Sizing a Grid-Tied PV System ...with Battery Backup

Intermediate

Inside this Article

OutBack Grid-Tied System Photo
Two stacked inverters are the core of this OutBack system, which integrates charge controllers and other BOS components.
Schneider Grid-Tied System Photo
A professionally installed grid-tied PV system with battery backup using the Schneider (Xantrex) XW inverter and BOS package, with a custom battery box.
SMA Grid-Tied System Photo
An SMA America Sunny Island system uses AC coupling to integrate battery charging with a second batteryless inverter.
Trojan Battery Photo
The Trojan L16 is a classic flooded lead-acid battery used in renewable energy systems—420 Ah at 6 V.
FullRiver Battery Photo
This sealed gel-cell by FullRiver Battery requires less maintenance, but more delicate treatment than flooded batteries.
Surrette Battery Photo
This 2 V flooded lead-acid battery by Surrette provides 1,766 Ah, giving high capacity with a single series string.
OutBack Controller Photo
Both OutBack and Schneider Electric make MPPT charge controllers that integrate directly with their inverters. SMA America’s AC coupling technology has built-in charge controlling.
Schneider Battery Monitor Photo
Schneider Electric’s LinkLITE battery monitor.
OutBack Grid-Tied System Photo
Schneider Grid-Tied System Photo
SMA Grid-Tied System Photo
Trojan Battery Photo
FullRiver Battery Photo
Surrette Battery Photo
OutBack Controller Photo
Schneider Battery Monitor Photo

Once the decision has been made to include batteries in your grid-tied system (see “Grid-Tied…with Backup!” in this issue), next is estimating the backed-up loads’ energy consumption, selecting the inverter, sizing the battery bank, and considering other balance-of-system (BOS) components.

System sizing starts with the loads, which determine system components and cost. Sizing a simple batteryless grid-tied system starts with a year’s worth of household energy use data (usually from electric bills) and a budget (see “Sizing Batteryless Grid-Tied PV Arrays” in HP138). Sizing the PV array (or other RE source) of a battery-based grid-tied system is done similarly, but sizing the inverter and battery bank requires more information and calculation.

Estimating/Measuring Backed-Up Loads

If you want a battery-based system, you’ll need to decide what loads will be backed up and how long you need them to operate.

Nameplate ratings—volts, amps, and possibly watts—are printed on most appliances. Remember: amps (A) × volts (V) = watts (W). Watts is an instantaneous measurement of how much power is needed (or produced) at a given moment. You’ll need to know how much energy (kWh or Wh) your loads use while operating over a period of time. Multiplying watts by the number of hours the load runs gives you its energy use in watt-hours (Wh) per day—then dividing by 1,000 will translate it to kWh.

Although nameplate ratings can be used for system sizing, they may lead to overestimating a load’s power needs, since amp ratings are sometimes given for surge peaks—not continuous draw. A more accurate way to measure a load is with a watt meter (see “Beyond Your Utility Meter” in HP138). These tools measure volts, amps, watts, and cumulative kWh for each load. Measuring each load you’re considering for backup will provide a better foundation for system sizing.

Inverter Sizing

The inverter in a grid-tied battery-based system must be sized to do two things: power all of the backed-up loads simultaneously and pass the energy from the renewable sources (PV array, wind generator, etc.) to the grid. To calculate the inverter power rating, sum the total backup loads. If surge loads (pumps, compressors, induction motors) are anticipated, the inverter should be sized to also handle the maximum combined surge loads. Most inverters can handle a surge twice their rated output for a few seconds. If more power is needed than a single inverter can supply, you can stack the outputs of multiple inverters to increase the total connected power and surge capabilities, or multiple inverters can separately feed separate loads.

