Battery Backup for Grid-Tied PV Systems

A presentation for solar industry professionals Presented by Doug Pratt, and SimpleRay Solar SMA Sunny Island System Large(!)Outback System

But first, a moment of thanks for northern California growers no, not for that. Thru the 1980 s more than half the PV sold in the U.S. ended up between Santa Rosa, CA and the Oregon border. Back when PV was > $10/watt, it was growers who embraced and financed the birth of the solar industry.

Think of electron flow from a PV array like water flow. Either put it someplace, or shut it off. Household loads, the utility grid, or batteries are all places to put electrons. Before ~1995 all PV systems were battery-based. There was no grid-tie. So the first grid-tie inverters from Trace Engineering were battery-based. Eventually we learned how to treat the utility grid as if it was a battery, and direct grid-tie was born. Direct grid-tie is simpler, cheaper, and more efficient, so it became dominant. But you always need either the grid or batteries to accept excess, or supply additional electrons as needed. In this example the house can run on Utility Grid, OR it can run on batteries.

What s usually the first question about battery backup? What s it going to cost? Additional retail cost to add backup to a std. direct grid-tie system (materials, labor, & markup) $6-8k minimum. So, is backup for everyone? Who really needs it? Batteries are for folks who need instant, reliable backup. Think data and medical. For everyone else it s convenience & bragging rights. Your average residential client is better (& cheaper) served with a generator and a large tank of propane $3-5k. For clients who insist on backing up EVERYTHING, generators are the best choice. Battery backup systems are only for critical loads. Battery Backup for Grid-Tied PV Systems Fridge, freezer, furnace, water, a couple lights and the security system. No swimming pools, or lights on the tennis court. A/C is negotiable.

But then, have convenience & bragging rights ever failed as a sales tool? Got a customer who s passed the $$ test for battery backup? There s two paths to battery backup, HAVE s or HAVE NOT s. Does the customer already HAVE a grid-tie PV system installed? Or have you already sold them a conventional grid-tie system? You ll want an ADD-ON system such as Sunny Island or Magnum. An ADD-ON system will support conventional grid-tie inverters, and fool them into thinking the grid is still up. Magnum is lower cost, will work with any grid-tie inverter brand. Sunny Island will work with any brand, but works better with Sunny Boy. Sunny Island systems will do split- or 3-phase. Sub-pumps are usually the most challenging load. Up to 3/4hp, no problem. 1hp & larger let s talk first. Battery Backup for Grid-Tied PV Systems

Solar Module Array 1 (12) Sharp ND-U235Q1, 235-watt modules. Wired as a single series string. What does an ADD-ON system look like? - + - + - - + - + - + Factory-supplied connection cable Utility Grid 120/240 vac MagnaSine MS4448PAE Inverter/Charger w/built-in 30A Auto Transfer Switch On 175A Magnum Mini-Panel Solar Module Specs: Sharp ND-U235Q1 Pm = 235w Voc = 37.2vdc Vpm = 30.1vdc Isc = 8.59A Ipm = 7.81A Series Fuse: 15A PV Source Circuits Isc of 8.60 x 1.56 = 13.4A Ampacity required. Typical wire type: 10ga CU wire, USE-2 in free air, or THHN-2/THWN-2 in conduit. Positive, Negative and Ground Magnum 175A Mini- Panel AC/DC Center on 30A30A off Output GROUND AC OUTPUT L1 L2 NEUTRAL OUT NEUTRAL IN AC INPUT L1 L2 30A 30A Input (Utility or Generator) N N Battery Backup for Grid-Tied PV Systems Inverting DC 54.5v 25A Magnum Digital Display DC Negative DC Positive - + 175A House Main AC Panel 20A, 2-pole circuit breaker (Best practice places inverter input breaker(s) at opposite end of busbar from main breaker.) Note: Sum of input breakers may not exceed 120% of busbar capacity In-line 2A Fuse Inverter Specs: SMA Sunny Boy 2500HF-US AC Volts 240vac Max AC Amps 10.4A MPPT Window 220-480vdc Max DC Volts 600vdc Max DC Amps 15.0A CEC Rating 96.5% To Supported Loads Critical Loads Sub-Panel To Supported Loads 8D Battery 8D Battery 8D Battery 8D Battery Battery Pack Four 8D sealed batteries. 48-volt, 245-Amphours Battery Cables & Interconnects, #2/0 awg CU AC Output Circuit Inv Max Amps of 10.4 x 1.25 = 13.0A Ampacity required. L1, L2, Neutral & Ground (Best practice limits voltage drop to 1.5% at inverter max amps output) Sunny Boy 2500HF-US + H H H H Integrated PV Combiner & 600VDC Disconnect 15A fuses supplied. #8 AWG (CU) system grounding electrode conductor. N N G G AC Disconnect Lockable, Visible 30 amp (Not required by NEC, may be required by local utility company or local AHJ)

