First you need to determine your average daily expected total electrical load. This can be done by going to each appliance you intend to use, determine it's power consumption per hour, determine how much time each day you intend to use the appliance, then do the math.
(All appliances and electronics will have the hourly load somewhere on it. It might be inside the refrigerator or microwave door, on the bottom of the digital clock, on the brick transformer between the device and the wall plug, or on the wall plug transformer itself. In most cases, it is somewhere between the plug and the area where the plug wire enters the appliance.)
Where A= Watts or Amps per hour, and B= Time in hours per day of expected use, C= total daily consumption for that appliance.
The problem comes when some appliances are DC and some are AC, and some list Amps, while others list watts. There is a formula for that too:
Amps x Volts = Watt hours
In general, if you daily total consumption is more than about 400-500Ah per day, you need to rethink your power requirements, the appliances you are using, and how much you will use them.
Once you determine this load, you need to size your battery bank to a minimum of 4 times your total expected daily load. However, once you get your daily load amount, you will need to add 20% to the total bank size to account for conversion loss.
Example: if you determine that your daily load is 175 amp hours per day on average, your daily load with conversion loss is 210 amp hours (175Ah x 1.2 conversion loss.) You will need a 840 amp hour battery bank, minimum (210Ah x 4.) For a battery bank this size that I will rely upon, I would choose two Rolls 6-CS-25P batteries rated at 820Ah, wired in series. I know that they would last at least 25 -35 years when properly charged and maintained.
A battery's life span is dependent upon two very important things:
1. Depth of discharge between full recharge
2. Time between complete full recharge.
The more deeply you discharge you battery before fully recharging it, determines the total lifespan of the battery. Most quality battery manufacturers use 50% discharge for rating life span. They also use 24 hour full recharge after discharge for life span ratings. If you wait 18 days to fully recharge your bank, you are doing long term damage to that battery bank.
3 cloudy and rainy days would force you to deplete your battery bank below 50% (if you use the 4x consumption battery bank size.) Based on your array output and consumption, your bank would remain at or below 50% discharge for 9 days. Doing this once a quarter can reduce the longevity of your battery bank by 3% per year for high quality Rolls batteries and by 10% or more per year for lesser batteries.
Typical "golf cart" batteries are not very good batteries. An off-grid home or boat that uses these types of batteries and maintains them perfectly can expect 5-8 years of life, before having to replace the entire bank.
Here is the math for an 825Ah 12VDC bank, perfectly maintained, 50% discharge (the following prices are from a 5-year old price sheet YMMV):
Trojan T-105 6V 225Ah "golf cart" battery = $115
8 batteries = 900Ah 12VDC bank
8 x $115 = $920 battery bank
Series/Parallel cabling = $400 minimum at wholesale prices
Total expenditure (not including shipping/labor) = $1320
Maintenance = Monthly
Life span = 8 years
####### ########### ##########
Rolls 6-CS-25P 6V 820Ah battery = $1150
2 batteries = 820Ah 12VDC bank
2 x $1150 = $2300
Series/Parallel cabling = $100 retail
Total expenditure (not including shipping/labor) = $2400
Maintenance = Yearly
Life span = 25 years
####### ########### ##########
Here is the fun part:
Replace Trojans every eight years. Inflation doubles (at best) the cost of the batteries every 15 years (but we will use 16 to keep it simple.)
First replacement: 8 batteries x 173 = $1384
Second replacement: 8 batteries x 230 = $1840
Total cost to use "golf cart" batteries = $4544
Times batteries needed maintenance = 300
Times you had to lug off old batteries = 3
Times you had go the chiropractor to get you back straightened out = 3
Times you had to contribute money to the cuss jar = 36
Total cost for the Rolls Batteries = $2400
Times batteries needed maintenance = 25
Times you had to lug off old batteries = 0
Times you had go the chiropractor to get you back straightened out = 0
Times you had to contribute money to the cuss jar = 0
How often you praised your good judgment = more than you can count.
Everything you ever wanted to know about the Rolls 6-CS-25P battery
Now, you need to replace the electricity you use on a daily basis. Your solar array should be at least 30% larger than your total requirement to account for cloudy days and environmental conditions.
You know that you have to replace 210Ah of energy each day, or 2520Wh. Depending on where you live (latitude) will depend on how much daily sun exposure you should expect. In my area of Texas, it is a factor of 4.5. That means that a solar panel will get 4.5 total hours of useable sun per day in a cloudless sky. So one 75W panel would contribute up to 337.5Wh; one 150W panel would contribute up to 675Wh.
One also needs to take into account charge loss of 20%. So my 75W panel will actually only contribute 270W directly back into the battery. I would need just over nine 75W panels to replace the energy used in one day IF I could count on perfect environmental conditions for the entire day. Since I cannot count on that, I would want to add more power capability to my array by at least 30%. So an absolute drop dead minimum solar array would be 12 75W panels or 900W array to replace my 175Ah per day consumption after taking all losses into account.
Charge controllers are sized by total AMPS passed through them. Xantrex (Trace), Morningstar, and Outback Power Systems make some of the best, and most versatile charge controllers. If they do not make one that has the capacity of all of your panels combined, then you will need two controllers and will divide your array into two.
In another example, your total daily load is 2600Wh, you are going to want considerably more than a 2800Wh array. Here's why:
If you get 3 cloudy and rainy days in a row, you can expect 1/2 the output from your array that you normally would.
Here is the math:
2800 x 3 = 8400
8400 / 2 = 4200Wh for 3 cloudy, rainy days
2600 x 3 = 7800Wh
You consumed 7800Wh but only put back 4200Wh
7800-4200 = 3600Wh in arrears
You have a 200Wh reserve based on your 8 110W panels in your area.
IF you have have perfect sunny days, and you continue to consume at your normal rate, it would take 18 (perfect) days to recover and fully recharge your batteries.
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