# Help! I Need Fomula/Computation how to Size-up Solar Power

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hi,

Please help me I need complete electrical computation/'formula or step by step procedure in sizing up solar power.

Info:

Demand Load: 1440Amps hour per Day
Days of Autonomy: 5 Days
Sun Hour Per day: 6 Hours
Depth of Discharge for the Battery: 80% Maximum
Ambient temperature for battery bank: 1.02 (multiplier)

Materials to be use:

SPV Modules: 100W, 12V nominal, 16.4V Maximum, 16.097Amps
Deep cycle Battery: 100 AH, 12vDC

Required:

No. of SPV Array or Watts or KW
No. of Battery or Total Amps Hour
Size of inverter (Minimum)

Question:

1. Base on the above data, If the Battery is 80% discharge, can the solar panel charge the battery at 100% (full charge) in 6 Sun Hour Per day of sun power.

Thank you

Robert
Email: pssrobert@yahoo.com

• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: Help! I Need Fomula/Computation how to Size-up Solar Power

bolero,
i must admit your info is confusing me so i'll just address what i see in the rest of your post. you state you'll take the battery to 80% dod and going that deep on the dod won't be good for that battery. just to be sure, you are planning to deplete (80%x100ah) 80ah of the 100ah capacity of the battery. injecting here that either 2 100ah batteries would be better or 1 200ah battery so that the dod is 40% rather than 80%. this equates to 80ah/6hrs=13.334amps needed to charge. if the pvs are giving 16.097amps over the 6 hours then i see no problem with the battery getting charged, although it may be close if reductions for stc ratings and charge efficiencies are taken into account. also, if any cloudyness or lessening of solar intensities or time available occurs for any reason then the battery(s) will not get charged in that same day.

as to your info, do the ah/day on the demand load represent 12vdc or 220vac? in either case the solar setup you have described will not be able to meet the power demand. same holds for a 5 day autonomy from the battery as that would mean, 80ah/5=16ah/day used. 16ah x 12v = 192wh/day used. this is not what is indicated in your info.
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Re: Help! I Need Fomula/Computation how to Size-up Solar Power

Amp*Hours is just Watt*Hours divided by your battery bank voltage.

So, for example, you wanted 16.4V worth of solar panels at 16.097Amps for 6 hours of sun:

16.4 V * 16.097 A = 264 watts

264 watts * 6 hours of sun = 1,584 Watt*Hours per solar day

1,584 WH / 12 volts = 132 Amp*Hours per day (at 12 volts)...

In reality, when all is said and done and all losses (panel eff~85%, MPPT charge controller ~92%, flooded cell battery ~80%, AC inverter ~85%, wiring 97%%) are taken into account, solar panels will deliver to an AC load about 52% of their name plate rating. So, based on the solar panel rating you gave (just as an example):

1,584 WH * 52% = 823 WH per day
132 AH * 52% = 66 AH per day

Regarding your battery, you have defined using it to 80% [typo, not 89%] depth of discharge--or down to 20% of remaining capacity...

For flooded cell lead acid storage batteries--deep cycling to 20% state of charge really reduces their life--normally it is better to discharge to 50% maximum (SOC, DOD). Also, flooded cell batteries do not like going for days below ~75% SOC. The lead sulfates (which occur during normal discharging) start to harden (within hours) and reduce the capacity of the battery--so you need to recharge your flooded cell battery back above 75% state of charge within a day.

If you use sealed AGM type batteries--they are defined to run down to 20% state of charge, and some brands say they do not suffer sulfate hardening. These are very nice batteries--but cost 2-3x as much as flooded cell batteries.

How fast a battery charges will depend on how many solar panels you have... Typically, your solar panels are sized to your load so that 1 day of sunlight equals about 75% of your 1 day of usage (your solar panels are about 1.33x your daily usage--remember that that you need to also include the 1/52% system efficiency too)... This would allow you to fully recharge your battery bank after one day of no-sun+normal usage in ~3 days.

