I've recently put together a very simple solar charging system for the travel trailer that I and a partner have parked in the campground at the Truckee Tahoe Airport. In doing so, I was reminded of how infrequently I have to worry about DC power output and usage.

In the airplane I have a power source: the alternator on the engine. Odds are that it'll keep running for a while, since it was recently replaced after suffering a sudden death in flight.

In the glider I have dual 12 volt 12 amp hour batteries, which will last quite a long time if I leave the transponder turned off. All the other instruments are very lower power. But even with the transponder on, I've got enough juice for any flight I decide to make.

But the camper has no way to generate power. It does have a newly installed deep cycle marine battery. It's a 90 amp hour 12 volt model. So how big of a solar panel did I need to get?

It depends, of course. It depends how much it'll be depleted when we use it and how quickly we'd like it to be recharged.

In order to figure that out, you need to grok the relationship between volts, amps, and watts. Solar panels are typically marketed based on their peak power production, measured in watts. The 12 volt batteries are often measured in terms of "amp hours."

The magic formula is:

Watts = Volts x Amps

So if we make a bunch of simplifying assumptions, the 12 volt, 90 amp hour battery requires 1,080 watts (or watt-hours, to be correct) of solar power to fully charge. The 85 watt Shell SQ-85P panel we bought could do the job in 12 hours, assuming optimal light and various other things that are never all true in the real world.

But if you assume 3 hours of good light per day (that's very conservative in the Sierra Nevada mountains), the battery will be back to normal in 4 days. So even if we were to run it down pretty far over a 3 day weekend and then come back the next Friday night, all would be well.

This concludes your physics/electricity lesson for the day.

I'll document the charging system in more detail if anyone is actually interested.

Posted by jzawodn at August 24, 2006 08:52 PM

Reader Comments
# John Abbott said:

Hey are you Jeremy Zawodny the MySQL guy?

Yes, I am interested in your charging system. I am thinking of putting together a solar system to run a furnace for a week in case of a blackout. I can sit in the dark if I am warm but cold and dark would suck. So, anything you could put up would be great!

on August 24, 2006 09:27 PM
# eric said:

Units Units Units! You're close, but not quite there.

Volts x amp hours = watt hours

I had to get up close with all this recently charging a Miata battery that had been left too long. In my case, it was about 48 hours at .7-1 amp off of a 15V nominal wall source. But ymmv.

on August 24, 2006 09:54 PM
# Jon Woolley said:

Who needs solar, when you have a Honda generator? Always works, and runs forever on a gallon of gas.

on August 24, 2006 09:57 PM
# Jeremy Zawodny said:


MySQL? Guilty as charged. (Oh, bad pun!)


Yeah, yeah... good point. Details matter. :-)

on August 24, 2006 10:04 PM
# said:

For those who learned all their electrical physics in metric, that formula is known well.

Electrity is often sold in kWh or kilo-watt-hours - which is directly convertable to joules (1 kWh = 3600 J)... Joules is another metric measure of energy used. It compares with BTU (which natural gas is often sold in) and horsepower (Watt is a unit of the same measurement)

on August 24, 2006 11:33 PM
# Owen Byrne said:

Well since you said above that "details matter," I have to complain about the use of "Ohm's Law" in the title. The equation you used is related to it, but isn't actually it. Ohm's law is V = I R (R = Resistance). Don't see any mention of resistance in your posting.

on August 24, 2006 11:59 PM
# Jeremy Zawodny said:

Owen: when I learned about all this stuff in school, it began with Ohm's Law and grew from there. I thought about saying "electrical learning nostalgia" but that seemed to be a bit... long. :-)

on August 25, 2006 07:10 AM
# Jeremy Zawodny said:

On second thought, I'm chaning that. It shortens the title and doesn't lose anything.

on August 25, 2006 07:11 AM
# Alex said:

I'd also be very interested in your charging system.

on August 25, 2006 07:36 AM
# John Montgomery said:

Owen's on to something. Amps * Volts only = Watts on simple resistive loads (space heaters, light bulbs). Really it yields a unit called a volt-amp which is like a watt but with no resistance. Resistance is, after all, futile. Anyway, it gets important when you're figuring out the load you're going to put onto a power source; a space heater behaves differently from the power supply in your PC (a variable load). I recently ran a subpanel for an upstairs addition and had to revisit my high-school physics in order to figure out how many amps needed to be on the subpanel and was reminded of this.

