GRID-TIE BATTERY BACKUP
Grid-Tie Battery Backup: Many solar customers are surprised and disappointed to learn that their battery-less grid-tie solar PV system not power their home during a utility outage. In areas where blackouts and extended weather-related outages are common, a battery backup system, like the one shown in the diagram above, can add substantial value.
Sizing and designing a grid-tie system with battery backup is more complex than designing a battery-less system. They perform two separate functions: offsetting the power purchased from the electric utility, just like a standard battery-less system and providing emergency backup power during utility outages. Both of these functions require separate design considerations and calculations.
The "grid-tie" part of the system is designed the same way as a grid-tie system is, using the average daily kWh power consumption and the yearly average peak sun-hours available where the PV array is located.
The "battery backup" part of the system is designed based on the power draw of the critical loads that need to operate during a grid outage, and how long the outage is expected to occur. These system are generally designed to run only critical loads located in a separate sub-panel. They are not designed to power the whole house, although this can be done at considerable additional cost.
Battery backup systems require specialized inverters and other components and most be carefully sized, so be sure to contact us for Technical Support assistance.
Inverters for Grid-Tie with Battery Backup
OutBack G-Series inverters and switch gear, as well as the new Radian inverter, can power loads individually from 2 to 8 kW and can be combined in a single system up to 80 kW. (See Battery-Based Inverters and Power Systems)
The Schneider Conext XW series of inverters offers grid-tie inverters with battery backup capability in 4 kW, 4.5 kW, and 6 kW increments. Up to 4 units can be paralleled for battery backup systems up to 24 kW. (See Battery-Based Inverters and Power Systems).
The SMA Sunny Island inverters, in conjunction with a Sunny Boy inverter and PV array, can be used to provide high-efficiency backup power in a grid-tied home or business. Backup systems can be configured with up to 24 kW single-phase output using up to 4 Sunny Island inverters or up to 72 kW of 3-phase output with up to 12 Sunny Island inverters and a Multi-Cluster Box. (See Battery-Based Inverters and Power Systems).
Follow steps 1 - 5 on the Grid-Tie PV System Design Worksheet (on Off-Grid Load page) to determine the size of the array required to provide the desired percentage of total power. Then calculate the inverter size and battery capacity needed using the worksheet below.
Worksheet: Inverter and Batteries for Grid-Tie with Backup System
Determine energy storage requirement for backup system.
________ Step 1: Find the power requirements (watts) for the appliances that need power during a black-out.
Make a list of the loads and appliances that need power during an outage. Refrigerators, safety lighting, etc. Only list the essential items, since the system size (and cost) will vary widely with power needed. The wattage of individual appliances can usually be found on the back of the appliance or in the owner's manual. If an appliance is rated in amps, multiply amps by the operating voltage (120 or 240) to find watts. Add up the wattage of all the items on the list that may need to run simultaneously to arrive at the total amount of watts. This is the "peak wattage" inverter requirement and will determine the minimum size of the dual-function inverter that you will need. If the PV array is larger than the peak wattage, then skip step 2 through 5 and size the inverter to the array as in a normal grid-tie system.
________ Step 2: Define how long of an outage the system must accommodate.
Power outages last from a few minutes, to a day or more. Again, this decision will greatly affect the system size and cost, so the desired length of time should be traded against the total loads supported. If the system needs to provide power for an indefinite period of time, use the array and battery bank sizing instructions for an off-grid system.
________ Step 3: Determine the amount of energy needed.
Multiply the power requirements (in step 1) by duration in hours (in step 2). The result will be watt-hours. For example, powering a 350 W refrigerator, a 150 W computer, and a 500 W lighting system for 2 hours would require 2,000 watt-hours (or 2 kWh) of energy storage.
________ Step 4: Calculate the energy storage needed.
Multiply the figure arrived at in step 3 by 1.7. In the example, 2 kWh x 1.7 = 3.4 kWh of energy storage in needed.
________ Step 5: Calculate battery capacity needed.
Divide the energy storage requirement from step 4 by the DC voltage of the system (usually 48 VDC, but sometimes 24 VDC) to get battery amp-hour (Ah) capacity. Most backup systems use sealed batteries due to their reduced maintenance requirements and because they can be more easily placed in enclosed battery compartments.
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