just to start off!

[sundawg]

Hi all i have been lurking for a bit and think i get it then zing.....i lose it all!.

What i want to do is have a portable system for camping, run a fan maybe a laptop, charge cell and camera batteries. Not all at same time.

I have the portable suitcase solar panels from harbour freight, 18 -24 volt output 400 ma max. with blocking diode. Also a portable jump start system that has...Rechargable 17 amp battery, 12v dc out socket, automatic cut off to prevent over charging, overload protection.
It can be charged 120v ac or 12v dc. Sooooo the question is will this work? I want to plug in an dc/ac inverter to run stuff, (AS per above)Is there something else i should have? something i need to know? Would really hate to fry my laptop......Thanks for any info!

[icarus]

Beware cheap modified sine wave inverters. Some devices get fried using them!

BTW 400mamp is almost nothing. Probably enough to keep a cellphone or other small battery topped up. Calculating charge efficiencies etc and it will leave you with little useful power for fans, laptops etc. (A typical modern lap top will burn ~ 60 watts. ~5 amps 12vdc)

Tony

Ps I'm sure Bill will chime in with some real numbers

[BB.]

Tony,

You are doing fine--Moderation is such hard work--wearing my poor fingers to the bone. :p

-Bill

[sundawg]

Beware cheap modified sine wave inverters. Some devices get fried using them!

BTW 400mamp is almost nothing. Probably enough to keep a cellphone or other small battery topped up. Calculating charge efficiencies etc and it will leave you with little useful power for fans, laptops etc. (A typical modern lap top will burn ~ 60 watts. ~5 amps 12vdc)

Tony

Ps I'm sure Bill will chime in with some real numbers

It is supposed to be 13 watts, is it safe to plug right in to the panel and run things? from what i gather NOT! Any recommendations on what i can use? Budget price and portable?

[BB.]

Safe--technically yes.

Will it damage your accessories... 80% chance of no... 20% chance of yes...

Because: Normal "12 volt car accessories" are designed to run on a running car (or parked) around 12-14 volts DC... A properly operating wet cell deep cycle battery can be charged (equalized) upwards of 15-15.5 volts--high enough to kill a "12 volt" adapter. Not all will be killed--but it is is certainly possible (and has been reported here over the years).

On the 120 VAC side--a MSW (modified sine/square wave) inverter will run 80% of things out there just fine... Some electronics (especially lower cost . massed produced items) and some motor driven items (like fridge) can be damaged by the square wave output of the inexpensive inverters. Also some receivers (TV, Radios) may have excessive noise/hash from the noisy inverter. The 10% (or less) of appliances that will be damaged by a MSW inverter--there is really no way to tell. Also, for inductive/motor based loads, they may draw 20-25% more power (wasted as heat) because of the MSW wave form.

You can get "true sine" wave inverters... They are much more expensive, but are probably worth the price if this will be for emergency (when things cannot fail) or for long term use (when you don't want to worry about failures).

(full disclosure, I have had a few MSW inverters over the years for powering a light, radio, or electric drill from the truck, and they have all worked just fine--I am too cheap to get the True Sine at this time).

The last issue with a small portable solar system is that they just do not deliver much power... When all is said and done, the losses and such in a battery / inverter off-grid backup system, you get, very roughly about 2 hours * panel rating of useful power per day (assuming 4-5 hours of sun for charging).

So, 13 watts * 2 hours = approximatly 26 watt hours per day... To power a cell charger and a few LED lights--not bad.

Or a 13 Watt CFL for 2 hours, or a 30-60 watt laptop for 30-60 minutes (really, much less because of the small battery).

And, regarding the battery--typically, they are pretty efficient around the 20 Hour rate (17 AMP*Hour / 20 hours = 0.85 amps at 12 volts for 20 hours... And since standard deep cycle batteries don't last very long if discharged more than 50%--then you are really talking about 0.85 amps for 10 hours--or ~85 watts.

And it would take your solar panel roughly ~4 days to recharge if you drained the battery by 50%.

So--in the end, systems like these do work--IF you do not expect more power out of them then they can actually supply to meet your needs.

Realize that you probably would need to replace the battery every couple of years, and that if you want long term backup/off-grid power--this will supply a some 12 volt LEDs with plenty of power for some decent lighting--and probably a small radio/AA battery/cell charger... But not much more.

