# General trouble with calculations

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As a newbie, I'm having a little trouble with some math... Was hoping someone here could help with some basic questions.

Example:

1 Compact flourecent 10W, 140mA
Deep Cycle 105 Amp Hours
350watt DC to AC inverter
2 15watt PV panels and a 7amp charge controller.
estimated 2 good hours of sun per day.

What other variables if any do you have to take into account in order to answer the following questions.

1) How long can you run this 120v CF while maintaining 80% reserve on the battery. If that 80% recommendation even applies to an off the shelf marine deep cycle.

2) Consumption of amps per hour

3) Charge rate? i.e. if battery is 10% depleated, how many days of charging are necessary with 2 hours of sun on the dual 15 watt pannels?

4) Is there substantial loss with the inverter and how is that calculated?

5) are the figures basically halfed if a second CF of the same wattage is added to the example?

I'm sure I am missing some key pieces of info but basically I am trying to come up with a graph or calculation that says with 1 CF bulb (10W 140mA) running for 1 hour, < X > amount of amps have been consumed and < Y > amount of time is required with < N > number of 15watt panels in order to get the battery back to starting level.

Are there specific technical numbers that I'm missing from this?

Any help is greatly appreciated. We are just starting out and being able to understand the above example would help us out greatly.

Thank You
Dave

• Solar Expert Posts: 5,183 ✭✭✭✭
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Re: General trouble with calculations

Dave, there is a bit more info needed for the board members to give you good feedback:

1. It sounds like you are using all 12 v equipment...right?
2. what are the specs of the panels?
3.what brand and model of charge controller are you using? Is it MPPT ?

As for your graph/spreadsheet, for a starter you need to use the formulas
Watts = volts * amps
ie 120 w = 12 v * 10 amps or 120 v * 1 amp

and Amps = watts / volts

these should get you started on your calculations.

cheers,
Eric

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
• Solar Expert Posts: 1,164 ✭✭
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Re: General trouble with calculations

The bottom line with what you have listed is that you most likely won't be able to run anything at all.

The major reason is that the inverter is using up more power than your panels will produce all by itself.

On the other hand, if you used a 12 volt CFL and ran directly from the battery you could get about 5 hours.
• Posts: 6,290 admin
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Re: General trouble with calculations

Just making a couple of quick assumptions about your panels, inverter etc, I wouldn't be at all surprised if this set up will require at least one hour of full sun, for every hour the CF is on. That also assumes the inverter will be off when the CF is not in use. By off, I mean actually disconnected from the battery, as some, or most with an on/off switch continue to offer a small drain on the battery even when turned "off". Your system is relatively small, so every wee bit of drain will have a big impact.
This is not an exact answer, just a rough ball park figure of where you are with your system, giving you an idea of where you go from here.
Cheers and all the best.
Wayne
PS: Windsun posted while I was building my post, so got ahead of me. I would go with what Windsun is saying.
• Posts: 6,290 admin
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Re: General trouble with calculations

I've got some addition info/specs that may or may not help.

The inverter is vector 300watt 700peak similar to this one

http://www.safehomeproducts.com/shp2/sc/shopexd.asp?id=158537

however its an older model and has slightly less output.

If I understand the replies previously posted, then am I to assume when this inverter is ON and is powering a 10w CF at 120V its really drawing its full rated output, even though a 120V CF draws way under 300watts? When I have used it in the past, I do disconnect the inverter completely from the battery when the light is not on because I figured that there was at least some draw on the battery even with the light off.

Charge controller is from Northern Industrial tools that came with the panel. Nothing fancy and not mppt.

http://www2.northerntool.com/product/200265524.htm

Panels are also marketed under northern tool and are each 15W
http://www.amazon.com/Northern-Industrial-Tools-Wattage-Panels/dp/B000K2HNZS/ref=pd_sim_auto_shvl_title_2/103-9334666-4347865

Battery is a 12V marine deep cycle with a label of 105 amp hours

I'm also confussed by windsun's reply. Lets assume for a minute that If I went with a 12v CF instead of this inverter/120V AC setup, you mentioned that I'd get about 5 hours?

windsun>>>On the other hand, if you used a 12 volt CFL and ran directly from the battery you could get about 5 hours.

