solar panel can go up to 40V, and this could damage the host.

From 1998–2011, the price of commercial Photovoltaic (PV) systems has dropped by 5–7 percent annually and analysis suggest that the price-drop will continue. It now costs between $4 to $5 per watt for a typical residential solar installation delivering 5kW. Larger installations cost $3 to $4 per watt with further reductions for megawatt systems.  

Maintenance Charger

A maintenance charger is usually power by a small solar cell and provides a trickle charge on a sunny day. These devices are useful to prevent sulfation of a lead acid battery when not used for a while.

When shipping maintenance charger, look for a unit that switches from trickle charge to float charge when the battery is fully charged. A prolonged trickle charge, even at a low current, could overcharge the battery and promote internal corrosion. Float charge, correctly adjusted, only replenishes what the battery loses through self-discharge. (See also BU-403: Charging Lead Acid)

Last updated: 3/16/2015

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On February 8, 2011 at 12:30pm
P_A_P_Y wrote:

You mentionned “Solar energy can be expensive. It costs $8 to $10 in materials to generate one watt. ”
I agree but after the material is yours…. and can produce many more watt…
For instance I bought a 1 W solar recharger for $30.00 Witch is 3 times more than you mentionned (it would of been cheaper to built one ) but i’m still using it after 1 year  

I do not know the lifetime of that small solar pannel but it might last long enough to bring down the ratio $ vs Watt you are mentionning ..

On February 9, 2011 at 3:38am
PV-Solar wrote:

About the cost of (electrical-) energy: It doen not matter what the cost is, it does matter what the cost (financial and non-financial) of an alternative is. e.g. calculate the cost of the energy in the battery of your wrist watch! and still nobody would consider to use a mains charger with extension cable. (but PV solar is a good alternative)
Depending on the cost of the energy, it makes sense to spent some more money on system efficiency. Inverting the low voltage dc from a solar module to 120/230V 50/60 Hz and bringing it down to the 15V dc required for your laptop is waising money. Unfortunately there are no manufacturers yet to solve this in a more energy efficient way. (but I am working on it)
Charging batteries with pv solar requires a totally different approach compared to mains charging. Main reason is that the primary energy, the sun, is not continuously available and when, sometimes not in the required amount (cloudy days) A charging technique that collects all the energy when available, even with some damaging effect to the battery has preference over an empty battery with damage by e.g. sulphation for lead acid.
With the lowering cost and increasing efficiency of PV modules, the number of applications where solar energy is the cheapest solution is growing. The concepts these systems are based on today will not longer be the optimum. Redesign will be necessary.

On February 15, 2011 at 2:50am
discussion forums wrote:

Thanks for the effort you took to expand upon this post. I look forward to future posts.

On May 13, 2011 at 4:23pm
Tyler Sproule wrote:

You wrote: “Producing 50W from solar panels to operate a laptop continuously, or charge a 50Wh battery in two hours will cost $400 to $500.”

How do you go about calculating how long it will take for a battery to charge up given the system?

On May 13, 2011 at 4:31pm
Pierre Legault wrote:

There are little calculators for that ,
The time to charge depends of the battery capacity mAh, and the charge rate current mAh

here is an on line calculator

On May 24, 2011 at 5:23pm
Don Rose wrote:


The costs per watt of solar panels have dropped a lot. It is quite possible to find quality panels for under $2.00 per watt.

Here is a website that retails to the public:


Thanks for all the information you have provided.

On September 3, 2011 at 9:39am
Subramanian, D.V wrote:

My LENZ II vertical axis wind turbine is located in a low wind area and works in gusty winds . Its output (D.C) varies from 2 Volts to 20 volts.in lulls rising to gusty winds. most of the time it is around 9 - 10 volts. How do I charge a 12 Volt battery?
Can you give me a schematic for an advanced charge controller which you say boosts low voltage and reduces high voltage of the source ?

On September 3, 2011 at 9:50am
Subramanian, D.V wrote:

My LENZ II vertical axis wind turbine is located in a low wind area and works in gusty winds . Its output (D.C) varies from 2 Volts to 20 volts.in lulls rising to gusty winds. most of the time it is around 9 - 10 volts. How do I charge a 12 Volt battery?
Can you give me a schematic for an advanced charge controller which you say boosts low voltage and reduces high voltage of the source ?

On September 23, 2011 at 1:28pm
abdulazeez wrote:


On November 22, 2011 at 10:44pm
Scott Brooks wrote:

You estimates may be out of date but if subsidies disappear there’s no telling how that will effect costs. people neglect to think about the costs of mining and manufacturing for all this good solar stuff. That takes real power both oil and either coal or nuclear. Then there’s availability of materials, if solar was massed produced for the masses the material sources could dry up or be peaked out.