Don't expect to plug a 300 watt inverter into this setup and run ~200 watt load for anything more than a few minutes (and you might kill the battery doing it).

-Bill

[sundawg]

Ok so i can understand the battery pack will power the laptop for an hour or so? Not sure what a CFL is?

"Or a 13 Watt CFL for 2 hours, or a 30-60 watt laptop for 30-60 minutes (really, much less because of the small battery)."

Could i just use a deep cycle battery? Would i charging regulator be necessary?

[icarus]

Tony,

You are doing fine--Moderation is such hard work--wearing my poor fingers to the bone. :p

-Bill

Bill,

I tend to err on the side of broad generalities, you on the other hand like to work things out to the decimal point. Both kind of useful in different ways

Either way is simple to show that a 13 watt panel won't put out much in the way of useable power. For quick numbers, I use a factor of half the wattage over 4 hour gets on close enough to estimate. Add in a factor on the number of sunny days on average for where you are and that gives a quick idea.


Tony
PS. You are doing a fine job of moderating. I moderate another site, and it could take all my time if I let it.

[icarus]

Ok so i can understand the battery pack will power the laptop for an hour or so? Not sure what a CFL is?

"Or a 13 Watt CFL for 2 hours, or a 30-60 watt laptop for 30-60 minutes (really, much less because of the small battery)."

Could i just use a deep cycle battery? Would i charging regulator be necessary?

CFL=Compact fluorecent light

Try these links to learn about batteries: http://www.rpc.com.au/products/batteries/car-deepcycle/carfaq4.htm#charge
http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries
http://www.batteryfaq.org/

Tony

[BB.]

Thank you Tony...

And I agree--getting anything closer than 10% is just a waste of digits with almost any power system.

Generally, I like the approximation that if you are with in a factor of 2x (or 0.5x) of the "answer" (or compared effect)--then you are pretty darn close.

If something is 10x (or 0.01x) difference--you can drop the smaller and only pay attention to the larger...

Sundawg,

Conservation is the keyword to operating anything on emergency/off-grid power... You can find laptops that take 60 watts to run, and you can find (or at least used to be able to find Thinkpads) that run on 8 watts... May not be the fastest PC out-there--but the ability to run a computer for 7+x longer on the same power source is nothing to sneeze at.

So--if you have a 60 watt computer--that will, realistically, kill your battery after 30 minutes or so (and require 4 days of full sun to recharge).

CFL=Compact Florescent Lamp (twisty bulbs).

Again, CFL's are another shot at conservation. Roughly a 13 watt CFL has the equivalent light of a 60 watt filament bulb.

Regarding the sizing of your battery--Very roughly, a good number to use is sizing your battery 6x your daily load... This gives you 3 days of "no sun" plus a 50% maximum discharge (for long battery life).

For example...
360 Watt*Hours=60 watt computer 6 hours
53 WH=13.25 CFL 4 hours [another example of why I should a calculator]
14 WH=7 watt battery charger 2 hours
==============================
467 Watt*Hours per day

Assume you are using an inverter (80% efficient), an 80% efficient lead acid battery, and a 90% efficient charger:

467 WH * 1/0.8 * 1/ 0.8 * 1/0.9 = 810 Watt*Hours

Battery (assume 12 volt battery, 20 Hour Rating):

6 * 810 WH / 12 VDC = 405 Amp*Hours

Those would be, roughly, the equivalent of 4-6 "car sized" batteries. Obviously, if you can cut the 60 watts of the computer down to 8 watts, you will dramatically reduce the size of your battery bank (and solar panels required to charge them-conservation).

Notice, that your solar panels need to almost be twice as large as the load to make up for all of the losses that are present in any power storage/conversion system.

For solar panels, assume 5 hours of sun (summer), you can approximate 80% of panel nameplate (panels are over-rated based on real-world experiences):

810 WH * 1/0.8 * 1/5 hours = 202 watts of solar power (to meet my imaginary loads)

This is neither a small or portable system (unless you have a handy trailer to install everything in).