Is that 5 hour figure you mention based on going from 100% charge to 80% charge on the battery? Seems like a VERY small number given such a small draw and a large marine battery.

Maybe I have completely missed the mark here... I was really striving for was a very small (portable) setup that would run at least (1) 120V CF for an estimated 12-24 hours per weekend, and be able to charge the deep cycle during the 5 week days. At the time when I ordered the PV panels, I could only afford two 15W so that was a complete guess on my part.... but I may have totally misunderstood the inverter piece of the puzzle. In my own mind, a 10W CF was going to be such a small draw that I could afford to have some efficiency loss on the inverter. I did NOT think that a 300W inverter would draw its entire 300W if it was only powering a 10W light. My other assumption which also may have been WAY OFF was that even with some efficiency loss by going from 12V DC to 120V AC was that a large marine battery (designed for a trolling motor) would be MORE than enough to power a 10W CF light for an amazing amount of time.
• Solar Expert Posts: 9,583 ✭✭✭✭✭
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Re: General trouble with calculations

(apparently, my original post yesterday never completed loading, but I see my comments were reflected in other replies - you need an hour of sun for every hour of light use, you have very tiny panels)

You can get some higher efficiency 12VDC CFL's here: http://stores.ebay.com/Glenergy-Solar-Lighting-and-Power I have used them in the past.

You could also eliminate the charge controller, get a refund, and add another solar panel. The 2 small ones you have will not supply much actual power, and stand no chance of damaging a large deep cycle battery, as long as you add water once a month.. Their full current is less than normal trickle charge. 15 watts @ 12v is only 1.2A It's unlikely you will ever see full output from the panels.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 264 ✭✭✭✭✭✭
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Re: General trouble with calculations
1 Compact flourecent 10W, 140mA
Deep Cycle 105 Amp Hours
350watt DC to AC inverter
2 15watt PV panels and a 7amp charge controller.
estimated 2 good hours of sun per day.
...
1) How long can you run this 120v CF while maintaining 80% reserve on the battery. If that 80% recommendation even applies to an off the shelf marine deep cycle.
First your energy budget:
* Input : 2 * 15W * 2 hours sun/day = 60 Watthours/day
.............20% * 12V * 105AH = 252 Watthours.
* Efficiency: without MPPT, say 80%. Your inverter is modified sine, efficiency around 90%, battery charge/discharge efficiency 90%. So, overall, say 60%.
* So, your total energy input is : 252 + 60*0.6 = 288 Watthours.
* Output 10W.
How long: 288 WattHours / 10W = 28.8 hours.
2) Consumption of amps per hour
10W / 12V /90% = 0.92A
So, it would consumes 0.92 AH per hour.
3) Charge rate? i.e. if battery is 10% depleated, how many days of charging are necessary with 2 hours of sun on the dual 15 watt pannels?
10% * 12V * 105AH = 126 WattHours
How long : 126 WattHours / (60*60% eff.) = 3.5 days
4) Is there substantial loss with the inverter and how is that calculated?
With modified sine-wave inverters, efficiency usually above 90%. Your old unit is probably around 90%, recent units can get up to 95%.
5) are the figures basically halfed if a second CF of the same wattage is added to the example?
Approximately, yes.
• Solar Expert Posts: 253 ✭✭
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Re: General trouble with calculations

If the following article is to be believed

http://www.fieldlines.com/story/2004/10/2/145353/321

then the efficiency of an inexpensive MSW inverter is low at the extreme low end of its power curve. I'm sure that's what the others were thinking instinctively when they recommended using 12V DC lights and skipping the inverter.
• Solar Expert Posts: 264 ✭✭✭✭✭✭
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Re: General trouble with calculations

Right, efficiency reduced with much lighter load. Apply 0.64/0.9=0.71 adjustment factor for the above figures then.
28.8 hrs --> 20 hrs
3.5 days --> 4.9 days

might still fit the bill with existing equipments, or use a 12V CFL as suggested or pick a 50W car lighter plug inverter to be more efficient then.

