Designing a Stand-Alone PV System: Page 3 of 6

Intermediate

Inside this Article

Ackerman-Leist Pole-Mounted Array
The Ackerman-Leist pole-mounted array stands at the garden’s edge, an integrated part of the family’s homestead.
A Watt-Hour Meter
A watt-hour meter gives precise figures on consumption for appliances already owned. Without that information, the values in the “Loads” table must be estimated.
Battery Bank
Contained in this simple battery box, three parallel strings of four 225 Ah Trojan T-105 batteries make a 24 V, 675 Ah battery bank.
Back of Pole-Mounted Array
This pole-mounted array offers unimpeded solar access at the site from 8 a.m. until 4 p.m.
Outback Power Controller
Step-down MPPT controllers can help decrease wiring costs by allowing PV array voltage to be higher than the battery bank voltage.
Xantrex Inverter
Select an inverter to handle the maximum loads that will be on at once in the home. Choosing the next larger size will help ensure your system can meet the demands of future loads.
Ackerman-Leist Pole-Mounted Array
A Watt-Hour Meter
Battery Bank
Back of Pole-Mounted Array
Outback Power Controller
Xantrex Inverter

Step 2: Battery Bank Sizing

The average daily load is then used to calculate the battery requirements. The batteries must be able to store the total daily load, in addition to the extra energy lost by inverting from direct current (DC) to alternating current (AC). Dividing the AC average daily load by the inverter efficiency (90% standard), inflates the average daily load that the batteries must store to account for efficiency losses from the inverter. While inverter manufacturers will commonly list “peak efficiency” (generally ranging from about 92% to 95%), we use a more conservative 90% to account for the fact that the actual operating efficiency depends on the AC load, which is constantly fluctuating. Hence, an inverter will rarely operate at the load level which results in peak efficiency.

The battery bank’s ambient operating temperature is also taken into consideration, since temperature affects a flooded lead-acid battery’s internal resistance and ability to hold a charge. As temperatures fall below 80°F, battery capacity is reduced. A battery temperature multiplier table can be used—check with the battery manufacturer for their specific correction factors.

Days of autonomy is also an important design criterion, as it dictates how many days the battery bank will need to sustain the average daily load when there is little or no sunshine to recharge it. It’s a compromise between having energy during overcast spells, how much time the generator will run, and the added cost of a larger battery bank. The more days of autonomy desired, the larger the battery bank. Generally three to five days of autonomy provides a good balance. Keep in mind that the larger the battery bank, the larger the PV array will need to be to recharge the bank sufficiently on a regular basis—or the more the generator will be needed to pick up the slack.

The last major design criterion for sizing batteries is the depth of discharge (DOD). While deep-cycle lead-acid batteries are designed to discharge 80% of their capacity, the deeper they are discharged on a regular basis, the fewer charge/discharge cycles they can provide over their lifetime. When choosing a DOD, strike a balance between longevity, cost, and the significant hassle of replacement. Many system designers will specify a 50% DOD to be used in the worksheet. Because several days of autonomy are accounted for, which increases the battery bank size, the actual depth of discharge during sunny weather will often be less than 20%. The DOD design value can greatly affect the cost of the battery bank. (For simplicity, the numbers from the load table have been rounded in the following equations.)

(1,800 AC Wh Avg. Daily Load ÷ 0.9 Inv. Eff.) + 360 DC Wh Avg. Daily Load = 2,360 Wh/day 

2,360 Wh/day ÷ 24 DC System Volts = 98.3 Avg. Ah per day

98.3 x 1.11 battery temperature multiplier x 3 days autonomy ÷ 0.5 DOD = 654.7 total system Ah

654.7 ÷ 225 Ah individual battery capacity = 3 parallel battery strings (rounded up from 2.9)

24 V system voltage ÷ 6 V battery voltage = 4 batteries in series

3 parallel strings x 4 batteries in series = 12 total batteries

The battery calculations indicate that a battery bank made up of 12 of the chosen 6 V, 225 Ah, flooded lead-acid batteries will provide adequate storage to meet daily energy requirements, inverter efficiency losses, operating temperature effects, days of autonomy, and the desired average depth of discharge. The number of batteries or series-strings of batteries connected in parallel should be kept to a minimum, preferably three or less. This minimizes the chance of unequal charging from one battery or string to the next. While using higher-capacity batteries would have resulted in fewer parallel strings, the Ackerman-Leists chose lower-capacity batteries for budgetary reasons.

