Sizing a Battery-Based PV System

Sizing the Inverter

The final step in sizing a battery-based system is sizing the inverter. When specifying an inverter for any battery-based application, you need to consider the voltage for the loads, the maximum power draw, charging capabilities (from an AC source), and the ability for the inverter to supply power when certain loads surge (draw a large amount of power for a very short duration).   

When I refer to inverters, I’m really referring to inverter/chargers (this is conventional terminology in the solar industry). Always verify that the inverter you’re working with is really an inverter/charger. Sure, you could design the system with a regular inverter and have the client buy a separate battery charger for use in conjunction with an AC power source (see the later section on generators), but this isn’t a great solution.

Besides, with all the reli- able inverter/charger choices out there these days, why add the hassle of specifying and installing an additional device?    The battery-based systems you install can use either a single inverter or multiple ones.

When installing more than one inverter, you have to make sure the inverters communicate with each other and work together (the different inverter manufacturers have specific methods for this communication).

Bear in mind that if you do add multiple inverters, their power output values will be additive. So if you have two 3 kW inverters installed in a system, they can work together to provide 6 kW of power to loads. 

Viewing voltage output

Your load analysis helps you know what voltages your client’s loads require. For most residential applications, this voltage is 120/240 VAC, which is the voltage required by AC household loads. Small commercial buildings typically have 120/208 VAC voltage requirements.

Note: The need for 240 VAC is limited to a select few loads, with the primary one being water pumping. Most inverters are available with 120 VAC output only; these have the ability to be stacked (connected together) to supply 120/240 VAC to the load centers (and some can be stacked to provide 120/208 VAC for commercial applications).

A small number of inverters are available with a standard output of 120/240 VAC; these don’t need to be stacked to provide the right voltage. So for residential and small commercial applications, an inverter’s output voltage isn’t a restriction. 

Calculating the power draw

Inverters don’t care about energy consumption. They’re simply concerned with delivering voltage and current (power) to loads. How long those loads run doesn’t concern them, which is why you need to take care to determine an inverter’s power output requirements by using the data that you gathered during your load analysis regarding the power draw of individual loads.

 Inverters for battery-based PV systems are always rated at their continuous power output value (just like grid-direct inverters), which means you need to make sure that the inverter(s) you select can provide the amount of power required by your client’s loads. You need to estimate which of the loads will be running at the same time and then add up those power values to deter- mine the minimum power rating for the inverter. 

Refer to the load analysis shown in Figure 1, and you’ll see that if every load were turned on in that home, the total power draw would be 1,931 W — that’s almost 2 kW. As the system designer, if you even think there’s a chance that all those loads would run at the same time, you’d buy an inverter/charger rated at a minimum AC output rating of 2 kW.   

If you add up the loads and they require more power than a single inverter can supply, you can consider adding more inverters and allowing them to work together to power the loads. Most inverters have this ability, to a limit. To be safe, just verify that the inverters you want to use can be stacked.   

Consider future loads that may require power so that the inverter installed today can handle growth in the next few years. Consider the users; the potential for growth in a system designed for a retired couple is a lot different from that of a system designed for a young family.

Staying in charge  

When connected to an AC power source (such as a generator or the utility), an inverter stops turning DC into AC and becomes a battery charger, taking the AC source and recharging the batteries for you. Inverters have a limited charging capability, though, so you should consider that value in your de- sign. Ideally an inverter can charge the batteries at a C/10 rate. 

All inverters list their maximum charging capabilities in amps so you can directly compare the charger portion of the inverter/charger to the battery bank when you divide the batteries’ capacity (in amp-hours) by 10 (the number of hours required to recharge the battery). 

Looking at surge ratings

Any load with a motor (such as refrigerators, washing machines, and well pumps) causes a brief power surge when it starts operating. If the inverter can’t deliver enough power to the loads during that brief surge period, the entire system may crash, and all the loads may go out. Fortunately, today’s inverters can surge three to four times their rated output to start motor loads.   

In order to account for inevitable surges, you need to estimate the amount of power the inverter will be providing just prior to the surge and then estimate what the power draw will be when the surge happens. By adding these values together, you can verify whether the inverter can handle regularly occurring surges.

For example, if the inverter is delivering 2 kW of power to all the loads and the refrigerator kicks on, requiring a surge of 1 kW, the inverter has to be able to deliver 3 kW during that surge time.  The amount of surge is specific to the appliance and is typically noted on the listing label as the maximum current draw.

If you can’t get to the listing label on the load, you can either estimate the surge by multiplying the load’s power draw by three or you can use a clamp meter and record the current draw as the load starts.

Evaluating inverter and array power output

For utility-interactive, battery-based systems, you need to consider all the items presented in the preceding sections plus the relationship between the array’s power output and the inverter’s power output. These systems want to send as much power back to the utility as possible, so you need to make sure the inverter can handle the power output of the array under all scenarios.   