I would have liked to have solar off-grid backup for my home... But, cost wise, because I have worked so hard to reduce my power usage (conservation, insulation, energy star appliances, etc.), that a Honda eu2000i (1,600 watt generator) would power my entire home (emergency loads) on ~2 gallons of fuel per day.

For less than $900 (generator) and 20 gallons of fuel (fuel stabilizer, dump in car once per year)--I have a reasonably small, inexpensive, and portable emergency power system that will last me several weeks (siphon fuel from the car--if needed).

So--my "off-grid" needs are for emergency use only (and it has been 50+ years since I have needed a generator for a power outage that lasted over several hours)...

If you are living off-grid, have need for power every weekend (or longer--like a back woods cabin)--solar certainly can make sense for those places.

Or, if you live in an area with ice storms / hurricanes / tornadoes where weeks of no power / no fuel is a yearly happening, off-grid capable solar may be worth the expense for you.

-Bill

[sundawg]

Great thanks so much for the info.. Guess i will give it a try and see if its what i need. Some tests and more reading Thanks!

[BB.]

I forgot to add--if you are gong to be using 120 VAC--get a Kill-A-Watt meter (http://store.solar-electric.com/kiacpomome.html) ($30 or less).

Absolutely the best (and cheapest) way to determine your load requirements (for 15 amp 120 VAC 60 Hz circuits) when looking at conservation or planning your off-grid/emergency power system.

-Bill

[icarus]

Just to add about CFL's

Please note that cfl's DON'T have to fit the stereotype of "twisty" "icecream cone" bulbs that put out harsh, cold light, take forever to start, don't work well in the cold.

People need to realize that cfl's come in almost every configuration from 3 watt candellabra base bulbs to +20 watt floods and spots. The come in many light colors from cool white to warm white, to full spectrum.

For too long people have been turned off because of the size, shape, and color of cheap cfl's. Even the "expensive" ones now are reasonably priced and you can't tell the difference from a conventional bulb.

If your local store doesn't seem interested in stocking different bulbs, try this link: http://www.nolico.com/saveenergy/ (interesting to visit, so you can see how many types are really out there!)

Tony

[sundawg]

Thats a great little helper! (Kill a Watt) Found that store on line, was going to stop by as its not too far away. Would Prob. go with led lighting.

Going to see about an inverter today. And camping this weekend! There is so much sun here in Az that i really think we all need to utilize it more. Unfortunately I can see it is cost prohibitive for most!

8)

[rplarry]

Sundawg
You might consider getting a small generator like the Honda eu1000i, great little generators, very fuel efficient and quiet. That an a long extension cord and your power problems would be solved. Not nearly so mluch stuff to drag around.
Hope this helps,
Larry

[sundawg]

Yikes kind of pricey! Looks like a great unit though.

[westbranch]

Sundawg, don't forget that with a little work you can get the LEDs to run on 12 v right from the battery. rather than using a 'driver' that converts 120 vac to 12 vdc. Just have to wire appropriately for the distances involved. I use MR-16 units without the transformer. Lots of fleEbay choices, start with the 1 - 3 watt versions, much cheaper than the 5 w state of the art versions. check out the frequency if you can there are a lot of different colours in the 'white ' category.

HTH ;)

Eric

[BB.]

Yes, the Honda euxxxxi family is not cheap... However, when you look at the price of fuel how much less you have to store/purchase for long term use, they look a lot better...

From thread on another forum (http://www.candlepowerforums.com/vb/showthread.php?t=207450)(don't think you have to register/login to read)...

Run a fridge freezer and a couple lights on a 4kW Generac = 12 gallons per 24 hours

Run same (~500 watts) on a Honda eu2000i = 2 gallons per 24 hours.

Difference is 10 gallons per day, or @$4/gal = $40 per day savings.

$850 (eu2000i) - $300 for cheap gen = $550 delta in price
$550/$40 per day = 13.75 days to break even.

14 days * 12 gallons per day = 168 gallons of fuel
14 days * 2 gallons per day = 28 gallons of fuel

Quiet generator running for 14 days--priceless! (reference to US Credit Card commercial).

So--get the minimum sized generator you can running the minimum loads you need. And get a quiet generator... The loud ones will drive you (and everyone around you--up a wall).

I believe that the Honda eu2000i is about the best choice for power output vs fuel consumption. You can get a second, larger genset, if you have specialized uses (like well pumping) that you just run, as needed, for short times during the day.

