Backup Power for Grid-Tied PV Systems

Intermediate

Inside this Article

Lit house showing backup power
Backup Power for Grid-Tied PV Systems
A small UPS
An uninterruptible power supply (UPS) provides fast, automatic backup, but is usually for smaller loads (like computers) and short periods of time.
SunnyBoy with SPS
SMA America Sunny Boy batteryless inverters with secure power supply (SPS) can provide up to 1.5 kW, but only during sunny conditions.
Small generator
A small gasoline generator may be an easy and inexpensive backup power option, but it doesn’t take advantage of an existing PV array’s output.
Larger, whole-house generator
A larger generator may be a permanently installed unit, with natural gas or propane fuel piped to it. Autostart is an option, but won’t activate online fast enough to keep computers up.
DC-coupled battery backup
This unique DC-coupled system splits the PV array to feed a battery backup system and a grid-tied-only system. An OutBack battery-based inverter (upper left box) is connected to four 12 V AGM batteries (lower left box) that provide 5 kWh of storage; a Xantrex grid-tied-only inverter is mounted to the right. This solution can be less costly than having battery backup for the entire array.
Morningstar controller
High-voltage MPPT charge controllers allow PV arrays originally wired for grid-tie-only to be integrated with a battery-based system by reducing array voltage to match battery voltage.
Schneider controller
High-voltage MPPT charge controllers allow PV arrays originally wired for grid-tie-only to be integrated with a battery-based system by reducing array voltage to match battery voltage.
AC-coupled battery backup
This classic AC-coupled system consists of 10 kW in the two gray SMA America Sunny Boy inverters; 8 kW in the Sunny Island inverters (yellow); and 10 kW of sealed AGM batteries (not shown) for storage.
Large AC-coupled backup system
This large AC-coupled battery backup system includes 26 kW of PV power connected to five Sunny Boy inverters (not shown) and four Sunny Island inverters (shown below).
AC & DC combo backup system
This combo DC- and AC-coupled system has 5.4 kW that are DC-coupled through an OutBack Radian inverter; 6 kW are channeled through an AC-coupled inverter (not shown). MidNite charge controllers serve the DC-coupled system, while Morningstar controllers operate a diversion load for the AC-coupled system (protecting batteries from overcharge). Surrette flooded lead-acid batteries provide about 34 kWh of energy storage.
AGM, 2-volt batteries
Sealed absorbed glass mat (AGM) batteries like these Full River 2 V (1,150 Ah each, for 55 kWh of storage at 48 V) are maintenance-free.
NiFe batteries
Nickel iron batteries are an old technology experiencing a renaissance. Their lower efficiency and higher upfront cost are offset by their longevity, making them possible candidates for backup systems.
Flooded lead-acid batteries
This 3,280 Ah battery bank of 32 Surrette flooded lead-acid batteries provides about 158 kWh of storage at 48 V—whole-house backup for a large on-grid home.
DC-coupled battery backup
A simple DC-coupled battery backup system can be straightforward to install, and most economical if battery backup is installed as part of the original PV installation. Here, a Xantrex 6 kW inverter is powered by a 4 kW PV array. Eight sealed AGM batteries in the cabinet provide about 10 kWh.
Lit house showing backup power
A small UPS
SunnyBoy with SPS
Small generator
Larger, whole-house generator
DC-coupled battery backup
Morningstar controller
Schneider controller
AC-coupled battery backup
Large AC-coupled backup system
AC & DC combo backup system
AGM, 2-volt batteries
NiFe batteries
Flooded lead-acid batteries
DC-coupled battery backup

A grid-tied PV system is great at reducing your electricity bills with clean on-site energy generation. But when the grid goes down, a standard grid-tied inverter also shuts off, leaving you without power, even with a PV-covered rooftop. If you want backup power, there are several options—it just depends on your needs.

Identify Your Needs & Loads

What kind of power outages are common where you live? Are they short power blips that occur several times a day for a second or two, and make all of the clocks reset, or do the outages last a few minutes to a few hours? How often do they occur? Perhaps there are days- or weeks-long outages that occur only every few years during major storms.

What do you need backup power for? Do you want power to back up all of your household loads, or just enough to charge a few cell phones and keep the fridge cold? If you buy a generator for backup power, it may not cost much more to back up the entire house than a smaller selection of loads. For example, the installed cost of a 15-kilowatt (kW) generator is not too much more than for a 5 kW generator—perhaps $8,500 versus $6,500. What does cost more is fuel for a larger generator. Another option is to install a backup battery bank. Having whole-house battery backup can be very expensive compared to having a smaller battery bank to back up a subpanel of “critical loads,” which are commonly lighting, refrigeration, well pumps, and communications devices (cellphones, computers, and modems).