Normally, our rule of thumb is that the battery bank should be 6x your daily usage. This allows 3 days of no-sun and keeps the battery to 50% maximum discharge for good cycle life.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Help! I Need Fomula/Computation how to Size-up Solar Power

hi niel,

Sorry if my data is confusing, I rewrite it again as follows:

Demand Load: 1440Amp-Hour per Day (At 12VDC)
Days of Autonomy: 5 Days (I don’t understand why 5 Days of Autonomy is not possible its (1440AH/Day X 5 Days = 7200AH)
Full Sun Hour Per day: 6 Hours
Depth of Discharge for the Battery: 80% (I know that 80% Depth of Discharge is not good for the battery it will only last about 200-500 cycle, But I believe it will only happen once or twice a month, that will be the worst scenario and I need to maximize the battery, because I don’t have Generator and no utility company on site)
Ambient temperature for battery bank: 1.02 (multiplier)

1. If I use SPV Module with rating of 100watts, 12VDC (Nominal Voltage) 16.4VDC (Max. Voltage) 6.097Amps, How many SPV modules do I need.

2. For 12V Battery how many do I need? and what is the Amp-Hour rating that you will recommend.

Thank You
Robert Lim
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Re: Help! I Need Fomula/Computation how to Size-up Solar Power

Thanks you very much bill

BB. wrote: »

Amp*Hours is just Watt*Hours divided by your battery bank voltage.

So, for example, you wanted 16.4V worth of solar panels at 16.097Amps for 6 hours of sun:

16.4 V * 16.097 A = 264 watts

264 watts * 6 hours of sun = 1,584 Watt*Hours per solar day

1,584 WH / 12 volts = 132 Amp*Hours per day (at 12 volts)...

In reality, when all is said and done and all losses (panel eff~85%, MPPT charge controller ~92%, flooded cell battery ~80%, AC inverter ~85%, wiring 97%%) are taken into account, solar panels will deliver to an AC load about 52% of their name plate rating. So, based on the solar panel rating you gave (just as an example):

1,584 WH * 52% = 823 WH per day
132 AH * 52% = 66 AH per day

Regarding your battery, you have defined using it to 89% depth of discharge--or down to 20% of remaining capacity...

For flooded cell lead acid storage batteries--deep cycling to 20% state of charge really reduces their life--normally it is better to discharge to 50% maximum (SOC, DOD). Also, flooded cell batteries do not like going for days below ~75% SOC. The lead sulfates (which occur during normal discharging) start to harden (within hours) and reduce the capacity of the battery--so you need to recharge your flooded cell battery back above 75% state of charge within a day.

If you use sealed AGM type batteries--they are defined to run down to 20% state of charge, and some brands say they do not suffer sulfate hardening. These are very nice batteries--but cost 2-3x as much as flooded cell batteries.

How fast a battery charges will depend on how many solar panels you have... Typically, your solar panels are sized to your load so that 1 day of sunlight equals about 75% of your 1 day of usage (your solar panels are about 1.33x your daily usage--remember that that you need to also include the 1/52% system efficiency too)... This would allow you to fully recharge your battery bank after one day of no-sun+normal usage in ~3 days.

Normally, our rule of thumb is that the battery bank should be 6x your daily usage. This allows 3 days of no-sun and keeps the battery to 50% maximum discharge for good cycle life.

-Bill
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Re: Help! I Need Fomula/Computation how to Size-up Solar Power

Robert,

The issue is that flooded cell lead acid batteries--the "fluffy" lead sulfate that forms while the battery is being discharged begins to harden within hours once the battery is below ~75% of state of charge (25% discharged)... If you stay below 75% discharge for several days--the battery will simple not last very long (regardless of how deep the discharge goes). And batteries are almost as expensive as the solar panels (or even more so in a 5+ day system)... So, if you end up replacing them every few years (or even more often)--then you almost may as well use a generator as the fuel costs will be similar to battery replacement costs.

1,440 AmpHours at 12 volts

Battery size:

5 days + 20% SOC:

(1,440 Amp*Hour * 5 days)*1.20=8,640 Amp*Hours (at 12 volts)

Use a big old forklift type battery:

Crown Industrial Battery 6-125-15

12 Volt, 1090 Amp-Hour
Regular price: \$4,452.00
Sale price: \$3,172.00

8,640AH/1,090AH = 8 batteries or >\$24,000 worth of batteries (US pricing at store)

To charge this battery, you should have probably around 10% of AH rating in solar panels, at a minimum (for proper equalization, no generator):

8,640 AH * 10% = 864 amps at "12 volts"...

864 amps * 15 volts = 12,960 watts charging current (recommended, you could go down to ~5% of battery rating or ~6,500 watts of charging current minimum).

I guess you have available 100 watt solar panels (Philippines?) and that is their rating? Vmp=16.4VDC; Imp=6.097Amps (guessing this is what you mean). Allowing for 85% solar panel name plate derating (dust, age, real specs. vs paper specifications):

864 amps * 1/0.85 * 1/0.92 solar charger * 1/6.097amps = 181.2 "100 watt" solar panels

The amount of useful power you will get from 181 100 watt solar panels from 6 hours of sun:

100 watts * 6 hours * 0.52 (eff derating) * 121 panels = 37,752 Watt*Hours per day

37,752 Watt*Hours / 12 volts = 3,146 Amp*Hours per day

1,440 Amp*Hours per day * 5 days = 7,200 amp*hours

7,200 AH / 3,146 AH = 2.28 days to recharge (if no load, after 5 days of no sun)

Or, if you have your daily load, after 5 days of no sun (and 6 hours of sun):

7,200 AH / (3,146AH per day - 1,440 AH per day of load) = 4.22 days to recharge (normal load, 5 days of no-sun)...

Is this what you are looking for?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Help! I Need Fomula/Computation how to Size-up Solar Power

Robert,

You also asked about an inverter... Assuming you are using 1,440 AH in an 8 hour day--that would indicate:

1,440AH * 12 volts * 85% (inverter eff) = 1,836 watt load 8 hours per day...

Inverters are rated on several things... Their output voltage, maximum "nominal" power, "peak" power, and Volt*Amps...

So, you need to know your average load (2 computers, 4 washing machines, 1 well pump, 3 lights, etc.) and add up the average power--and size your inverter to run around 60-70% of that load [clarify--size the average peak load at ~60-70% of Inverter Capacity]... Plus you need to look at the surge current (say the starting power required by a well pump). And add the average+surge current such that the peak rating of the inverter will run your starting loads.

Lastly, if you are running a bunch of motors, you need to look at the Volt*Amp ratings vs the Inverter's VA rating (this is not "power" but Power/Power-Factor)... Basically, motors being inductive take "extra current" and need heavier wiring--but it is not "power" from the solar panels. An electrician can help you, if needed, to measure your loads.

Also, you need to know what the battery bank voltage will be... Some rough rules of thumb... I like to recommend that the inverter's input current be kept below 100 amps. You can wire for larger currents--but it is expensive for all of that copper... It is easier to get a higher voltage inverter and battery bank than to run at a low battery bank voltage (remember power=voltage*current, so if you double the voltage, you 1/2 the current).

My two cents is that a 12 volt battery system should be 1,000 watts maximum inverter. 24 volt system, 2,000 watt max. And a 48 volt system would be 4,000 watts maximum (keeping DC battery wiring to less than 100 amps).

With modern inverters and solar charge controllers, you can get a 48 volt setup for the same (or even less overall costs) vs a 12 volt system (that most people are familiar with).

Your system, being so large, you should be only considering a 48 volt system (for example, you will only need 1/4 the number of solar charge controllers vs a 12 volt system).

Lots of guessing going on here--I hope we have given you some useful information to think over... Our first recommendation is always to "conserve"... Reduce your power requirements as much as possible first by using energy efficient appliances and turning off unneeded loads, and look for alternatives like better insulation, and such--where applicable.

Using US pricing--generally we pay around \$0.10 to \$0.30 per kWhr for utility power... Off-Grid solar power costs, very roughly, \$1.00+ per kWhr. So if you can save using those extra kWHours in the first place by conservation, then you don't need to build a larger system to generate them.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Help! I Need Fomula/Computation how to Size-up Solar Power

Hi,

Thanks again bill, now I see the whole picture, and yes I’m from the Philippine, actually generator is not possible it’s a 2Miles walk from main road to the Cabin. As of now I’m conducting a test on a 1kW windmill and I notice, its runs continuously at night for 3 to 4 hour every day, the wind is not that strong in daytime, so I think I’ll use it to charge the battery (solar at day and windmill at night), so I can reduce the quantity of the battery and the solar panels which is very expensive.

Regards
Robert