The voltage and frequency of the inverter must also match the loads. Inverters for the United States are available in 120 and 240 VAC output at 60 Hz. If you only need to back up 120 V loads, then a 120 V inverter will be your most economical choice. However, purchasing a split-phase, 120/240 V inverter (or stacking two 120 V inverters for 120/240 V output) gives the flexibility to power both 120 and 240 V loads.

The second inverter selection factor is the ability to send renewable-made energy to the grid. If you have a PV array rated at 4,000 W, the inverter needs to be able to process the full amount. While it is true that the PV array will produce less power under normal operating conditions (due to module heating, dust/dirt, wiring inefficiencies, etc.), there are conditions (cold temperatures and clear skies) when the array can produce full power. Select an inverter that will handle the larger of the two factors: Array output and maximum combined backup loads.

Battery Bank Sizing

After sizing your inverter to fit the loads and RE sources, next size the battery bank to power the loads for a given amount of time, making sure to match the battery and inverter’s DC voltage.

Begin battery bank sizing by determining how many days you want to power your backed-up loads without a charging source (days of autonomy). Since the grid will be your primary energy source, for this calculation, you only need to consider the amount of time the grid won’t be available. Using historical information, you can estimate the average length of time your site experiences utility outages. You can also add in a safety factor to account for extreme situations. In arriving at a number, assume that there won’t be a source of energy to recharge the batteries. Finally, if the days of autonomy exceed three, consider adding an engine generator as a third energy source. This will help keep overall system costs down, and result in a more reasonably sized and better utilized battery.

Batteries can only supply a limited amount of energy before they are depleted. Some deep-cycle batteries may be discharged up to 80% for about 2,000 cycles, or at 50% for about 4,000 cycles. A battery-based backup power system may undergo 10 cycles per year—often less. So sizing a battery based on an 80% deep discharge rate is appropriate for this type of infrequently cycled system. (For an off-grid home that cycles batteries often daily, a more conservative approach may be necessary.)

Once you know the desired days of autonomy, possible battery discharge level, and the energy requirements of your loads, you can size the battery. Start with the total Wh per day for backed-up loads and divide by 0.85 to correct for inverter loss (assumes an 85% efficient inverter). This results in DC Wh per day, which we will divide by our nominal battery voltage—usually 24 or 48 V (dictated by your inverter’s nominal DC input). That computes the total DC amp-hours (Ah) per day. Divide this by 0.80 to account for the maximum DOD of 80% (i.e., to leave our battery 20% full after a day of discharge). Then multiply this total by your days of autonomy to get the adjusted DC Ah total.

Battery capacity in Ah is rated at several different discharge rates. For general sizing, use the Ah capacity at a 20-hour discharge rate; this is a realistic discharge rate for batteries in an RE system. Once a battery has been selected, divide your autonomy-adjusted DC Ah per day by the Ah capacity rating of your chosen battery to determine the number of parallel battery strings needed. If this number is greater than three, select a larger-capacity battery to promote a balanced charge across all strings. Each parallel string of batteries will contain as many individual cells as needed in series to reach your nominal battery voltage.

For example, assume that the backup load daily requirement is 8,000 Wh. First, correct for inverter loss by dividing by 0.85. This gives you 9,412 DC Wh per day. Dividing this by the nominal battery voltage of 48 equals 196 Ah per day. For an 80% depth of discharge, divide by 0.80, which equals 245 Ah per day. Multiply that by 4 days of autonomy (this home tends to be among the last reconnected to the grid after an outage) for a total of 980 Ah—the minimum size battery needed.

Eight 350 Ah, 6 V batteries in series yields 48 V at 350 Ah. Divide the 980 Ah total capacity by 350 Ah series battery capacity, which equals 2.8. Round up to 3 since batteries cannot be divided—the total bank (consisting of 24 batteries) will provide a capacity of 1,050 Ah. If the number of paralleled strings needed is greater than three, then source a higher-capacity battery as the basic building block.