Batteries are rated by their capacity in amp-hours and at the rate that they are charged/discharged. In most PV systems, the appropriate Ah rating to use is based on a discharge over 20 hours. Unlike shallow-cycle vehicle batteries, deep-cycle batteries in PV systems are charged and discharged over 24 hours, and the weather, level of solar irradiance, and energy usage patterns all influence the charge/discharge scheme. In this system example, the battery could provide 225 Ah of stored energy—if discharged 100% over 20 hours. If it were discharged faster, the capacity would be less, and vice versa. Be sure to check with the battery manufacturer, as they provide battery-specific Ah capacity values based on different charge/discharge rates. Choose the 20-hour rate when sizing and selecting batteries, unless a specific load profile dictates otherwise.

Comments (8)

Brady Sheridan's picture

I have been reading quite a few of these articles and comments, great magazine and information source! I wondered if I could get someone to do a logic check on what I think I have learned for a small off grid home?
A) What you consume in electricity daily and the average daily sun hours etc. determines the size of the solar array in watts produced per hour and the battery bank of storage you require in amp hours.
B) The total electricity used in watts at one time, as well as the type of load (120/240), determines the size of the inverter you will require.
C) Your solar array setup, how many panels in series, how many parallel strings, is determined by the panels, watts, amps, volts and the charge controller.
After using a number of calculators and links to calculators provided by this site has brought me to the following possible design:

Daily Consumption: 8,000 watts (wood heat, propane HW & cooking)

Battery Storage: 38,400 watts; (48 volts @ 800 amp hours; 2 days storage at 50% dod, generator bu)

Solar Array: 3.6kwh = 16 x 230watt panels arranged 4 to a series of 4 parallel strings (3.5 hrs winter sun, 5.5 spring/fall, 8.5 summer)

Charge Controller: Midnight Classic 200 (Classic Sizing Tool used to calc the solar panel arrangement)

Inverter: Magnum MS4448PAE (based consumption and the 240v deep well pump)

Does this look like I am understanding correctly?

Michael Welch's picture
Hi Brady. It seems like you have a good handle on this. Maybe your battery is a little large for my liking -- these days solar has gotten so cheap that folks are using smaller batteries and larger arrays to fill the batteries quicker and even get some energy on cloudy days. I run my small off-grid home with under 12 kwh of battery, with a much smaller PV array and no backup generator. An array can meet the day's usage and fill up a battery, discharged to your max DOD, in one day of full winter sun. There are lots of different opinions on how to approach battery & array sizing, and this one is how I've been thinking about off-grid systems lately. Are you going to do your own installation? If not, your installer will run through your figures with you (and they may have different equipment that they prefer). If you are buying from a retailer, a good one will also help you check your calcs. Let me know if you'd like a recommendation for an installer near you, retailer, or consultant. Email michael dot welch at homepower dot com Times of year are very important -- the winter time has a lot less sun, and is a time when more electric energy is used. Also critical is to get your usage absolutely correct. Underestimating and your system won't perform well. On the other hand, folks not used to off-grid living will often underestimate their ability to conserve energy, and also figure in inefficient appliances, rather than looking at buying new, more efficient ones. Yes, and another factor is surge power, like when you first start a well pump it takes a lot more power (for a few milli-seconds) than once it is running. Generally, inverters within the normal power usage range required will also handle surge. But what if your pump and your fridge start at the same time? Personally, I recommend 25% DOD, to make the batteries last as long as possible. They are heavy and a pain in the butt to change out. And also for that reason, I recommend industrial-type batteries (metal-casings & 2 V individual cells, usually but not always) rather than commercial-type (like the L-16 category). More expensive, but cheaper in the long run -- specially if you value the time and effort involved in swapping batteries.
Brady Sheridan's picture

Thanks for reading and replying. I now see what you mean about the large battery storage. I guess i did as an occupational hazard, "accountants are to always prepare for a loss" thus the overestimate for batteries.
Since you stated that i also found another sizing tool, after keying in all the numbers, it shows my battery status as being at less than 10% state of discharge for the most of the year. Definitely over sized. I ended up using such a large array because i was offered an incredible deal but i had to buy the whole pallet at $0.78/watt (and that's with our Monopoly money north of the border)
Its the only purchase i have made yet till I fine tune the system design.
I also inherited an Onan 20kw genny, all the more reason not to oversize the batteries.
I would like to install as much as I can myself, as I am really enjoying the whole process. I suspect I will at least end up with a local electrician/PV installer reviewing everything for safety etc.
As far as batteries I was thinking the similar to you, forklift batteries, 2v cells. Thanks again for the input, much appreciated.

