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