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  • Solar Off-Grid Battery Design

    Recommended Design Practice of Off Grid Solar PV Systems

    The design process is a fairly simple and straight forward that doesn’t require a lot of technical knowledge. Just Jr. High School Algebra. The most difficult calculation is what is your daily WATT HOUR usage. What follows is based on using MPPT Charge Controllers because they are the most efficient and economical solution for systems requiring a 200 watt or higher solar panel wattage. The initial steps are:

    · Determine the load in energy for a 24-hour period. Not the watts, but the watt-hours.
    · Determine the size of the solar array to be used.
    · Determine the battery voltage, Amp Hours, and type like AGM or FLA
    · Determine charge controller size in amp based on panel wattage and battery voltage


    The following example is a rough estimate to take to a system designer to discuss cost and objectives. He/she will then fine tune the system based on actual components, cable distance, etc… Our basic objective in this simple example is to provide 4000 Watt Hours of usable power in a 24 hour day in two different cities (Tucson AZ, and Seattle WS) with 90 % availability. The reason I am using two cities is to demonstrate location is extremely important and greatly weighs in on system cost. FWIW this is a system I have designed many times using remote cell radio sites. The transmitter is continuous 200 watt load, and the system will be a typical sized system that can be used in a home application. The only difference is radio sites are designed for 99.99% availability and this one will only be 90%. Getting from 90 to 99.99% greatly increases the cost with a larger solar array, larger batteries.
    NOTE: A generator is required for any off-grid system so monthly Equalization Maintenance can be performed, and provide power during cloudy spells of more than 2 days.


    In this example the worst case is simple to determine because the load is continuous 24 x 7 x 365 of a 200 watt light bulb. So the worst case is the months of December and January when the Solar Insolation is at its lowest point of the year. In some instances, the worst case for the load might be in the summer when you have things like fans and refrigeration. . Therefore you make two designs, one for winter and one for summer, and then use the larger of the two systems.

    So in this example we need to determine the energy needed in a 24 hour period. This is done with watt-hours. To determine the watt-hours is straight forward of Watts x Time (in hours). So 200 watts x 24 hours = 4800 watt-hours or 4.8 Kwh in a day or 24 hours. Make note of this number as it will be needed latter.


    To account for overall system losses in the wiring, charge controller, battery charge efficiency, and inverter you multiply the total 24 hour load energy by 1.5 So 4800 x 1.5 = 7200 Watt Hours or 7.2 Kwh. Now take note of this figure.
    FWIW if using a PWM Controller the Fudge Factor is 2


    Most solar map data are given in terms of energy per surface area per day. No matter the original unit used, it can be converted into kWh/m2/day. Because of a few convenient factors, this can be read directly as "Sun Hour Day” The number you want to use in this example is for December since December days are the shortest. Tucson is shown to receive 5.6 kWh/m2/day in December.. For Seattle, the number is 1.2 Kwh/m2/day. So we need to note 5.6 and 1.2 for our Sun Hour Day as it will be used to determine the solar panel array wattage.
    NOTE The huge difference location makes. This gets real expensive real fast.


    The size of the array is determined by the adjusted daily energy requirement using the Fudge factor number divided by the sun-hours per day.
    Panel Watts = [Daily Watt Hours x 1.5] / Sun Hours
    So for Tucson 7200 wh / 5.6 sh = 1285, round up to 1400 watts.
    For Seattle 7200 wh / 1.2 = 6000 watts
    NOTE. Note the huge difference; it is because of the Solar Insolation. Location matters and will greatly affect system cost.


    · First determine battery Voltage. This is based on Panel Wattage
    · 0 to 600 watts = 12 Volts or higher
    · 601 to 2000 watts = 24 volts or higher
    · 2001 to 5000 watts = 48 volts or higher
    · 5001 watts and higher = Forget it or higher than 48 volts. You cannot afford it.

    Determining battery size is extremely simple Standard Protocol for lead acid batteries is a minimum 5 Day reserve capacity. This sounds like a lot but is necessary. In praties a 5 day reserve only nets you 3 days of Run Time in cloudy weather. You do not want to take your batteries down more than 50 to 60 % Depth of Discharge. Limiting daily discharge to 20 to 25% will maximize your battery dollar getting the most bang for your buck.
    So with this said the battery capacity is calculated to be 5 full days. So in real application you have 2.5 days of usable capacity to allow for cloudy days before reaching the 50% discharge danger point. Once you are at 50% time to run the generator or shut down and go dark until you are fully recharged which will take 2 or 3 full days before you can turn the power back on.

    Battery Amp Hours = [5 x Daily Watt Hours] / Battery Voltage
    Total battery capacity is the exact same for both Tucson and Seattle. 5 days x 4800 watt hours = 24,000 watt hours or 24 Kwh

    Tuscon can be a 24 or 48 volt system. The least expensive is the 48 volt system because you can use a smaller less expensive Charge Controller. For Tuscon Battery Amps Hours = 24,000 wh / 48 volts = 500 Amp Hours. At 24 volts is 1000 Amp Hours.

    Seattle is not really doable even at 48 volts because there is no charge controller large enough for a 6000 watt panel. It would require two charge controllers.


    Again Very Simple. MPPT Amps = Panel Wattage / Battery Voltage.

    Tucson is 1400 watts / 48 volts = 30 amps or 60 amps at 24 volt battery. A 30 amp MPPT Controller is a lot less expensive than a 60 amp controller.

    Seattle as you can see is not doable economically. 6000 watts / 48 volts = 125 amps or two very expensive 60 amp MPPT controllers.


    We need to re-visit batteries for a moment. Flooded Lead Acid (FLA) are the least expensive and last the longest of the lead acid chemistry. However they have one drawback and that is they have the highest internal resistance. What this mean is the maximum charge rate they can be charged with is C/8 where C = the battery 20 hour discharge rate Amp Hour Capacity. So the maximum current we can apply to a FLA 500 AH battery is 500 AH / 8 h = 62.5 amps. So for the Tucson we can use FLA batteries.

    More bad news for Seattle because the charge rate exceeds C/8 for FLA batteries. 125 amps is a C/4 charge rate. So in Seattle you are forced to use AGM batteries. AGM is a lead acid battery which is a Sealed Lead Acid (SLA) or sometimes called a Valve Regulated Lead Acid (VRLA). AGM batteries have lower internal resistance but they do not last as long as FLA batteries. To add insult to injury AGM batteries cost roughly 200% more than FLA an dlast half as long. That means long term battery cost is 400% higher than FLA.

    Now here is the real fun and educational part. Based on the USA national average price of $0.101/Kwh each of these two design examples generate 4.8 Kwh/day and cost or $0.50/day or $915 in 5 years. Batteries need replaced on average every 5 years. So the Tucson system major equipment cost (panels, charge controller, and battery only) roughly $8000. After 5 years you are looking at a $4500 battery replacement cost. So for the first 5 years you have volunteered from paying $0.101/Kwh to $0.75/Kwh or a 740% rate increase.

    You folks in Seattle are really in for a shock. You initial equipment cost are $24,600 so for the first 5 years you are now paying $2.25/Kwh or a 2227% rate increase. In 5 years you are looking at $9000 battery replacement cost.

    Location and battery voltage matter. AVOID 12 VOLT SYSTEMS if you can
    MSEE, PE

  • #2
    revised off grid calcualator

    here is a revised calculator with a bit more information on it.Off grid calculator excel 97 version
    NABCEP certified Technical Sales Professional


    [URL][/URL] (Voltage drop Calculator among others)