And to be competitive with fossil or nuclear it has to be manufactured in places like China sending all those alleged green jobs overseas.

On December 18, 2011 at 2:01am
amjum wrote:

So far I dont think you can replace all your electric consumption wih solar panels aloine, http://www.southlondon-roofing.co.uk

On February 19, 2012 at 9:09am
Dick Triller wrote:

If you don’t want your children to suffer from asthma, cardiovascular illnesses and or perpetuate unknown illnesses related to polluted air, then the sacrifices made whether in money or effort toward cleaner environment is a must. Whatever it costs to reduce the carbon emissions is worth it. Find a way. Iceland has three times as much energy than they can use, due to geothermal and hydro available resources. They can use their surplus energy to make hydrogen fuel, which can power many green production facilities and operations. We need to start using coal and nuclear more responsibly. That mean perfecting nuclear before we proliferate. Solar is a great idea. Send your kids to school and get them interested in finding solutions for our insatiable appetite for energy. Wind and Solar cost as much as the coke and smoke your kids will be spending their hard earned pay on, if they don’t stay busy. Education, sports, arts and music will inspire creativity and maybe even spawn a Newton or Einstein solution to our energy needs. Creativity and conservation is a great idea. I like solar, I like wind and I love waves. Surf’s up!

On May 22, 2012 at 7:08pm
Kevin wrote:

I am trying to understand ,why there is a difference in price between different deep cycle batteries.If the reserve capacity ,and amp hrs are the same is there a difference in construction,that would mean the more enxpensive would last longer.

On October 2, 2012 at 4:10am
Gabriel wrote:

I have a 1250va inverter rated 24V. Currently I also have 2 200amps battery. Can I connect the batteries in parallel for longer runtime?

On November 10, 2012 at 12:46pm
Beverley Cummins wrote:

I have a solar system at my cabin now. Can I hook up wind power to the same system without having to buy new deep cells and a new converter?

On January 3, 2013 at 7:06pm
marc wrote:

what about flywheels? A while back (10-15yrs) in popular mechanics I read of a frictionless flywheel that worked by magnetism. They built a large one which would store all of the energy into mechanical energy this huge heavy metal wheel could run for days without any energy input supplying a steady rate of energy to the house also resolving the problem of surges and such as all devices charge the flywheel any excess energy generated by flywheel would be used to charge batteries for lulls in sun or wind power again battery power use would be minimal as all that would be required would be to maintain wheel to optimum speed.I wonder if that company has continued to develop this idea if so they must be pretty far ahead of by now.was looking int o chargers but came across this thread I have always been interested in alternate independent off grid systems I think we need to think in combination with other types of technologies to make it all work better.

On January 18, 2013 at 2:37am
BigBob wrote:

Marc- Have a look down at the bottom right area of your alpha keyboard: you should spot some punctuation keys. I ran out of breath reading your long sentence.

On February 10, 2013 at 4:59am
Theorw wrote:

While the article is very informative i have a few complains.Why use an inverter and then plug the ac power supply of the laptop/phone when they both need dc current?Instead a step up or step down DC/DC converter can be used to output the desired voltage.I recently made a portable charger that can get 2v+ and output 5 volts close to 1 amp which is more than enough for a mobile phone to charge in less than one hour.Input can be a photovoltaic cell with 3volts/1,5-2 amp that can be easily made with parts from ebay with less than 15 euros.Or even make a 6v configuration and feed the phone directly.
The same can be upscaled for laptops with a 14volt/4amp configuration and feed the laptop directly on DC.
The inverter would only add cost and reduce efficiency if its only portable applications we are talking about. smile

On February 14, 2013 at 11:47am
Theorw wrote:

@ Beverley Cummins
No reason why not, but if you dont get more cells, where will the extra power get stored? raspberry
You could either turn off some panels or increase your consumption.And by converter do you mean inverter?If thats the case then it would depend on the power rating of the inverter.If your current setup of solar panels produces less power than the inverter rated power then you could add the remaining power as wind power.But if you plan to add more you will need a bigger inverter.

On November 1, 2013 at 12:53pm
Jane wrote:

While installing a home solar system can be expensive, there are also lots of incentives provided by federal and state governments that can make it much more affordable. For example, in New York the incentives can cover about 70% of the cost of a system. That would make the payback for the average residential PV system about 5-6 years, and since a system is estimated to last about 30 years, you would essentially be getting 25 years of free energy from the sun.

