Designing a Stand-Alone PV System: Page 5 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 4: Controller Sizing

With an array size specified, a charge controller is next—sized to safely handle and regulate the array’s incoming power to prevent overcharging the batteries. A charge controller needs to be selected based on the maximum array watts, nominal battery voltage, and desired features. A MPPT controller allows the array to maximize the energy put into the batteries, particularly under cold conditions (high array voltage) and low battery voltage. These controllers also have the ability to step down a higher array voltage to a lower battery bank voltage which, in turn, helps keep wire size and costs down for long wire runs. It can also reduce the number of series fuses and the size of the combiner box. To prevent damaging the controller and potentially voiding its warranty, the maximum open-circuit voltage (Voc) of the array must never exceed the charge controller’s maximum voltage rating at the lowest expected ambient temperature.

12 modules x 80 W each = 960 W (max. W controller must handle)

960 W ÷ 1,500 W max. controller W rating at nominal battery voltage (24 V) = 1 charge controller required (rounded up from 0.64)

22.1 V module Voc  x 4 modules in series x 1.25 temp. multiplier (per NEC Table 690.7 for record low temp. of -35°F) = 110.5 VDC maximum PV array Voc

110.5 max. Voc < 150 VDC, the controller’s maximum Voc rating

*Max. system voltage was calculated using the module’s Voc temp. coefficient

Although charge controllers are most commonly rated by the amount of current (amps) they can deliver to the battery bank, it is often simpler to compare the calculated array watts with the controller manufacturer’s recommendation for maximum array watts (STC) at the applicable battery bank voltage. More often than not, the maximum array watts for different battery bank voltages are listed on the controller’s spec sheet, allowing the designer to simply divide the system’s array size (in watts) by the controller’s maximum allowable watts, to determine how many controllers will be needed.

Another option, especially when a controller spec sheet does not list the maximum allowable watts, is to use the manufacturer’s controller string-sizing tool on its Web site to determine allowable array configurations. If no string-sizing tool is available, make sure that the calculated array size meets the given controller specifications, mainly “maximum input current.” In the example here, the controller spec sheet does specify an STC nameplate rating of 1,500 W for a 24 VDC battery bank. Lastly, the above calculations also verify that at the coldest expected low temperature, the maximum array voltage will not exceed the controller’s maximum open-circuit voltage rating.

Step 5: Inverter Sizing

A battery-based inverter must handle all the household AC electrical loads that could be on simultaneously (AC total watts). An inverter must also be able to handle the expected surge or in-rush of current that some large loads draw upon startup. While a conservative method for estimating surge requirements is simply to multiply the total AC watts by three, realistically, many household loads do not surge. In this sizing example, likely only the clothes washer and well pump will surge significantly, although we also include the base load of the other appliances that may also be consuming power. Always be sure to compare the surge rating of an inverter with the expected surge requirements of the system.

Other design criteria include matching the inverter’s input voltage with the nominal battery voltage, choosing the desired AC output voltage (120 or 240 VAC), considering environmental conditions (indoor or outdoor, mountainous or coastal, etc.), and weighing different optional features, such as an internal battery charger.

2,356 W total AC loads = minimum inverter continuous watt rating (round up to 2,500 W typical inverter size)

[(1,560 W pump + 480 W washer) x 3] + 316 W base load = 6,436 W minimum surge rating

Desired AC output: 120 VAC

Desired features: Integrated AC-DC battery charger, digital display 

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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