I would only get the eu1000i if space/weight is a big factor for you--or you are going to be running very small loads of a 100 watts or so... The eu1000i is a bit on the small size if you want to run a fridge with a few other loads. Also, I am not sure that it is any more fuel efficient with reasonably sized loads than the eu2000i.

For the eu2000i running 500 watts * 24 hours = 12 kWhrs per day (little over 2 gallons of gas per day)... My 3.5 kW worth of solar panels on grid tie generate an average of 13k Whrs per day (over 1 year)... For emergency usage--it is hard to beat the price and portability of a 50 lb generator and 30 gallons of fuel.

A couple of sources that have been recommended by many folks over the years:

www.wisesales.com (http://www.wisesales.com/)
www.mayberrys.com (bought 1 eu2000i years ago from them) (http://www.mayberrys.com/honda/generator/html/invertgenerator.htm)

As always, neither I nor NAWS have any connection with the above links... Do your own due diligence when shopping on or off-line.

-Bill

[wxh3]

Assume you are using an inverter (80% efficient), an 80% efficient lead acid battery, and a 90% efficient charger:

467 WH * 1/0.8 * 1/ 0.8 * 1/0.9 = 810 Watt*Hours

Battery (assume 12 volt battery, 20 Hour Rating):

6 * 810 WH / 12 VDC = 405 Amp*Hours

-Bill

I'm a bit confused by the battery and charger efficiency included above.

First, shouldn't the charger efficiency be included in figuring the solar requirement, not the battery usage?

Second, does the battery efficiency mean that the capacity of the battery is 20% less than the spec capacity? Isn't battery efficiency already figured into the 20aH capacity? Are you saying for example that a 100AH battery will really only provide 80AH at the 20 hour rate?

[mike95490]

Are you saying for example that a 100AH battery will really only provide 80AH at the 20 hour rate?

No, if you USE 100AH from the battery, you have to recharge with 120AH. The charging process is not 100% efficient.

Same with a charger, all the "warm" air you feel from a charger, is waste heat, from internal losses. (500W input = 450W output & 50W of heat loss)

[wxh3]

No, if you USE 100AH from the battery, you have to recharge with 120AH. The charging process is not 100% efficient.

Same with a charger, all the "warm" air you feel from a charger, is waste heat, from internal losses. (500W input = 450W output & 50W of heat loss)

So technically, charger and battery inefficiencies should be factored in sizing solar charging and not battery usage/size. (I understand BB did include this in solar sizing as well since it was computed directly from the daily battery usage.)

[westbranch]

wxh3, going back to Bills comments about the Honda genset, (note my sig line,)

there is another advantage if you go that way, it produces TRUE sine wave and there are a lot of other things you might want to power that do not 'like' the somewhat dirty power from a standard (cheapo) Briggs and ScrapIron type genset, AND it is sooooo quiet!

HTH
Eric

[BB.]

W'

Technically, you are correct--I should not have included the 90% of the solar charger efficiency in the battery sizing caculations... I did this for two reasons--one to just keep down the "extra" math... And two, +/-10% is not really going to change much of anything. So--I tend to "error" on the conservative side (batteries would be ~10% larger than you really needed).

Also, depend on the charge controller (MPPT vs PWM)--the efficiency will be different (rough, between 95% and 80%--PWM losses are a bit more complex to determine--depends on solar panel Vmp, voltage drops, and battery voltage--all of which change with temperature)... And, no charger runs 100% max current--the last 10% or so of the charging cycle is at less than 100% max current--so panel/charger efficiency drop way down (instead of 60amps--the controller may drop down to 10 amps for last part of charge/floating--assuming off-grid with batteries--grid tie is different).

Enough hand waving... Close enough for a first cut at requirements. ;)

-Bill

[sundawg]

Ok i was curious of what the solar suitcase was doing, I used a multi meter and checked the voltage coming out in full sun. It read 45vdc !!!And 27 in the shade....Can that be correct? And what can i use to regulate that as it seems to high to use for charging????

[BB.]

I could not find anything on Harbor Freight that matched your description... So it is difficult to say how your system is setup.

Some rough rules of thumbs...