Before you invest in an electrical backup system, consider non-electric options. It is usually much cheaper to switch to propane or natural gas for heat-related appliances (stoves, space heaters, water heaters, etc.) than backing up the electric ones with a large battery bank. For example, backing up a 1,500-watt electric space heater to run for a three-day power outage during an ice storm will cost upwards of $20,000 in battery storage alone. An efficient wood or propane heater would cost just a fraction of that.

An uninterruptible power supply (UPS) can be an option for smaller equipment for short power outages. These self-contained plug-in units usually store 15 minutes to an hour of energy for a computer or other small electronics, and switch on automatically—quickly enough to keep computers running so you don’t lose data. If typical outages in your area are short blips that cause all of your computers and modems to reset, then a UPS can be an inexpensive solution. Price: $50 to $500.

Secure Power Supply (SPS). During a utility outage, SMA America’s line of SPS transformerless inverters provide up to 1,500 W of 120 VAC (without requiring a battery bank), dependent upon how much the PV array is putting out at the time. With its dedicated outlet, you can charge cellphones, run other small electronics, and possibly have enough power for a refrigerator.

There are two main disadvantages. The first is that operation requires flipping a switch, then plugging the desired load into a special outlet. The second is that, during the night, there’s no backup power. On the plus side, this feature only requires a little extra wiring. The major advantage is that there are no batteries to pay for, maintain, and eventually replace. This inverter is available in seven different capacities, ranging from 3 to 7.7 kW. The additional cost for labor and the SPS outlet ranges from $100 to $300.

Generator—Autostart or Portable. A fossil-fuel-powered generator can be a good fit in many cases. Generators are well-suited for powering large loads, and can be less expensive than a large battery bank. The drawback is engine maintenance, dealing with generator emissions, noise, and fuel cost. Fuel availability can also be an issue—when a major storm strikes, it may be difficult to get fuel.

Generators can range from a small portable generator used with an extension cord to power a few electrical loads to a large, automatically starting generator with an automatic transfer switch. With this approach, your PV system is still not being tapped during an outage, so all of that potential energy is being wasted.

Portable engine-generators can cost as little as $200 for a 1,000-watt unit; higher-power (10,000 W) and higher-quality ones start around $2,000. The cheaper ones are suitable for expected use of a few hours a year; choose a higher-quality one if you expect to be using it for a few days a year or more. Portable generators exhaust carbon monoxide and other harmful pollutants, and can be very dangerous if run indoors—even in an open garage, which is a temptation if it’s still raining or snowing outside. They should also never be connected to the home’s permanent electrical system unless proper transfer switches are used, since they might shock or even electrocute utility workers trying to repair power lines.

Just as with inverters, generators come in many different power sizes. Portable generators start as small as 600 W, which can run a small fridge, a computer, and some electronic devices, and go up to around 15 kW, which could run most appliances in an entire house, including air conditioning and well pumps, if a suitable transfer switch is used. A small one might run for 8 hours on just 1 gallon of gasoline, while a 15 kW unit might use 15 to 20 gallons of gasoline for that 8 hours. Permanent residential backup generators range from around 6 kW to more than 20 kW.

Permanently mounted generators that hook up to propane or natural gas lines for fuel and have automatic transfer switches can range from about $6,500 installed cost (for a 5,000 W unit) to $10,000 or more for higher-quality ones in the 15 to 25 kW range. Quality does not vary as widely here as for portable units, but still remember that the cheapest ones are only designed to provide backup power for a few hours a few times a year—not to run several hours a day for many days straight.

Battery Backup. A grid-tied PV system with battery backup can operate as an off-grid PV system. The inverter rapidly responds to power outages, switching quickly enough that computers don’t go offline, so most people won’t even notice the grid is out. The transfer time is much faster than an auto-start generator, which will take several seconds to start and come up to power delivery speed.

In this type of system, when operating in backup mode, there are no fuel costs—so long as the array receives enough sun to recharge the batteries each day, it can operate indefinitely. Of course, when outages occur during winter storms, large loads and many sunless days can deplete your battery bank.

The primary limitation of this approach is the size of the battery bank and inverter. If you have large loads, you need lots of batteries and more inverter capacity, which can greatly increase the system cost.

There are two options for adding batteries to an already-existing grid-tied PV system that don’t necessitate rewiring the array. In either case, you’ll have to add a battery-based inverter and connect the existing PV array to the new system components.

The first option is to use a DC-coupled system, which uses a high voltage charge controller to convert the high-voltage DC generated by most grid-tied PV arrays down to the lower voltage (24 or 48 V) common for battery-based inverters. The second option is to use an AC-coupled system that uses the battery-based inverter’s output to provide the AC needed to keep the grid-tied inverter operating. The grid-tied inverter gets connected to the backed-up critical load panel. An AC-coupled system needs a way to keep batteries from becoming overcharged during a utility outage, since there isn’t a charge controller to regulate energy coming from the PV array. The method depends upon the chosen equipment—see “Adding Battery Backup to Your PV System with AC Coupling” in HP168.

