small system

Re: small system

Hi '91... Here is my copyrighted way of looking at such a question. :roll:

First, since you gave me a battery bank size--I am going to make all calculations based on the battery and how everything else hangs off it.

First 4x L16 @430 AH at 100 Hour Rate... I prefer to use 20 Hour rate--that is closer to how most PV systems "use" the battery bank. Call it 6 volt @ 420 AH.

I will "ignore" the 12 vs 24 volt and just work in "watts" for the most part at this point... A large bank like this should be at 24 volts unless you have specific reasons for 12 volt.

So, first rule of thumb is 5% to 13% rate of charge. 5% if system is mostly in "float/standby". 10% or higher if in daily use--Also Trojan recommend 10% rate of charge (of 20 Hour rating):

• 4x 7.25 volts charging * 420 AH per battery * 1/0.77 panel+controller derating * 0.05 rate of charge = 791 Watt Solar Array minimum
• 4x 7.25 volts charging * 420 AH per battery * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,582 Watt Array "nominal"
• 4x 7.25 volts charging * 420 AH per battery * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,056 Watt Array "cost effective maximum"

So the first thing to say is this is a pretty hefty size battery bank. At 12 volts it would be 840 AH and if you were using a backup AC battery charger (24 volt arrangement would be 1/2 the current):

• 840 AH * 0.05 = 42 Amps minimum
• 840 AH * 0.10 = 84 amps nominal
• 840 AH * 0.13 = 109 Amps typical max continuous
• 840 AH * 0.25 = 210 Amps not to exceed (watch battery temperatures)

A typical AC battery charger, non-power factor corrected would need around a generator of this size, minimum for a 10% rate of charge. Use 10% below, but range from 5% to 13%+ is typical range:

• 84 amps * 14.5 volts * 1/0.67 typical charger power factor * 1/0.80 typ. charger efficiency * 0.10 rate of charge = 2,272 VA minimum generator loading

Sizing the AC inverter... Typically I would suggest C/8 for maximum continuous discharge (flooded cell bank) and C/2.5 for maximum starting surge:

• 4x 6 volt * 420 AH * 1/8 rate of discharge * 0.85 inverter efficiency = 1,008 watt max nominal AC loads
• 4x 6 volt * 420 AH * 1/8 rate of discharge * 0.85 inverter efficiency = 3,226 watt max starting surge AC loads

Normally, I would recommend a max continuous inverter rating of 1,200 watts for a 12 volt battery bank. To run a 1,200 watt 12 volt inverter:

• 1,200 watts * 1/10.5 volt batt cutout * 1/0.85 inverter eff * 1.25 NEC derating factor = 168 Amp minimum for Wiring/Breakers/Fuses

If your AC loads are not too heavy (no refrigerators or microwaves), I would suggest a MorningStar 300 watt 12 VDC TSW inverter (600 watts for 10 minute surge). It is a great inverter and is a good reason for a 12 volt battery bank.

Now, we suggest a battery bank has 1-3 days of storage and 50% maximum discharge... For for a 2 day bank and 50% maximum discharge, your "nominal" daily battery load would be around:

• 4x 6 volts * 420 AH * 1/2 days * 0.50 max discharge * 0.85 inverter eff = 2,124 Watt*Hours of battery load per day (suggested)

Lets say you are around Boulder Colorado and get a good 4.5 hours of sun minimum (get upwards of 6 hours per day for a couple summer months). PV Watts fixed array, Boulder:

Month Solar Radiation
(kWh/m 2/day)
1 4.43
2 4.89
3 6.05
4 6.09
5 5.99
6 6.08
7 6.06
8 6.24
9 6.25
10 5.67
11 4.60
12 4.29
Year 5.56

So, for the three different suggested arrays:

• 791 Watt Solar Array minimum * 0.52 system derating * 4.5 Hours of sun per day = 1,850 Watt*Hours per day
• 1,582 Watt Array "nominal" * 0.52 system derating * 4.5 Hours of sun per day = 3,702 Watt*Hours per day
• 2,056 Watt Array "cost effective maximum" * 0.52 system derating * 4.5 Hours of sun per day = 4,812 Watt*Hours per day

Anyway--That is the basics of system sizing... Before we get in the details of 3 vs 4 panels and such--What is your expectations of energy usage vs system ability?

-Bill