0Bills Photovoltaic Batteries Information

The choice of battery type is not a simple decision with many different factors to take into account but we would always recommend that a comparison is made using the above considerations and looking at the total cost over the life of the system and not simply choosing the lowest initial cost option which in many cases may be more expensive over the life of the system.

Equally critical is the size of the battery with one too small providing insufficient benefit and one too large being a significant additional unrequired expense. Detailed below are some of the factors that need to be considered when determining the size of battery required:

Solar Battery Sizing

Attribute	Comments
Essential Load Energy Usage	For a grid-backup solution the most important thing to consider is the loads that need to be supported when the grid has failed. It is not generally practical to consider powering all the loads in the property, e.g. an electric oven, geyser and pool pump will all consume considerable amounts of electricity and would require a very large battery to run even for a short time. A good way to consider this is to generate a list of essential energy loads to be backed up and the amount of time they’re needed in a typical day. An essential load is basically something energy must always be available for. This could be something normal like a freezer or burglar alarm, or something site specific like a fish tank. If no power was available, would it lead to loss of (fishy) life or just defrosted ice cream? In the UK, power cuts are relatively rare but for more remote locations or other countries it is definitely worth considering. A lot of loads won’t require their maximum power all the time, so you can add a factor to take that into account. Once that’s done, you’ll have an accurate baseline of energy consumption and be able to consider the appropriate battery capacity. Load Power (W) Time Factor Daily (Wh) Lights 200 5 1 1000 Fridge 150 24 0.3 1080 Freezer 150 24 0.2 720 WiFi Router 10 24 1 240 Phones 50 1 1 100 Fish Tank 30 24 1 720 TV 170 4 1 680 Other 100 24 1 2400 Total 7690
Battery Operating Time	The next critical decision is to decide the number of hours that the system needs to power the essential loads for. Typically a planned grid outage due to load shedding will last for 4 – 6 hours whereas a failure due to a grid fault will typically last for between 1 and 24 hours. The decision on how many hours to allow for is largely driven by the budget available as the cost of the battery pack will be directly related to its size and its size will be directly related to the number of hours chosen. Usually a system will be sized to support the essential loads for between 12 – 24 hours.
Space Available	Especially when choosing a lead-acid battery the space available to hold the installed battery and the strength of the floor may be a consideration that imposes a limit on the maximum size of the battery that can be installed. With a Li-Ion battery this is unlikely to be a major concern as a Li-Ion battery will be much smaller and lighter than a similar usable capacity of lead-acid battery.
Charging Time and Rate	The battery will be charged from the surplus energy available from the PV system, this is the difference between the energy generated by the solar PV system and that used by the loads during the daylight hours. It is therefore important to ensure that the battery can be fully recharged during a typical day of sunlight, especially in the winter months. A battery pack which is too large relative to the PV system will not get fully recharged and therefore not be fully available to provide power in the event of a grid failure.
Maximum Depth of Discharge	Each battery pack will have a recommended maximum depth of discharge, e.g. lead-acid might be 50% and Lithium Ion might be 90%. Having determined the total energy required to be generated from the battery pack with the equation : ‘essential loads energy in 24 hours divided by 24 multiplied by the required battery operating time’ then the gross battery capacity needs to be determined by dividing by the recommended DOD. e.g. 7690W / 24 * 12 hours / 90% DOD = 4272kWh.

How to Equip a Grid-tied Solar System With Battery Backup?

There are three scenarios how you can add solar batteries to your existing solar panel system:

• AC Coupling: Splice your AC wiring to add a storage-ready inverter and batteries
• DC Coupling: Splice your DC wiring to add a storage-ready inverter and batteries
• Inverter Replacement: Replace existing inverter with a storage-ready inverter

We’ve noticed a surge of calls lately from people looking to add battery backup to their existing grid-tie solar system.

Many of these calls come from our home state of California, where PG&E has announced rolling blackouts to limit the impact of wildfires. With the prospect of scheduled blackouts looming, solar owners have been pushing to add battery backup to their systems to keep the lights on during grid outages.

Unfortunately, this process isn’t as easy as simply hooking up a new battery bank. Grid-tie inverters are designed to convert DC (direct current) from solar panels, but they are not designed to integrate with a battery bank. You’ll typically need to add new components to make your inverter work with your batteries.

It’s also not cheap. Batteries are the most expensive part of a solar system. Between an appropriately-sized battery bank and a battery-based inverter like the Outback Radian, you’re looking at something like 10 grand minimum to add batteries to an average-sized grid-tied system. (We wanted to make this really clear upfront, since people who call us often get sticker shock when we tell them the backup power package can cost more than the system itself!)

If you are concerned about recent blackouts and want the most cost-effective solution, your best bet may be a power generator. It’s going to cost less upfront, and it may be easier to pair it with your existing system because there are less restrictions on system sizing. A gas generator is usually large enough to back up most or all of your household, where an inverter and battery bank is usually sized to power only the essential appliances, because large battery systems can get expensive quickly.

Power generators have their own downsides: they are noisy, less environmentally friendly, require maintenance and a fuel source. But there is no question they are the most cost-effective option upfront.

Batteries have higher upfront cost, but are maintenance-free and much more versatile. The main appeal is storing and managing energy produced by your panels so you can recharge with solar during a long-term power outage. Another benefit of using batteries is that they can turn on and provide power to your home almost instantly, usually under 1 second and without any interruption to your appliances. A gas generator will take a few minutes to start the engine, warm up and begin providing power.