Presumably, the batteries will be full and the inverter will want to take all the array’s power and send it somewhere. First, it’ll satisfy any loads within the house or office, and then it’ll push the current back into the grid. Either way, the inverter will be taking the array’s power and processing it.

Incorporating a Generator

Most people installing a battery-based PV system want to incorporate a generator into the system. Why? For utility-interactive systems, a generator provides peace of mind. In other words, it guarantees that the building will have power regardless of the length of a power outage.  

Many clients who have the grid present and either have an existing generator or plan to incorporate a generator want to back up their entire home or office through the MDP.

Be careful, though; if the generator isn’t hooked up correctly, the utility-interactive inverter may see the generator as the utility and try to send power back into the generator.

Although this scenario is good for a utility, it’s dangerous for a generator. Work with the inverter manufacturer to make sure that the inverter can’t send power back to the generator.

For off-grid homes, many people consider a generator to be a necessity because producing 100 percent of energy needs solely from a PV source is difficult. Generators are used in off-grid systems primarily to charge the batteries when the PV array can’t keep up.

Note: Generators are also perfectly suited for equalizing flooded batteries as a part of the required maintenance for any battery-based system that uses flooded batteries. 

The sections that follow outline the fundamental features of generators as well as the basic requirements for sizing a generator for off-grid systems so you can recommend the right generator for your client’s needs. 

Generator features  

When the time comes to determine the best generator for your client’s system, consult with a reputable generator dealer and consider the following features:

Engine speed: Lower speeds generally equate to higher overall life. Look for a generator that operates at 1,800 rotations per minute (RPM).

Fuel source: Try to match the generator’s fuel source to one that your client will have handy. For instance, many off-grid homes use propane for cooking and water heating. Propane is a good fuel source for a generator as well.

Remote start: The inverters used in battery-based PV systems can incorporate remote and even automatic generator charging. Make sure the generator has a remote-start feature as well. If the generator doesn’t, your client will have to walk to the generator to turn it on every time (which isn’t a lot of fun in the middle of winter).

Output voltage: You need to match the generator’s voltage to that of the inverter. A generator output voltage of 240 VAC is most common, but some generators can be configured to 120 VAC. (I explain how to size an inverter earlier in this article.)   

Generators are a source of frustration for many people because they require regular maintenance and are prone to Murphy’s Law — they break down only when you absolutely need them. So, when working with a generator distributor, verify that it offers full warranties for generators used in any stand-alone, battery-based PV system.

Some generator manufacturers have a blanket statement that their warranties don’t apply for stand-alone applications. Make your distributor aware that this generator will still be used as a back- up, only it’ll be backing up a PV system and not the grid. 

Generator sizing

The generator portion of the sizing calculations for battery-based systems is often the part of the design that doesn’t receive enough attention — largely because a generator may already be in place.  You may install a stand-alone, battery-based PV system where a generator is already in place and the owner doesn’t want to switch to a different one.

Another scenario I’ve seen is where the generator used by the construction crew to build the home becomes a permanent resident and is incorporated into the PV system. Using generators that aren’t fully designed into the PV system is far from ideal, but it’s a reality for many systems. 

When the generator can be properly designed into the system, there are a few key parameters to keep in mind: The amount of current available for running loads and charging batteries is a major consideration. After the generator is turned on, the inverter locks onto that power source and passes generator power through to the loads in the house or office and uses whatever’s left over to charge the batteries.

Therefore, the generator’s power output needs to equal at least the amount required by any simultaneously running loads plus the maximum amount of power the inverter can use to charge the batteries. So if the home draws 2 kW and the charger needs 3 kW to properly charge the battery bank, the generator should be sized at 5 kW at an absolute minimum.   Encourage your clients to run their major electrical loads — washing machines, vacuums, and the like — when the generator is operating in order to drown out the noise from the generator.

Generators are rated by their power output, a value that’s typically at 240 VAC. If you’re running a single inverter at 120 VAC, you’ll probably only get half of the inverter’s rating, which means that a 5 kW rated generator can only deliver 2.5 kW when operating at 120 VAC. If you were to run only 120 V off of a 240 V generator, that’d cause damage to the generator eventually because the generator’s output wouldn’t be properly balanced. A select few generators can be rewired to get their fully rated output at 120 VAC, so there may be a way around that issue — but I have an easier method for sizing a generator.  

Base the generator’s power output off of the inverter’s power output. As an inverter manufacturer taught me years ago, a generator’s power output is based off of the unit being pushed downhill with the wind at its back. (This is a kind way of saying that the rating system is overly optimistic.)

If you size the generator’s output by three to four times the inverter’s output, you should be able to meet the needs of your client’s loads and battery-charging requirements. So if you size an inverter at, say, 2 kW, the generator should be a minimum of 6 kW to 8 kW.

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