But in general, amps * volts == watts.

on August 25, 2006 09:38 AM
# Marc said:

Luckily, this is DC. With AC, you can get into mumbo jumbo with phase and RMS.

I think I'm going to listen to some AC/DC. I'm on a highway to hell...

on August 25, 2006 10:32 AM
# Dirk Spiers said:

Also interested in the charging scheme

on August 26, 2006 04:22 AM
# Yaniv Golan said:

Or, instead, you could just plan your trip so that most of it is done on piezo-pads-paved roads:


on August 27, 2006 08:14 AM
# Cliff Winsor said:

I am very interested in your charging system for my own very similar camper application.

I would love to use my generator ... but it makes it hard to enjoy the pristine outdoors of the wilds with all that racket! Besides, I don't know what magic fuel Jon Woolley is using, but my generator definitely DOES NOT run forever on a gallon of gas!

on August 28, 2006 09:13 AM
# Peacocker Marc said:

I heard of solar panels that can be put on boats, Rvs etc... but the ones i find only charge at a trickle....in milAmps..
Im looking for something to put on a bassboat that will help maintain charge on the electric motor battery, 12v 100amps...
doesnt have to charge it, but at least keep charging during the day..with the ones i've found, at 12v with 185mil amps.. it wouldnt be needed... to slow,,

on September 12, 2006 11:51 AM
# Jeremy Zawodny said:

You're talking about a very big panel!

on September 12, 2006 12:10 PM
# Randy said:

"(1 kWh = 3600 J)"

A joule is one watt second, so 3600 joules is a watt hour. A kilowatt hour is therefore 3,600,000 joules, or 3,600 kJ, not 3,600J

Just being pedantic...

on October 18, 2006 02:44 PM
# Veta Aquino said:

I am very interested in your charging system Jeremy for a larger applicationat home. Thanks

on January 21, 2007 02:51 PM
# said:

Got a 14 watt panel and a 120 amp hour battery.the area is kenya in east africa.Nairobi the city under the sun.i have a 500watt sinar inverter charger that runs a 32" 152 watt samsung lcd for an hour a day. and two 12 volt 11watts energy saver lamps that run for 3 hrs a day.how many solar panels do i need as i figured out the battery would be full in 32 days with the panel i have.all advice appreciated the power grid is not very reliable but improving. Ps usually have to recharge the battery from the mains every 4 days.

on March 16, 2007 08:19 AM
# Rene Mark Fonda said:

I have a 27 foot trailer on a remote site and what you have and how it set up sounds loke just the thing.
My air cond. is a 30 amp system. Have a micro wave, 10 cu. ft,. refridge/freezer, standard lighting trailer scenario, 6 gal hot water heater and a stereo system.

Please advise.

on September 22, 2008 11:52 AM
# Peter Dorian said:

Rene Mark Fonda 9/22/08 comment is the same as mine.
Please reply how many Watts solar panels needed.

We are looking at Northern Tool Sunforce 60 Watt Kit[pp.29]
4/15 Watt ABS framed amorphous Solar panels,1 7 Amp charge controller, 200 Watt inverter, DC plug, battery clamps, ABS adjustable 4 panel frame and all 12 v line connections.

Each panel is 13" x 38" x 3/4" which is good on the RV roof
if we secure it to the luggage rack with zip ties OR
install the 4 panels on the canopy of the golf cart to be able to reposition for best sunlight.

The golf cart has 6/new 6.8 volt deep cycle Trojan batteries=48 volts for storage.

My idea is to use these batteries for solar energy storage
for our 27ft RV for my wife and myself.

How do I go about connecting the 6 6 volt batteries to the RV 30 Amp power line? I do have a female 30 Amp pig tail in my tool box that could be connected to a larger inverter?

Did I explain myself enough? Will 60 watts be OK?

Many Thanks, I HOPE;


on October 17, 2008 10:00 AM
# Tamara said:

Hi Jeremy,

How many panels and what watts and voltage should they be in order for me to get a 90 amp output?