Add-On kits will work with new or existing conventional grid-tie systems. Existing grid-tie system requires no alterations. Any brand of conventional grid-tie inverter is okay. Magnum Single Add-On Kit Delivers up to 4,400 watts. Limited to ~ 4,000 watts of PV. Will start ½ or ¾ hp sub-pumps. $5.3k. Sunny Island Single Add-On Kit Delivers up to 6,000 watts. Limited to ~ 8,000 watts of PV. Will start ½ or ¾ hp sub-pumps. $8.6k. Magnum Dual Add-On Kit Delivers up to 8,800 watts. Limited to ~ 8,000 watts of PV. Will start 1.5 hp or smaller sub-pumps. $7.4k. Sunny Island Dual Add-On Kit Delivers up to 12,000 watts. Handles up to ~ 15,000 watts of PV. Will start 2.0 hp or smaller sub-pumps. $13.5k. Kits all include inverter, AC/DC power center with appropriate breakers, DC cabling, 9.5kWh sealed battery pack, & battery boxes. These kits require the dealer to install a new Critical Loads AC breaker panel. Breaker panel is locally-supplied. 705 Raymond Ave, Suite 220, St. Paul, MN 55114

HAVE NOT systems are starting with a blank slate. No PV, & client wants backup. Direct battery-based inverter systems are usually less costly than Add-On systems. Direct battery-based systems can be either off-grid or grid-tied. There are many battery-based off-grid inverters. There are a few that grid-tie. Outback, Xantrex, & Sunny Island inverters are battery-based, & grid-tie capable. Not all Outback models have grid-tie software, must have a G in the model #.

All Battery-Based systems use a Charge Controller between PV array and battery. We program the Controller to get the batteries up to 56 volts. We program the Inverter to hold the batteries at no more than 53 volts. The inverter grabs any excess electrons, turns them into AC, shoves them out to the grid. Normally, this internal transfer switch is closed. If Grid fails, switch opens (~ 30 milliseconds). Utility Grid is cut off, but Supported Loads still get power thanks to PV input & stored battery energy. All battery-based, grid-tied inverter brands use this system. (Outback, Xantrex, Sunny Island)

The internal transfer switch is a bottleneck. At 60A capacity, we can t (usually) backup the whole house. So pick your critical loads, and run them off the Critical Loads breaker panel on the Inverter s AC output. For those clients who insist on running EVERYTHING... Battery Backup for Grid-Tied PV Systems...a 15-20kW generator & a large tank of propane are a better (cheaper) solution. Available Direct Grid-Tied, Battery-Based Inverters Overview: Modest Systems: Outback Single GFX Delivers up to 3,600 watts @ 120vac. 60A transfer sw. Will start ½ or ¾ hp sub-pumps. Available pre-assembled. Outback Dual GFX Delivers up to 7,200 watts @ 120/240vac. 60A transfer sw. Will start ¾ or 1 hp subpumps. Available pre-assembled.

A max of 2 Outback GFX inverters per grid-tied AC system. Off-Grid, up to 10 FX inverters can be used in a single system. Larger Systems: Outback GS (Radian) Delivers up to 8,000 watts @ 120/240vac. 60A transfer sw. Will start up to 1.5 hp subpumps. Up to 10 inverters per system! No pre-assembly. More power, less wallspace. Xantrex XW Series 4.5 or 6.0kW units @ 120/240vac. 60A transfer sw. Will start 1.5 hp & larger subpumps. Up to 4 inverters per system. No pre-assembly.

3-Phase Systems? Possible w/ 3 Sunny Islands (18kW). But $$$!! Up to 12 Sunny Islands possible with SMA Multi- Cluster (72kW). $$$$$!! So we CAN do batterybased 3-phase, but it s large and very expensive! A backup generator is probably a better choice.

Battery Choices for Backup Going Big Isn t Smart 1-2 days of critical loads battery capacity is prudent. More than 2 days is foolish. Most outages are less than 4 hours. Batteries are expensive, and getting more so. Used or not, they need regular replacement. 7-10 yrs. typically. 12-15 yrs. for top-of-the-line. A generator & large tank of propane costs less for longer outages. What Kind of Batteries? Lead-Acid or Lithium-Ion? Lithium-Ion is perfect when you need light weight & small size. But it s still 4-5 times more expensive than lead-acid. So it s Lead-Acid. But wet-cell or sealed? And if sealed, AGM or Gel types? All lead-acid batteries need exercise to stay healthy. Sealed batteries, particularly AGM types, have their chemistry tweaked to tolerate inactivity. Wet-cell batteries suffer a short ugly life when put in backup service. Four 8D-size AGM batteries w/ 9.5kWh of useable storage works well for most systems.

Review Cost: $6-8k minimum. Can easily be more, depending on system size. Add-on packages are an easy upgrade for existing grid-tie systems. Add-on packages are easiest way to add backup to new grid-tie systems. Add-on PV size limited to ~ 8kW w/magnum, or 15kW w/ Sunny Island. Not ALL the client s PV has to be backed up. We can drop an inverter or two. All new system? Direct grid-tie, battery-based systems are lowest cost. Direct battery-based systems range 4kW to 80kW. 1-2 days of battery capacity is plenty. Use a backup generator for more days. 3-Phase battery backup possible, but expensive. Think remote islands or eco-resorts. These are typically off-grid systems. Please feel free to call us for advice, support, and design services.