# Backwards requirements calculator?

Solar Expert Posts: 109 ✭✭✭✭
I want to play with numbers. For example if I give you X AH battery size and X Watts PV array size how many watts/hour per day does that give me to use? I know there's many factors involved however is there a general rule of thumb to simplify the process without going through all the mathematical steps which I have allready. Thanks for any info.

i.e 200 W panel with a 100 AH battery would allow me to use how many watts per day?

If this sort of logic is not possible I understand, I'm just curious..

Re: Backwards requirements calculator?

Starting with the panel size... You can use this link to see how many watt hours (or kWhrs) per month/year based on parameters you input...

You will need your location (pick one from program nearest you), and, if you are off grid generating AC power--you will want to use a derating factor of ~0.52 (assumes flooded cell batteries and AC inverter--you can use the default of 0.77 for grid tied systems). The rest of the numbers you input based on your panel setup (tilt, facing E/W/S, tracking or not, etc.).

The one drawback to this program... It assumes that the minimum panel size is 1kW--so you will have to do a conversion for smaller panels (for example, x200Watt/1,000watt to get the kWhrs per month for a 200 watt system or 2/10 or 1/5 for this example).

For many reasons (mostly to do with long battery life and efficiency), the starting rule of thumb for battery sizing is 6x the daily battery load (pretty much assuming that you charge during the day, and consume power at night). Gives you 3 days of "no sun" and a 50% maximum depth of discharge.

If you get outside of the Rules of Thumb--then there are other issues that need to be looked at to define an optimum system.

I think also that somebody here made up an Excel spread sheet to work out some of the panel/battery/charging calculations for setting up an off-grid system...

I took a quick search and could not find it just now... Maybe somebody else remembers the post.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 109 ✭✭✭✭
Re: Backwards requirements calculator?

Thanks for that link and information. This solar stuff is truely a rewarding experience from researching to buying and installation to upgrading and maintaining, I spent this weekend converting a 3-way light in my bedroom to use the DC from my distribution power bar Was quite fun, although I no longer have a 3 way on/off switch since I used the traveller wires to send power to my other switch. Anyways, I keep learning and its great to see stuff working.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Backwards requirements calculator?

bb,
the calculator you speak of was from bad apple and it would appear it has dissappeared from the post in the thread he created for it.
on further investigation into it i found this link, but didn't try it. i'll copy and paste it here.

Attached FilesSolar Calc.zip (14.9 KB, 44 views)
• Solar Expert Posts: 109 ✭✭✭✭
Re: Backwards requirements calculator?

Neil, I can't get the spreadsheet to work with Excel 2007 on Windows Vista, does it only run on office 2003? I would have to install that version ?
Re: Backwards requirements calculator?

Opened fine in Windows XP with Excel 2003--Can't imagine that a later version will not work...

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 109 ✭✭✭✭
Re: Backwards requirements calculator?

It opens fine but when I enter some values in winter month for example, it doesnt update the other spreadsheets.. shouldn't need to refresh anything since its excel?
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Backwards requirements calculator?

i haven't played with it to tell you how it works with vista. you may need to address questions to its creator and that wasn't me. the instructions do say to fill it in for each of the worksheets, but i don't know for sure if he intended it to carry the info to the other worksheets or not. i lean not.
Re: Backwards requirements calculator?

This is the first time I have looked at this program... For me, the ouput graphs did not update until I had put in data/numbers in all of the Green Areas for PV and Battery pages (used <Enter> key to update the cells--may not have been required).

I have not looked at what is going on--so I don't know if everything is correct/makes sense or not... If you like the program, you might try sending a message to Bad Apple (he has not logged in since early September--so he may not be available).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: Backwards requirements calculator?

Here’s a quick “back of the envelope” backwards calculator for an off-grid system:
(Energy Requirement (kWhrs/day) / 63% end-to-end system efficiency) / local isolation = ~ PV array size (kW STC)
For example, to deliver 5.0 kWh/day in Massena, NY, from a south-facing PV array tilted up at latitude plus 15 degrees (~60 degrees) on an average day in December:

((5.0 kWh /day) / 63% ) / (2.5 hrs/day) = 3,2 kW STC, or 3,200 W STC

Ref: http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/sum2/94725.txt

The 63% end-to-end system efficiency factor is derived from the following values:
• PV module / array operating efficiency: ~85% of STC spec
• System wiring efficiency: 97%
• Charge controller efficiency: 96%
• Battery efficiency: 85%
• Inverter efficiency: 93% for 48 V inverters
Multiplying these values through results in an overall system efficiency spec of ~63%. YMMV.

HTH,
Jim / crewzer
• Solar Expert Posts: 9,583 ✭✭✭✭✭
Re: Backwards requirements calculator?
cizzi wrote: »
i.e 200 W panel with a 100 AH battery would allow me to use how many watts per day?
.

The simple way, is, allow for 5 hours of sun, daily.
PV = 85% of label [ 170W ]

5 x 170 = 850 W hour daily harvest.

There is about 15% loss when recharging a battery, leaving you 722.5wH to use.

You can split that between different uses, lighting, laptop computer, LCD TV,
an inverter will generally add 10% conversion losses, so what you can power directly from 12V is better than up converting to 120VAC
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 ,