GP
• Posts: 6,290 admin
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Re: General trouble with calculations

Thank you very much to everyone who has posted or will post. These calculations do help. When thinking up this system, instead of using very accurate numbers, I rounded everything with what I assumed was a large factor for error...

I rounded the .140mA light up to a full amp per hour figuring that was enough to cover any loss in the DC to AC conversion.

12V x 105ah = 1260 watt hours. In order to keep the battery at 80% or above the maximum I should use is 20% of that total, which is 252 watt hours.

So, using that calculation I've got a total of approximately 25 amps and 252 watt hours to play with on any given weekend, while maintaining a discarge level not more than 20% of the total battery.

That should equal a maximum usage time of.

1 light for 24 hours or 2 lights for 12 hours. (approximate)

Now comes the part about putting the energy back into the battery? I picked the two hours of useable light per day as a pretty conservative guess and again I figured that there was enough of a margin of error here that putting between 50 and 60 watts back into the batt per day during the week, would be enough to recover.... By the time friday rolled around again, I would be back at 100% charge on the battery.

An example such as this:

Sat: 1 light, 12 hours @ 1 amp per hour -12amps -120wh
Sun: 1 light, 12 hours @ 1 amp per hour -12amps -120wh

Total for weekend -24 amps and 240watt hours

Estimated In
Mon:2 15W panels 2 hours, 60watts. (5 amps, 50 watts)
Tue: 2 15W panels 2 hours, 60watts. (5 amps, 50 watts)
Wed: 2 15W panels 2 hours, 60watts. (5 amps, 50 watts)
Thurs: 2 15W panels 2 hours, 60watts. (5 amps, 50 watts)
Fri: 2 15W panels 2 hours, 60watts. (5 amps, 50 watts)

Total in for the week +25amps, +250watts with charging being halted by the controller if its reaches 100% prior to Fridays.

So...to me, that scenario works under the following conditions

1) that I factored in enough error on the CF light drawing a maximum of 1 amp per hour of use. Including any power lost in the inversion process as well as the inverter itself.
2) battery is kept efficient i.e (temp)
3) I factored in enough error with the two panels being able to put in at least 5 amps and 50 watts back into the battery per day... again factoring in some efficiency loss

This is all very good stuff and thanks to everyone for chiming in on it... By the summer of 08, we are really hoping to get a much larger and "purpose built" off-grid system installed at our remote cabin... i.e. 100w pv panels, batteries designed for solar, high end inverters and charge controllers but since we ran out of \$\$\$ actually building the cabin, the system we are discussing here was a very temporary system to get us some "clean" and safe lighting.

I will use some new found info like the inverter specs to work up a worst case scenario on the actual consumption side (68% efficiency at 15w load) and put in a larger margin of error if necessary.

Do most agree with the above numbers? Do-able or not do-able?
• Solar Expert Posts: 264 ✭✭✭✭✭✭
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Re: General trouble with calculations

Look like you have got those equipments. Don't know if it's worth it, but saving all the troubles and headaches and use this solar security light
(if the link doesn't work, just go to the main sight and search for "solar security")
GP
• Posts: 6,290 admin
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Re: General trouble with calculations

BTW, I do want to stick with inverters and learn the ropes with 120V conversions as that is our primary goal. I don't want to go down the road of 12V appliances or lights.
• Solar Expert Posts: 5,183 ✭✭✭✭
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Re: General trouble with calculations

You may want to consider a 'hybrid ' system of 12 v lights and 110 v ac appliances. There are some real advantages to this as long as they are planned for... IMHO 12 v for lights is it...

I dont remember just when they started, within this year, but there are several strings on the 'hybrid system ' topic... weighing the pros and cons

Cheers
Eric

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada