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

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

As Niel said... Your load information does not make sense.

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

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

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

While I believe off-grid solar can be a big help for your location... Don't overlook the size/weight/cost of the system...

A properly sized generator can supply you will ~5+kWhrs per gallon of fuel. That is ~3 days of power for approximately 1 gallon (6 lbs+container) of fuel if this is ~1,800 WattHours per day... (example a 50lb Honda eu2000i 1,600 watt generator). I am not sure of your AC loads--so I am still guessing about sizes; although, there is enough information/formulas for you to plug in and get your own answers.

And, by the way, if you want to add information about your wind turbine (make/model, how much power per month, any maintenance or other issues)--please feel free. We don't have a lot of wind turbine users here on the forum (or at least people who have been successful with wind turbines).

-Bill