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Info: Grid Tie with Battery Backup

STRAIGHT GRID TIE
or
GRID TIE WITH BATTERY BACK-UP
WHICH ONE SUITS YOU BEST? 

As grid outages become more common, there is a growing desire for solar electric systems that provide power during utility power outages. For the safety of line workers, grid-tied solar electric systems must not feed power back into the utility lines during an outage. Battery-less inverters simply shut down when the power goes out. Grid-tied battery based inverters include a transfer switch to isolate them from the grid and use a battery bank to supply power to loads designated for backup and connected to a separate output from the inverter.

 

There are several ways to design and deploy inverters with backup capability. Means of connecting the backup loads to the system can range from simple to complex. Backup power can be arranged for longer or shorter times depending on the loads and battery bank capacity.

 

DC coupled PV system

The most common arrangement for grid-tied solar with backup is where the PV array powers a DC charge controller to charge a battery bank. Whenever the battery is fully charged, the voltage rises to a set point and any power available over that voltage is available for use. The inverter output is connected to both the backup (aka critical loads) sub-panel, and to the grid inter-tie in the main electrical panel. Under normal conditions, it functions like any other net metered inverter. During an outage, an automatic transfer switch in the inverter opens the grid interconnect, isolating the building from the utility to prevent backfeeding the local power lines. The inverter then draws from the battery bank to provide AC power to the backup sub-panel.

DC coupled PV system

The term “DC coupled” has only come into use recently to distinguish this configuration from “AC coupled” which is a newer arrangement that has come into use. When starting from scratch, a DC coupled system is typically the most cost-effective and reliable configuration to use. Note however, that the PV array is not managed by the inverter, but by the charge controllers, many of which are limited to 150 VDC input. Inverters that can be used for DC coupled grid tied systems include the OutBack GVFX, GTFX, and GS (Radian) series, the Schneider Electric XW series, and the SMA Sunny Island series. Any charge controller can be used with a DC coupled system so long as it is compatible with the solar array and the battery.

Grid-tie solar AC coupled

In an “AC coupled” system, a battery-less grid-tie inverter is connected to the solar array, while a battery based inverter is used to produce power from the battery bank during a utility outage. In this configuration, the AC output of the grid-tie inverter is connected to the backup (aka critical loads) sub-panel rather than the building’s main panel. The backup sub-panel is also connected to the AC output of the battery inverter. The AC input of the battery inverter is then connected to the main panel as in a DC coupled system.

 

Under normal conditions, the battery-less grid-tie inverter is passed through the battery based inverters built-in transfer switch, to the main panel and utility inter-tie, without loss of efficiency. Likewise, grid power can pass the other direction when needed to power the loads in the backup sub-panel. During a power outage, the battery inverters transfer switch isolates itself from the utility connection and provides AC power to the backup sub-panel, drawing energy from the battery bank. The battery-less grid-tie inverter will shut down at the start of a blackout, but will turn back on (after a mandatory 5-minute waiting period) when AC power from the battery inverter is detected and supply AC power to the backup sub-panel, and if enough power is available, will be used by the battery inverter to charge the batteries. Note that once the battery bank is fully charged and the loads are served, the battery-less grid-tie inverter will need to be throttled back, diverted or shut down in order to prevent damage to the batteries.

 

An AC coupled system offers a few advantages compared to the traditional DC coupled system. The battery-less grid-tie inverter can use the higher voltage from the solar array, reducing the required wire size, and is generally more efficient than a battery based inverter. Additionally, if a direct girdtie system is already installed and battery backup is added later, it is often more convenient and cost effective to leave the existing system in place. Countering the advantages, AC coupled systems are typically much more expensive and complex to design and only certain inverter technologies can be used in such systems.

Battery backup separate from solar

It is, of course, possible to have a battery backup system that is separate from the solar grid-tied system. In parts of the world with frequent short outages, many people just install a battery based inverter and battery bank without any PV whatsoever. Similar to a UPS, the battery is kept charged by the inverter from the utility AC power, and during a power outage will supply power from the battery to AC loads. This  type of system has no way of being recharged during an outage. The Schneider TR2424 is an ideal lo cost modified sine wave inverter that can provide emergency power from batteries. The simplest of installations can use extension cords (temporary applications). There is no wiring required because during normal times a wall plug provides the power to charge the batteries. The number of back-up batteries will determine how long power will last. Solar panels can also be added to charge the batteries.

Battery backup separate from solar

Backup / critical loads considerations

The most common and accepted method of connecting backup loads is through a backup subpanel that serves specific circuits. The backup sub-panel is typically installed next to the main panel so that the circuit wiring can be easily transferred from the main panel to the sub-panel. These circuits are then automatically powered through the backup system during a utility outage. Note that only complete circuits can be wired this way, not individual appliances. Appliances can occasionally be moved from other circuits onto a backed up circuit or loads can be moved off of backed up circuits if they are not needed during an outage. If it helpful for the end user, unique styles or colors of receptacles can be installed on backed-up circuits.