If you do decide that battery backup is the way to go for you, this article covers the 3 approaches you can take to get it done:

1. AC Coupling
2. DC Coupling
3. Replace grid-tie inverter with hybrid inverter (storage-ready inverter)

Scenario #1: AC Coupling

Grid-tied inverters need the power grid to operate—they constantly sense grid voltage and frequency and will shut off if it falls out of range.

In an AC coupled system, the grid-tied inverter is paired to an off-grid inverter and battery bank. The off-grid inverter provides a second power source, which effectively tricks the grid-tied inverter into staying online. This allows you to charge your batteries and run essential appliances during a power outage.

The best option for AC coupling is the Outback Radian. The newest firmware supports frequency shift AC coupling, which will work with any inverter certified to EC or UL 1741 standards.

This feature causes the off-grid inverter to shift its frequency to control the output of the grid-tied inverter. The Radian limits the power coming in from the solar array when needed to prevent overcharging the batteries.

Here are the basic sizing guidelines for picking an inverter:

• The Radian should have at least 25% higher nameplate capacity than the grid-tied inverter.
• The GS8048A can AC couple with grid-tied inverters rated up to 6 kW (5 kW max for Fronius inverters)
• The GS4048A can AC couple with grid-tied inverters rated up to 3 kW (2.5kW max for Fronius inverters)
• Requires MATE3s remote with updated firmware for both the inverter and remote

See the Outback site for more info on using AC coupling to add battery backup to an existing grid-tied system.

Pros of AC Coupling

This is the easiest way to retrofit your system, especially a microinverter system. The battery bank connects to the Radian, which is installed between the grid-tied inverter and your load panels. The existing grid-tied inverter does not need to be removed.

Cons of AC Coupling

Strict guidelines for inverter and battery size make the process of sizing the addition a challenge. The system will perform poorly or not work at all if the inverter or battery bank are undersized. In addition, if the existing grid-tied inverter is large, an AC coupled system can get very expensive.

Compatible With:

• Most grid-tied inverters on the market

Scenario #2: DC Coupling

In a DC-coupled system, the solar array is connected directly to the battery bank using a charge controller.

This is how off-grid systems work, and it could be done to a grid-tied system if they are using a 600-volt string inverter. This works with the SMA Sunny Boys, many Fronius inverters, or any other 600-volt string inverter.

This Morningstar 600-volt charge controller is designed to retrofit grid-tied systems with batteries. It can be combined with any one of our pre-wired power centers that doesn’t have a charge controller.

The 600V charge controller would be installed between the existing PV array and your grid-tied inverter. It includes a manual switch to switch between grid-tie and off-grid modes. The downside of this method is it can’t be programmed—the switch has to be physically turned to start charging the batteries.

The battery-based inverter can still automatically turn on and power your critical appliances, but the PV array won’t charge the batteries until the switch is turned. So, you have to remember and be on site to turn on the solar charging. Otherwise, you might find your batteries are drained and you won’t be able to recharge from solar.

Pros of DC Coupling

In comparison to AC coupling, DC coupling works with a broader range of off-grid inverters and battery bank sizes.

Cons of DC Coupling

The manual transfer switch means you have to be available to initiate the PV charging. If you forget or aren’t there, your system will still provide backup power, but the battery bank won’t recharge from solar until someone manually flips the switch on the controller.

Compatible With:

• Most residential string inverters rated for 600 Volt max input

Scenario #3: Replace Your Grid-Tie Inverter With a Hybrid Inverter

The last option is usually the most expensive: you can remove your existing grid-tie inverter and replace it with a storage-ready inverter instead.

This approach is going to be the most flexible option—it works for all existing grid-tie systems. There are a handful of inverters on the market designed specifically to accommodate energy storage for grid-tie systems:

• The Fronius GEN24 can replace many SMA, classic Fronius and other 600V-1000V string inverters
• The StorEdge or the SolarEdge Hybrid can replace standard SolarEdge inverters
• Microinverters would need to be removed and replaced with any hybrid or storage-ready system

Ideally, you want to replace your existing inverter with one that is about the same size and can use the same array wiring.

In many cases, this solution is preferable to adapting your existing system with AC or DC coupling because these inverters are designed from the ground up with energy storage in mind. They include some cool features, like storing energy and selling it back to the utility during peak time-of-use (TOU) periods to take full advantage of your local net metering policy.

This approach is tough with micro-inverters because it takes more work to rip out the old ones and retrofit every panel with a replacement. The labor is a bit more expensive and time-consuming, so an AC-coupled solution is often a better alternative for microinverter systems.

Pros of Replacing Your Inverter

• Works for any system
• Storage-ready inverters come with additional features

Cons of Replacing Your Inverter

• Most costly option, especially for microinverters

Compatible With:

• All systems

Wrapping Up

Retrofitting solar systems with new equipment can be tricky because of all the different equipment options and various methods for incorporating energy storage. New equipment can change the electrical characteristics for the entire system, and that could introduce faults if the components are not designed to work with each other properly.

If you need help picking the right products to add battery storage to your existing grid-tied system, drop us a line. We have been designing solar panel systems since 2007 and have more than 3,000 solar battery systems in our reference list. Call us +44 333 772 0506 or +1-786-600-1814 or fill out this form to request a free consultation with a member of our design team. We’re happy to help you work out the details.

ZEROhomebills.com offers a range of batteries and self consumption solutions.