Marty Rosenzweig's picture

Very straight forward article but am I missing something here? Where is the "array sizing" adjusted for the C/10 optimum charge rate for those batteries? 3 X 225 Ahr /10 = 67.5A. necessary for the battery charging (plus accommodation for the load amps).
Even at C/15 that's 45A. I speak from experience since I designed an identical system for my house in Mexico. After about a year, the battery inertia (don't know what else to call the increased internal battery resistance) made bringing up to full charge those three strings more and more difficult and I experienced somewhat premature battery failure in less than 4 years. Even the addition of 300 W.more panels were not satisfactory. I've settled on 2 strings (450 Ahr.) with the 1200 watt array and two days of autonomy. We'll see how long the new batteries last under these conditions.
It would be beneficial to hear how the Vermont design is currently holding up and to track the battery life in the future.

Justine Sanchez's picture

Hi Marty,
Thanks for your comments! Joe has already covered the key aspects to battery longevity. I would also just like to add a few additional thoughts...while we can dial in the charge rate of an battery charger utilizing a generator (or the grid if avail) to charge batteries, from my experience an array is usually sized to simply replace lost energy from the battery bank on a daily basis. If we oversized the array to meet a specific charge rate (including during the winter months), we likely would have a significant portion of our array producing excess energy the rest of the year...and in an off-grid setting no place to utilize that excess other than a dump load. Historically it is the job of a backup generator to get those batteries topped off when the array (sized for estimated daily energy consumption) cannot.

A few other notes...Not sure what the rest of your system is comprised of but an MPPT charge controller will help wring some extra amps out of those modules, especially during cool sunny days. Also keeping the parallel battery strings to a minimum (one is ideal) will help keep battery bank charge/discharge imbalances from reducing battery longevity. And of course making sure the batteries are regularly maintained (watered, tops cleaned, connections checked for corrosion or loose hardware, etc.) will also increase battery life.

Best,
Justine
Home Power Magazine

Joe Schwartz's picture

Hey, Marty. We'll check in with Khanti and see if we can get some information on the performance of his system/batteries to date. Few thoughts related to battery charging:

First, historically, most off-grid PV systems were not designed to meet a battery manufacturer's optimal charge rate. The cost of modules was simple too high for most people to afford/achieve a c/10 charge rate for example. With the falling cost of modules higher charge rates in the range of c/10 are becoming more common.

Designing a system to meet a battery manufacturer's optimal charge rate is significantly less important to battery longevity than the following:

1. System's should be designed to replace all of the energy used on a (sunny) daily basis including system efficiency losses.

2. The battery bank should be fully recharged as often as possible, at least once a week and more frequently is always better. It should be completely recharged every sunny day.

3. Systems should be sized to limit the average daily depth of battery discharge to around 20% and again, less is always better. For example, my off-grid system is designed for a daily depth of discharge of 10%.

4. Off-grid system owners need to avoid the common scenario of falling into a pattern of cycling their batteries between say 80-60% capacity on a daily basis and rarely recharging them fully. Run the engine generator when necessary to avoid this scenario.

5. Flooded batteries should be equalized regularly per the manufacturer's recommendations.

-Joe

Geoffrey Kaila_3's picture

Hi Khanti,

I refer to your article designing a stand alone PV system in HP 136. I have two observations; 1. Cound't you have used a higher voltage than 48V on the PV side to maximize the MPPT benefit. 2. When I use the PWM sizing method I get the same # of modules. I thought the MPPT method would reduce the # of modules. Please clarify. Thank you and best regards,

Geoffrey kaila

Art Drayton's picture

Nice article - particularly like the emphasis at the start of energy efficiency first! Cheers - Aussie Art

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