The additional cost for a battery backup PV system (compared to a grid-tied-only PV system) can range from $3,000 to tens of thousands, depending on how many loads need to be backed up, and for how long. For a two-day-long outage without sun, a backup system with 4.5 kW of 240 VAC inverter capacity and 15 kWh of storage capacity using sealed AGM batteries­—which could back up a fridge, well pump, computer and modem, and a number of LED lights, as well as some miscellaneous small appliances—would cost from $10,000 to $14,000.

Battery Backup, Plus Generator. Adding a generator along with a bank of batteries provides another backup method, and reduces the size of the battery bank and inverter capacity needed. This can be especially helpful during sunless periods by keeping the batteries from being overly discharged. Fuel usage during long outages is less than with a generator-only system—even if there is no sun, the generator only has to run a few hours a day to recharge batteries, rather than running 24 hours a day to serve loads. Typically, there is at least some solar input contributing to battery charging.

What’s It Worth?

A common method of assessing the economic advantages of a batteryless grid-tied PV system is to figure the time that it takes to recuperate initial costs. Computing payback for a backup power system can be a little more complex. Adding backup capability to your grid-tied system will cost more (sometimes a lot more) than a batteryless system and, since it uses the same PV array, it won’t generate any more energy. What it does generate is reliability and resilience. If you are running a business, you may be able to come up with a value for lost sales, lost data, or lost perishable merchandise. But valuing reliability at a residence can be more difficult. Spoiled food in the fridge might have a value, but what is the exact monetary value of having lighting at night or the security of communications?

Comments (6)

Greg Smith_0_0's picture

Whole home anything is expensive. If the homeowner can size the battery system to backup sensible loads like cell phones, laptops, fridge, lighting, etc., then a smaller battery system can be used along with the solar inverter to have near perpetual backup. Same thing with a generator.

The problem is that people do not want to change their lifestyle when the grid goes down and therefore require larger backup/generator systems, which cost more.

Frank Heller's picture

Actually, there are a growing number of dedicated solar-battery appliances which are reliable. They include the now common sidewalk lighting; simple security systems using horns and lights; and vehicle battery chargers. I live along the coast and cruising sailboats have engineered these systems for a long time. You may want to visit a large marina and see what they have for solar and even hydro & wind power backups, see Sail-Gen water turbine for one example of the many out there.

Frank Heller's picture

Backup battery systems sized to the amp load are more convenient than ever

A more than adequate standby generator in the 10- 15 kW range, installed, running on natural gas, installed and maintained locally is now fairly inexpensive.

The tricky part is sizing it to your load. GE has an on-line tool to determine your load.

You also want to make sure you have a transfer switch which cuts off the utility power when it fails and turns on your generator that is compliant with their requirements.

It is also possible to fill a battery from solar panels and draw upon it first and then turn on the generator when the battery falls below a certain level. A bit more complicated, but it can be done.

Robert Georgantas's picture

The estimated prices for a backup generators are MUCH too high. A whole house 16KW running off utility natural gas cost only about $6000. Of course, if you need to have gas tanks and piping installed, or a natural gas line run to the house, the price would be much higher to install. But in most cases homes will already have gas service.

Greg Smith_0_0's picture

Great article, Zeke. It is always good to see those pretty yellow inverters, although I do know of another company with an all-in-one solution for battery backup/off grid :)

Frank Heller's picture

Hi...this can get complex when going off the grid. I just assessed a large--4 kW system that powered a lodge. It included a non-working hydro power plant, a new solar PV array and a large generator all tied to a large 24 volt Surrette battery bank through an expensive Outback set of inverters and controller.

Lack of an operation's manual and poor maintenance created a lot of problems when restoring the hydro as an active component....the propane generator would run at night and diminish the 'wilderness experience'.

Balancing solar and hydro input was tricky since the solar was installed after hydro failed(turgo fins broke off); and AC feed went into a controller and then into batteries. Solar came with a $12 k Outback system of 4 inverters, shunt, etc.

Basically, I concluded that what this organization wanted/needed was an 'on/off' switch, and regular, perhaps off site electronic monitoring and maintenance. Interesting they sort of had one with a large manual discharge butterfly valve for the hydro, shut it and water doesn't flow, i.e. no power.

Have to balance system and provide for load shedding, both water and air heaters were used; and batteries as they age need to be monitored; getting 15 years is desired, but off-grid replacement is expensive for these large batteries. Just be mindful of the long range implications of whatever you do and who is going to maintain a complex hybrid system.

Show or Hide All Comments

Advertisement

X