Choosing a Charge Controller

A charge controller’s main function is to keep the battery from being overcharged and potentially damaged. The two common styles of charge controllers used with these systems attain this goal differently. Either type can be used in battery backup grid-interactive systems.

Pulse-width-modulation (PWM) charge controllers regulate charging by adjusting the width and frequency of the full current pulses sent to the battery. The closer a battery is to full, the farther the pulses are apart, effectively lowering the charging current.

Maximum power point tracking (MPPT) charge controllers have several advantages. Their software algorithms can operate a PV array at its MPP over a wide range of operating conditions and at a voltage much higher than the battery voltage. This improvement increases power harvest by up to 30% (with greatest gains attained with cooler site temperatures) and allows longer distances or smaller wire sizes between the PV array and charge controller.

A charge controller should be sized to pass all the array’s current to the battery. A 60 A controller charging a 12 V battery can only pass 750 watts but if configured to charge a 48 V battery, it can pass nearly 3,000 W. Oversizing the controller slightly can be beneficial since the controller will not have to work at the upper limits of its capabilities all the time and it can harvest any unexpected wattage that could come from extra irradiance or environmental conditions.

If the PV array is capable of producing more power than one charge controller can handle, consider upgrading to a larger-amperage controller, installing multiple controllers, or increasing the battery bank voltage to get more wattage out of each controller.

BOS Components

Bypass breakers allow bypassing the inverter and battery-based system to power all loads directly via the grid. Most commonly, these are installed for inverter or other removal and repair. Bypass breakers are usually located in the battery/inverter power panel, and limited to 60 A or smaller.

A battery meter is critical to understanding a battery bank. Much like a fuel gauge in a car, a meter will report your battery’s state of charge and help you determine the need for other charging sources or to conserve during an extended power outage. These meters are your window into your battery bank’s world and commonly display the battery voltage, amperage in or out, and state of charge.

Most systems include a production meter to measure the amount of energy produced by your renewable energy system. Incentive programs commonly require a utility-grade kWh meter. In a batteryless system, the production meter is installed between the inverter and the grid connection to measure all energy flowing between the inverter and the grid. But in a battery-based, grid-tied system, this would only measure the net difference between your system’s renewable production and energy consumed by the backed-up loads.

To get a true reading of how much energy is being produced, a special kWh meter must be used. A “Form 12S” kWh meter measures both the grid input/sell circuit to the inverter, and the inverter to backed-up loads circuit. For 240 V systems, two of these meters are needed.

All electrically live parts are kept in enclosures of some type. The backed-up AC loads have their own panel; DC inputs or loads have another; and batteries are in their own enclosure. Batteries can discharge massive amounts of current if shorted—via a dropped wrench or other conductive materials—so their terminals must be protected. When charging to a high voltage batteries give off hydrogen gas, which must be vented outdoors to lower the risk of fire or explosion. The battery enclosure must be sealed but vented passively or mechanically from its high point. Incoming air should be introduced at the bottom of the enclosure, and wire conduit should also extend to the bottom.

Putting It All Together

A grid-tied with battery backup system is one of the most complicated RE systems to install—if you plan to hire a local installer, do your homework, and ask pointed questions about their related experience. The recent boom in PV installations has led to many new companies that have done only batteryless grid-tied work, which is much simpler and requires comparatively little design work. A North American Board of Certified Energy Practitioners (NABCEP) certification is good sign that the prospective installer has been tested on the basics of battery-based systems, but not a sure sign of an extensively experienced installer in that field.

Access

Flint Richter (flint at rockygrove.com) lives and writes off-grid in Arkansas’ Boston Mountains. He is a partner and NABCEP-certified PV project manager with Rocky Grove Sun Company, and a contracted instructor for Solar Energy International. He is teaching his young daughters the difference between a solar module and a solar panel.

Battery Backup Inverter Manufacturers:

OutBack Power Systems • www.outbackpower.com

Schneider Electric • www.schneider-electric.com

SMA America • www.sma-america.com

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