Thanks for yuor help,

on March 16, 2009 06:38 AM
# SkyWatcher said:

Hi folks, there is a method that raises the charging efficiency of solar cells as well as other charging sources and its called " Slycell ". the method breaks up the solar panel into segments so say we have a 18 volt panel, we tap into the panel to give us 6 seperate voltage sources of 3 volts and then we use each 3 volt solar cell grouping to charge individual 2v lead acid cells or any other cells. This is a quote from inventor " The internal resistance of each of the charging sources is matched to the internal resistance of each battery cell. The configuration provides substantially faster charging of the battery cells than is possible using convential battery cell array charging configurations". In the article the data showed a solar panel that would normally take 6 hours to charge the battery takes less than 1 hour under the same solar conditions using the Slycell charging method.

on March 17, 2009 05:09 PM
# SkyWatcher said:

Just an addition the name given to this invention is actually called 'SYLCELL' and not slycell and the original inventor is Daniel Wallace and the patent is US Patent #4,651,080. sorry about that.

on March 18, 2009 12:52 AM
# said:

Hello :

Please consider adding solar charging kits for Marine, (RV) Recreation Vehicles, (LSV) Low Speed Electric Vehicles,(NEV) Neighborhood Electric Vehicles , and Golf Carts to your inventory & product list.

New developments in low voltage controller technology have made solar charging systems for golf carts , (LSV) & (NEV) street legal vehicles both very efficient and cost effective. And with the 30% solar tax credit and upcoming plug-in electric vehicle tax credit for (LSV) & (NEV's), these systems and vehicles are attractive investments.

SunGo Power Systems manufactures the most powerful and lowest cost (per watt) charging systems on the market. We offer 50, 90 and 187 watt charging systems and our controllers feature the new maximum power point tracking technology (MPPT). We also offer a solar powered (LSV) & (NEV) kits that fall under both the solar tax credit & the plug in electric vehicle tax credit.

Our dealer pricing is as follows (we only sell wholesale): the SG-50 is $595 , the SG-90 is $795 and the SG-187 is $1,650 (Dealer Cost Per Unit), and we offer discounts for quantity orders (call for pricing.) MSRP is $995, $1,095 and $2,195 respectively. We have mounting kits for all common golf carts sold by the big three and many other manufacturers, and kits for most (LSV) & (NEVs).

SunGo guarantees its products with a 30 day satisfaction guarantee & 2 year end user warranty. If you or your customers are not completely satisfied with your SunGo product, return it and we'll give you a full refund for the unit and shipping costs.

Please call or write if you would like more information about becoming a SunGo dealer.

Best Regards,

Seann Connolly


(352) 753-7796

on April 17, 2009 06:04 PM
# Saul said:

I am putting a off grid solar/wind system together for my house and I am a little confused of weather I am maximizing the components that I need, Especially the battery.

2 wind generators/24volt
10 100 watt solar panels / 24v

I am thinking 24 volt battery 20hr and?
We use 3000kwh a month

pure sign 3000watt is it big enough?


on April 29, 2009 05:29 AM
# Dev Jadhav said:

hi, i m interested to know a 36 cell panel will produce how much amount of electricity per hour under the intense sun

on June 5, 2009 03:35 AM
# said:

Newbie question here...
I want to set up a system that will run my pool filter off of a battery that I charge w/ a PV system. During the summer I run my pool filter for 8 hours a night (from mid night to 8 am) and in the winter I cut that down to 4 hours. How do I figure out how much of a battery I need for this? I figure once I know how much energy it is burning per night I can then figure out how many cells I will need to charge it.

on February 8, 2010 01:51 PM
# said:

Hey all
I am still alive

on May 17, 2010 07:51 AM
# jill said:

good day!how many wattage of solar panel i need in 10kwh a day in used.i want atleast 8-10 hours reserve power from battery bank.10000 watts of inverter is enough?how many batteries i need for this solar power system?please help me.

on June 29, 2010 07:10 AM
# said:

I would like to build a solar system for my 14'by 70' mobile home water heater. The water heater I am installing would be 40 gallon with 3500 watt elements. How many volts would I multiply to amperes/hour to equal 3500 watts? How many panels or how many cells/panel would I have to have to equal that amount?

on July 5, 2010 06:47 PM
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