Solar panels will vary in output voltage from Vmp=15-17 volts (V max-power for 12 volt battery charging, panels hot) to Voc=30-34 (open circuit--panels cold)... So, while the voltage sounds high--if your system is setup to output 24 volts--then getting 50+ volts open circuit (with cool panels) is certainly not impossible.

Solar panels themselves are not regulated output (solar panels look more like a current source than a voltage source--if you are familiar with electronics)--for stable voltage output, they use the storage battery to keep the voltage flat (battery absorbs during periods of high voltages and output during periods of low solar voltage).

The solar charge controller is used to control current flow into the battery/system so that the battery is not overcharged--but the solar charge controller itself is not designed to regulate the solar panel voltage for a stable output without an attached storage battery...

So, if you measure your "system" voltage, without a battery, using a typical DVM/Multi-Meter--What you will be measuring is pretty much the unregulated output voltage of the solar panel--and as long as the input current rating of the battery is not exceeded and the solar panel voltage is higher than the battery--everything is probably fine...

And, if there is a charge controller in the circuit--you may actually measure zero volts--because many solar chargers will not turn on unless there is a charged/working storage battery attached.

Lastly, it is also possible that your volt meter is not working correctly... Connect it to a car battery (or similar) and see if you get reasonable readings (~12.8 volts with the car off, ~14.2 volts with the car running).

-Bill

[sundawg]

This is the panel

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?function=Search

meter checks out fine.....

[mike95490]

This is the panel

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?function=Search




"Sorry, the item number is not a valid item"

[BB.]

Link is not working...

If you are just talking about a "bare panel"--then those voltages will not cause any problems for any lead acid storage battery (0.400 A x 1/13% to 1/3% recommended charging rates) of 7.7 Amp*Hours to 33 Amp*Hours (20 AH rating of battery, assuming no other charging sources). And assuming you have some sort of charge controller to prevent over-charging.

-Bill

[mike95490]

this looks close (suitcase & solar)
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95000
18 Volt to 24 Volt Solar Charger
""Only use this Charger with charging bases from Harbor Ffreight Ttools that accept
a 5.5 x 2.1 mm plug and are from 12 V – 18 V. Do not use on another brand, another voltage, or a charging base designed for a different plug.""
So, unloaded, 40V is not unreasonable. (manual says 400mA max)

Now I'm hooked on the solar umbrella !
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95453

[sundawg]

"Sorry, the item number is not a valid item"


95000 item number in search came up right away

[sundawg]

yes that is direct from panel plug, The battery pack is 17 amh and built in over charge protection. Bought the black and decker 400 watt inverter and am plugged in to battery pack right now. Will see how long it runs for, every thing seems fine so far :)

[sundawg]

Yes so now i want just the lights too.... Theroetically i could use an 18 vdc dc regulator straight from the panel then.......like this
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2306

[BB.]

If this Item can charge 24 VDC (http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95000) (say really closer to Vmp=30 volts max for charging a 24 volt lead acid battery), there is nothing wrong with connecting to a 12 lead acid battery (of at least 8 AH capacity)...

The "draw back" to a Vmp=30+ VDC panel and charging a 12 volt battery is the old electrical equations for Power:

P=V*I

P=30 volts * 0.4 amp = 12 watts
P=15 volts * 0.4 amp = 6 watts...

Basically, you are "wasting" 1/2 the output of the solar panel--unless you can "chop the wiring in half" and change it to 15 volt output for charging a 12 volt battery (the old series/parallel) question.

Without knowing the actual specifications of the panels--it is difficuilt to determine the optimum connection/voltage/battery combination (or, you can do your own experiments to figure out Vmp for the panels).

This is why a MPPT type solar charge controller is so nice... Basically, instead of "wasting" the difference in voltage*current (between 30 volts and 15 volts in my mythological example)--the MPPT controller acts sort of like a variable "DC" transformer between the panels and the voltage--the MPPT controller "maximizes" the P=V*I for the solar panel and converts that into the maximized P=V*I of the battery bank.

However, this conversion does "waste" some power--and is not practical with currently available MPPT controllers for very small solar panels (like 12 watts in this case).

My guess is that at 24 volts--the panel current starts to drop off towards zero--it probably maximizes around 17-18 volts--which is just fine for charging a 12 volt battery.