On December 28, 2013 at 1:26pm
RHG wrote:

I agree with Theorw that an inverter is not always necessary or desirable. A lot of appliances can be run directly off 12V DC. You can buy a universal laptop step up/step down DC/DC converter from Maplin for less than £20. Any ‘in-car’ 12V cable or even USB can be used from a battery or charge controller. You can buy a pocket-sized Xpower 75W inverter for less than £20 as well. It all depends on your power needs. Ben Law’s woodland house featured on Grand Designs was 100% 12V solar and wind powered.

On May 31, 2014 at 6:49pm
Ferdie wrote:


We are planning to put up a 1MW system in the poor provinces here in the Philippines to help out the under privilege folks.  Based on calculation, we will need 668 pcs. 250W solar panels and 209pcs.  24V 200AH Batteries.  Im still not sure how many charge controllers and Inverters we need to get started.  We prefer the MPPT type controllers and the best Inverters.  Can you help?

On September 4, 2014 at 10:16am
Mark wrote:

I have three type-24 12-volt lead-acid batteries in my RV that I would like to leave unattended for 10 months without worrying about boiling or freezing the electrolyte. Two are in parallel for the RV loads, and one is the engine battery.  I’ve disconnected them from all loads, so I have no parasitic discharge, just the internal self-discharge.

Assuming a 70 AH battery and a 5% self-discharge per month, I calculate that each battery’s self-discharge is equivalent to a 5 mA load (70 amp-hours x 0.05 per month / 720 hours/month = 4.86 mA).

For $60 I can get a 5-watt panel with included 3-stage Battery Tender, or for $15 I can get a 2-watt panel without charge controller (Sunsource says the controller is not needed on such a small panel).  Opinions?

Seems like 2 watts is more than enough to overcome self-discharge and is small enough to not be boiling electrolyte.  And should I use separate panels for the two systems, or just parallel all three batteries despite their being different ages and manufacturers?


On October 19, 2014 at 11:15am
Basat wrote:

what is the relation between the use of different solar panel with charging batteries ?
Can we use lower rated solar panels to charge batteries ?Will it require more charging time ?

On October 23, 2014 at 1:38pm
albino salazar wrote:

I am working on a project that uses a DC generator to charge a 12 volt 8 amp battery.  Can you give me information of the criteria on the size of generator that is should use and also the type of charge controller to use.

On November 18, 2014 at 12:45pm
Bonez wrote:

what is a charge controller and where can i buy one

On November 26, 2014 at 4:26pm
Cadex Electronics Inc. wrote:

Article Updated

On December 29, 2014 at 6:50am
Ernie Foster wrote:

Hi, I have a TPS-102 2.4W a@ 17V solar panel. I want to use it to keep a 12v battery charged to use with a small heating fan. My intention is to make a greenhouse heater, with a thermostat and timer (to go for a few hours at night or when it dips below a certain temp, charge permitting) and combine all the components in one box. I gather it is not good to charge at the same time as using battery to power a fan heater, will I have to work out some kind of automatic switching to alternat between charging and usage ?  Really excited about attempting this project but am feeling rather out of my depth. Thanks for any advice. Ernie

On January 24, 2015 at 2:11pm
simon hamilumbe wrote:

I am struggling to make my solar system work. But batteries are dying fast. Details of my Solar system.
1. 2 Panels 200 Watts each.
2. 1 mppt charger Controller.
3. 8 Willard Batteries 12 Volts and 105Ah. (Lead Acid)
4. 1 12Volts Inverter.
The batteries drain fast.I use the system for the following.
1. Lighting 10 bulbs 8 Watts.
2. TV 197 Watts.

Now can i use the charger to charge these batteries as they are being used to avoid draining and leaving me in darkness. I want to be using the Industrial Charger. the Charger will be connected to the power the Solar system is generating.
please i need power in my house and watching TV at my free time.

Thank you in advance for your help.

On January 25, 2015 at 12:46pm
Telmo M. wrote:

@simon hamilumbe
Hi Simon!

I think you bought the wrong batteries. It seems to me that those “Willard” batteries are starter batteries. You should have choosen Deep Cycle Lead Acid batteries.
The Starter Batteries are meant for be used in cars, tractors (...). If you cycle them, they will wear out in 10 cycles or so.

Deep Cycle batteries are physically different inside. They are meant for use in systems like yours, which will discharge and charge them lots of times.
You can read more about these two types of batteries in this site - “BU-201: Lead-based Batteries”.

I hope I could help!