 

It is possible to have a backup system power an entire home or building if either the backup system is very large or the loads are unusually low. This is not often done because it requires a backup system large enough to power all loads in the house. Otherwise, the loads running when the outage occurs may overload the inverter, effectively defeating the backup system’s purpose. It also introduces technical and potential legal complexities as a larger transfer switch and line side tap are typically required.

System sizing – Backup power duration

For a grid-tied battery based system, the sizing of the solar array is much the same as sizing it for a non-battery grid-tie system. The limitations on the solar array are still: available installation area, maximum offset of utility power, and budget. Often a solar array sized for grid-tie will be more than large enough during a power outage. For AC coupled systems, the battery based inverter typically needs to be larger than the battery-less grid-tie inverter for best results. 

 

The battery bank size is determined by the length of time and the size of the loads to be run during an outage. Backup systems can be made to supply power for minutes, hours, or days. If outages are brief or if a backup generator is planned, the battery bank only needs to be large enough to carry the loads for a few hours. Most often, a backup time of one half day or multiple days is desired. If there is not much sun, a generator can be run for a few hours per day to keep the battery bank charged. If the system is to be capable of running indefinitely, it should be sized like a true off-grid system.

Schneider Electric Conext XW

The Schneider Electric Conext XW inverters output 120/240 VAC in each unit and up to four inverters can be wired in parallel for up to 24kW of power. The XW inverters have two AC input circuits, one for the utility connection and one for a backup generator. Both connections are 120/240 VAC. Available integration panels and accessories from Schneider Electric and/or Mid-Nite Solar and help simplify design and streamline installation.

 

Conext XW inverters can be used in AC coupled systems, and recent versions include a frequency shift feature for control of the grid-tie inverter for battery charging. When the frequency is shifted out of specification, it causes the grid-tie inverter to drop off line. The capacity of the grid-tie inverter should not exceed 100% of the XW inverter capacity.

Batteries

A battery banks are typically 225 A-hr or larger. A 48 volt, 225 A-hr battery bank, when adjusted for inverter efficiency and maximum 90% depth of discharge, will provide about 10 kWh of energy over a 20 draw down hour period. The battery bank must obviously be sized to serve the loads, but also must be able to accept the highest possible charge current from the PV array if it is not otherwise regulated. We recommend Trojan flooded lead acid batteries with water miser caps rather than sealed and Power Pulse desulphator for the best value.

Solar Energy

 

What is solar energy?

A solar energy system creates usable power from sunshine. There are two basic kinds of systems: Photovoltaic or PV uses sunlight to generate electricity. It’s the same technology found on pocket calculators, just on a larger scale. PV systems can be designed to generate the majority of the electricity used in your home, or just a portion of it.

Does solar work only on sunny days?
Solar systems work even when it’s cloudy. Naturally, cloud cover reduces the amount of solar radiation reaching the panels, but the system will produce some electricity and/or heat on all but the most overcast days.

How does solar help the environment?

Fossil fuels are a leading cause of global warming and air pollution. Solar energy reduces the amount of fossil fuel that is burned, thus reducing the pollutants and CO² that get into the atmosphere. Solar energy systems that are recommended by Solar Energy World have the added benefits of very high efficiency, durability, and service life – they’re engineered for sustainability. This means fewer replacements and repairs, saving even more energy and precious resources.

How long will my system last?
If properly installed, it should last 30-40 years. Systems that were installed in the 1970’s are still fully operational today. Technology has evolved so the systems from the 1970’s may not be as efficient as today’s technology.

How does a solar energy system benefit me personally?

In many ways:

  • It reduces your energy bill, and in many areas the utility will buy back any extra electricity you generate.

  • Federal, state, and utility financial incentives are also available in many areas.

  • It adds to the value of your home.

  • You gain energy independence.

  • Perhaps most importantly, it’s a good investment in a sustainable future for yourself and your loved ones.

 

How can solar help the economy?

Right now the US has to import oil and natural gas to cover our energy needs. The cost adds up to many billions of dollars, and all of that money leaves the country. But solar energy is generated locally. The energy dollars stay at home, creating economic growth and benefiting your community.

How does electricity buyback work?

This is often called “net metering” and is available from many utility companies. If you generate more electricity than you use, the excess goes back to your utility company, spinning the meter backward and giving you a credit for the electricity your PV system generated.

If the power goes out, will my solar system keep making energy?

No, for safety reasons, your solar system will automatically shut off if the power goes out.

What regular maintenance do I perform
With systems that we recommend, very little. PV systems are inherently very lowmaintenance, requiring the system owner only to wash the solar modules down with water when they get dirty so light can get through.