-Billl

[BB.]

Yes so now i want just the lights too.... Theroetically i could use an 18 vdc dc regulator straight from the panel then.......like this
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2306

That controller appears to have a ~17 volt limit... It is likely that it would be destroyed if connected directly to your solar panel... However, it should work just fine for connecting to a 12 volt battery which is connected to your solar panel.

-Bill

[sundawg]

If this Item can charge 24 VDC (http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95000) (say really closer to Vmp=30 volts max for charging a 24 volt lead acid battery), there is nothing wrong with connecting to a 12 lead acid battery (of at least 8 AH capacity)...

The "draw back" to a Vmp=30+ VDC panel and charging a 12 volt battery is the old electrical equations for Power:

P=V*I

P=30 volts * 0.4 amp = 12 watts
P=15 volts * 0.4 amp = 6 watts...

Basically, you are "wasting" 1/2 the output of the solar panel--unless you can "chop the wiring in half" and change it to 15 volt output for charging a 12 volt battery (the old series/parallel) question.

Without knowing the actual specifications of the panels--it is difficuilt to determine the optimum connection/voltage/battery combination (or, you can do your own experiments to figure out Vmp for the panels).

This is why a MPPT type solar charge controller is so nice... Basically, instead of "wasting" the difference in voltage*current (between 30 volts and 15 volts in my mythological example)--the MPPT controller acts sort of like a variable "DC" transformer between the panels and the voltage--the MPPT controller "maximizes" the P=V*I for the solar panel and converts that into the maximized P=V*I of the battery bank.

However, this conversion does "waste" some power--and is not practical with currently available MPPT controllers for very small solar panels (like 12 watts in this case).

My guess is that at 24 volts--the panel current starts to drop off towards zero--it probably maximizes around 17-18 volts--which is just fine for charging a 12 volt battery.

-Billl

what type of specifics are required on the panel.....i did look at the MPPT but they dont seem practical for this small a set up....Also expensive...

[BB.]

The best "small" MPPT controller is going to be this one (lots of discussions elsewhere here):

Morningstar SunSaver MPPT Solar Charge Controller (http://store.solar-electric.com/mosumpsochco.html)

It is a 15 amp (output to battery bank) controller that can handle ~200 watts of "12 volt" and ~400 watts max of "24 volt" panels...

Just to clarify the above statement (thanks Niel!): The SunSaver's ability to handle 200 or 400 watts is based on the voltage of the battery bank ("12" or "24" volt battery bank at 15 amps). The input panel voltage, as long as it is (roughly) 2 volts over the battery bank voltage and less than the controller's maximum input voltage, does not affect the charge controller's maximum solar panel wattage.

Still way too large and expensive for your needs. Staying with a simple PWM controller (or equivalent analog) (http://store.solar-electric.com/mochco.html) will be just fine.

Regarding your panels--knowing the Vmp (voltage, maximum power), Imp (current maximum power) and Voc (voltage open circuit), is the basics required to figure out the battery and charge controllers needed.

It appears, that they are also plugging these into electric drill chargers and such--just substituting for the "12-18 volt" wall wart (fixed from "wort" which is a beer brewing term--thanks to Mike) transformers (or DC wall worts)... Should be just fine as Harbor Freight has confirmed their tools (probably would work with other similar brands of tools/battery charger sets--but who knows and wants to donate their equipment to test?).

-Bill

[BB.]

Added clarification note that SunSaver (and most/all? MPPT charge controllers) are limited by output current to the battery bank in terms of maximum panel wattage. If you double the battery bank voltage, the maximum wattage will double (assuming that the input solar panel voltage is high enough to charge the batteries, and that the panel/battery ratings are within the controller's ratings).

Thanks Niel.

-Bill

PS: Thanks to Mike for reminding me that the correct term for Wall Wort is actually Wall Wart transformer...

[icarus]

The Bluesky series of Mppt controlers have a good reputation as far as I know. I am very happy with mine. http://store.solar-electric.com/sbchco2512vm.html

Tony

[sundawg]

Just to let you all know the battery pack lasted 3 hrs running my laptop. it was discharged to 30% when the inverter started to whine. That was from a 100 % full charge. Not to bad.