Designing a Stand-Alone PV System: Page 5 of 6


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 (5)

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.

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.


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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