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Power - Calculations for Solar Arrays and Battery Backup

Calculations for Solar Arrays and Battery Backup

Please feel free to use the calculator below to do your own calculations for solar arrays and battery backup.


Use the following equations to determine solar array size and battery backup requirements based on 12V nominal system voltage.

Power consumption for each piece of equipment Power = voltage × current W = V × A
Daily amp-hour load for each piece of equipment Amp-hour load = power / voltage × hours of operation per day (h) Add up amp-hour load for whole system. Ah = W / V × hours of operation per day (h)
Efficiency and loss-corrected load for whole system Corrected load = amp-hour load / 0.8 Ah(corr) = Ah / 0.8
Worst case equivalent sun hours (ESH) (h) Determine from charts or meteorological data.
Solar array current requirement (A) Solar current req. = Corrected load / ESH (h) A = Ah(corr) / ESH (h)
Number of solar panels in parallel (N) No. of panels = solar current req. / current output per panel N(panel) = A / A/panel
Autonomy or reserve time (Ah) Autonomy = no. of days × corrected load / % discharge rate Ah(res) = N(days) × Ah(corr) / % discharge rate
Battery bank (backup) size Battery bank = autonomy (Ah) / Ah per battery Bank = Ah(res) / Ah/battery



A permanent station with a Trimble receiver, cellular phone, modem, and computer located in Boulder, Colorado, approximately at latitude 40´02´´ N, 105´14´´ W, elevation 1602 meters.

Power consumption:

 Equipment  Power  Hours/day
 Trimble 4000SSi receiver    9W   24
 Cellular phone  15W     4
 Cellular phone modem  10W     4
 Computer  20W     4

Daily amp-hour load:

 Equipment  Calculation  Ah
 Trimble 4000SSi receiver    9W/12V x 24h  18.0
 Cellular phone  15W/12V x   4h    5.0
 Cellular phone modem  10W/12V x   4h    3.33
 Computer  20W/12V x   4h    6.67
 Total  33.0

Efficiency and loss-corrected load:

33 Ah / 0.8 = 42 Ah.

Worst case equivalent sun hours (ESH) for Boulder, CO:

3.5 hours

Solar array current requirement:

42 Ah / 3.5 h = 12A

Number of solar panels in parallel:

Three solar modules are currently being used in UNAVCO designs:

  • Siemens PC-4JF rated at 75W (4.4 A 17.0 V)

  • Siemens M75 rated at 48W (3.02 A 15.9 V)

  • Solarex MSX-30 rated at 30W (1.68 A 17.8 V)

Choosing the M75 module, four panels are needed to cover the solar array current requirements: 12A / 3.02A per panel = 3.97 panels

Autonomy or reserve time:

Choosing five days of autonomy with 50% maximum discharge:
5 × 42 Ah / 50% = 420Ah battery bank

Battery bank (backup) size:

Three types of sealed deep cycle batteries are currently being used in UNAVCO designs:

  • Deka 90Ah

  • Deka 66Ah

  • Deka 30Ah

As an example, if you choose the Deka 90Ah, five batteries are needed to provide the required autonomy. 420Ah / 90Ah per battery = 4.67 batteries.

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Comments (23)
Comment by A.zah on Tue, Feb 2nd, 2021 at 10:41 PM
hi..how to determine the ESH value if not charts to be referred to? is 3.5hours can be the fix value?
Comment by meesum tamar ali on Fri, Apr 3rd, 2015 at 6:11 PM
Thanks for you information
Comment by Ketan on Mon, Feb 2nd, 2015 at 6:52 AM
please send me the Excel sheet for this power calculation sheet on my email ID. ketan_7309@yahoo.in
Comment by Eric Aquereburu on Mon, Jul 7th, 2014 at 10:26 PM
This method is very good the solar backup system sizing.
Comment by vijay patil on Wed, Feb 12th, 2014 at 5:29 PM
Simple and helpful
Comment by eric on Tue, Jan 7th, 2014 at 9:16 AM
Hi have 8 kw radio repeater how to calculated solar battery power please help me .It s need 48 v for my radio repeater thanks
Comment by Anonymous on Mon, Dec 9th, 2013 at 7:51 AM
Please provide us, how to calculate Load analysis? What are the perimeter we have to take into account to calculate the Total load for Solar power Plant.
Comment by anwar mousa on Sun, Nov 10th, 2013 at 6:56 PM
I like the calculation procedure
Comment by pankaj misal on Sat, Oct 26th, 2013 at 7:08 AM
sir, my load is1.5kwh in24hursin30days so,haw maine solar panal,12v,150ah,batterys,charge controller(capcity),invter(dc to ac)capcity? please send me total enformasoin.
Comment by SANJAY VERMA on Wed, Oct 23rd, 2013 at 6:35 AM
Very usefull n simple method of calculation....
Comment by patricien tsangya on Wed, Sep 25th, 2013 at 12:53 PM
Thank you for your help
Comment by Maruthupandiyan on Mon, Sep 23rd, 2013 at 11:15 AM
Very useful information for calaulation of required battery & solar panels
Comment by soji on Wed, Aug 28th, 2013 at 10:23 AM
How many 80 watts panel will it take to charge 4 bank of 200ah 48V. Meaning 16 pcs of deep cycle bat connected 4 in 4 series and later parallel all.
Comment by vimal kumar on Sat, Aug 3rd, 2013 at 11:09 AM
sir, i have 200watts panelswhich are commected in series of 2 x200 and 3 x200 what type of inverter i should use and the battery back up for 1000 watts ECH is 6 hours. Pls suggest me
Comment by john on Mon, Jul 1st, 2013 at 6:08 AM
Hi I need more information How to calculated the Solar panel need and the size of battery use and Cable size for the Solar system Rdgs John
Comment by venkat on Thu, Apr 4th, 2013 at 5:57 AM
Our device are oprated in 220v in India can we use this process Instead of 12v we can use 220
Comment by MUKESH SHAHRI on Fri, Feb 8th, 2013 at 2:22 PM
Comment by c v Prasad on Wed, Jan 16th, 2013 at 7:41 AM
dear Sir please let me know in steps for calculating the battery back up for 3 KW Load for 4 hours capacity and invereter.
Comment by shiva on Mon, Dec 31st, 2012 at 9:39 AM
These calculations are very usefull to me thank u
Comment by A.RAJASEKAR on Sat, Dec 1st, 2012 at 12:13 PM
Sir, above calculation is perfect. Now I need the calculations for the 1kw solar panel.( ie) fromthe panel to the controler,inverter,batteryand then load. (load may be connected for two days 9hours/day)
Comment by Frederick Blume on Wed, Oct 31st, 2012 at 5:28 PM
12 is the system voltage as determined in this case by the battery and charge controller. Panel voltages vary and are usually 20V or greater in full sunlight.
Comment by v.s.dev on Wed, Oct 31st, 2012 at 11:53 AM
while calculating the solar panel requirement , you are dividing the wattage by 12 and then multiplying by no of hours. I want to confirm whether 12 denotes the panel voltage. please confirm
Comment by vijay chakrawar on Thu, Dec 30th, 2010 at 9:09 AM
Very useful information for calaulation of required battery & solar panels.
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Last modified: 2019-12-27  16:36:35  America/Denver