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

• 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 DIY 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. In a DIY solar scenario 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 DIY 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.

Diesel or Petrol 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 power 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 – DIY Solar

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 DIY solar 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 – DIY Solar

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 – DIY Solar

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.

Investing in a solar system is a smart solution for homeowners. The latest solar panels and photovoltaic (PV) systems are easy to install, maintain, and operate, with long-term performance and energy savings.

Getting Started with DIY Solar System Sizing

Before you begin to size a solar system, you’ll want to figure out the main constraints on the project and use those restrictions as the starting point for the design. You can approach the project from one of three angles:

• Budget constraints: Build a system within your target budget.
• Space constraints: Build a system that is as space-efficient as possible.
• Energy offset: Build a system that offsets a certain percentage of your energy usage.

Estimating of Your Energy Usage

Before you begin to size a solar system, follow these steps to determine your home’s average electricity consumption and PV needs:

1. Calculate Your kWh Usage

1. Gather the kilowatt-hours (kWh) usage from your electric bill. You’ll want to have full 12 months of usage to be able to look at peaks and valleys in usage over a year. Energy consumption spikes in the summer and winter with heavy use of your A/C and heating units.
2. Determine your average monthly kWh usage. Add up your kWh usage for 12 months and divide by 12 to figure out your average monthly consumption. Your grid-tied system will tend to overproduce in the summer with peak sun exposure.
3. Figure your daily kWh usage. Divide by 30 to determine your daily kWh usage.

To determine your home’s energy usage more accurately, use our home appliances power consumption table to find out how many kWh your appliances would use per month.

If your utility provides a favorable net metering policy, the energy your system generates can be banked with the utility as a credit that can be used later. Not all utility companies give you credit; check with your local provider.

2. Look Up Your Peak Sun Hours

Average peak sun hours vary greatly depending on your location and local climate. You’ll want to determine how may peak hours of sunlight you’ll get so you can make the most of the solar power:

1. Look up your peak sun hours, through a sun hours chart to determine the number of hours per day the sun produces peak sunlight.
2. Find the nearest city to you and write down the daily average of peak sun hours.

3. Calculate the Size of Your DIY Solar System

To figure out how to size your solar system, take your daily kWh energy requirement and divide it by your peak sun hours to get the kW output. Then divide the kW output by your panel’s efficiency to get the estimated number of solar panels you’ll need for your system.

For example, if you live in New Mexico, you average six peak sunlight hours per day. You’ll need 6.2 kW DC according to the formula:

Using the example above with a 6.2 kW DC system, you can multiply this number by 1,000 to confirm that you need 6,200 watts of solar panels.

Fine-Tuning the Estimated DIY Solar System Design

To make the solar system sizing estimate as accurate as possible, you’ll also want to take into account the type of roof mount you’ll need, the direction your panels will face, and the appropriate size panels to fit your design.

1. Select Your Mount Type

A roof mount is the simplest and most cost-effective solution since it costs less than other racks. To determine if you can use a roof mount:

2. Choose the Right Solar Panels

If you have a small or odd-shaped roof, solar panel size is an important consideration when deciding on the size of a solar system. Take these factors into account:

• With a large usable roof area, you can buy more larger panels (at a lower cost per panel) to get to your target energy output.
• If your usable roof area is limited or partially shaded, using fewer smaller high efficiency panels will ultimately be the most cost-effective, long-term solution. You can add more panels later on to accommodate increased energy needs.

3. Calculate Solar System Output

Once you know how much area you have for solar panels, and what angles and direction you will be working with, use a PV watts calculator to figure out how much power your system will put out on a monthly basis:

1. Enter the address and hit the orange arrow to the right.
2. Once you are on the System Info page, enter the DC system size from the previous section.
3. Choose a standard module.
4. For array type, select “fixed” for roof mounts, or “open” for ground mounts.
5. Leave the system losses at around 15%.
6. Enter the slope of your roof in degrees, and the azimuth. Azimuth is the degrees relating to north and south, with north being zero and south being 180.

Click the arrow to the right to show your monthly solar system output. Once you know what size solar system you need and system output, you can cross-reference that with the amount of space available to fine-tune your solar system sizing assessment.

Choosing Grid-Tie DIY Solar Equipment

A fast resource for selecting your grid-tie solar is through our grid-tied solar packages. Here are a few viable options to consider after you size a solar system. Note that the imported panels are more cost-effective, so you get roughly 10% more production for the same price.

Grid-tie DIY solar systems with EU-made panels:

• 6.2 kW system with 310W Mission Solar panels and SolarEdge inverter/optimizers
• 6.2 kW system with 310W Mission Solar panels and Enphase IQ7+ micro-inverters
• 6.2 kW system with 310W Mission Solar panels and SMA central inverter

Grid-tie DIY solar systems with imported panels (China):

• 6 kW system with 335W Astronergy solar panels and SolarEdge inverter/optimizers
• 6.7 kW system with 335W Astronergy solar panels and Enphase IQ7+ micro-inverters
• 6.7 kW system with 335W Astronergy solar panels and SMA central inverter

Grid-tie DIY solar systems with USA-made panels:

• 6.2 kW system with 310W Mission Solar panels and SolarEdge inverter/optimizers
• 6.2 kW system with 310W Mission Solar panels and Enphase IQ7+ micro-inverters
• 6.2 kW system with 310W Mission Solar panels and SMA central inverter

If you’re having trouble deciding which products to buy, check out these articles covering that ground as well:

• Best solar panels
• Best grid-tied solar inverters

Of course, sometimes it’s easier to talk to someone with experience and have them walk you through the design process. The fastest way to get a thorough evaluation of your solar needs is to call us at +44 333 772 0506 or +1-786-600-1814 and connect with one of our designers. We’d love to help you design the perfect grid-tied system for your needs.

Need Help with DIY Solar System Sizing?

Of course, sometimes it’s easier to talk to an expert who knows how to size a solar system and can walk you through the design process. Once you’re ready, we do encourage you to schedule a free design consultation with us so that we can double-check your sizing, find compatible products, and ensure the system works within your constraints (budget, build space, and energy offset).

The fastest way to get a thorough evaluation of your solar needs is to call us at 1-786-600-1814 and connect with one of our designers. We’d love to help you design the perfect grid-tied system to meet your solar requirements.

Renewable energy sources like solar and wind are ideal for powering equipment in remote locations. In this article, we’ll outline a step-by-step process for sizing an off-grid solar system so you can generate electricity even when you’re miles from the nearest power line.

Step 1: Determine Energy Requirements for a DIY Solar System

First, you need to know how much energy the equipment uses on a daily basis. This is measured in watt-hours or kilowatt-hours per day. For example, let’s assume the equipment consumes 10 watts of power and operates 24 hours a day:

10 Watts x 24 hours = 240 watt hours per day or .24 kWh per day

How do you find this information? Check the data sheet or manual for your equipment to find out how much power it consumes (in Watts), and then multiply that by the number of operating hours per day. If possible, use a meter to measure the power consumption for an accurate real-world measurement.

If you are using an inverter to produce AC power for a load, remember to account for the inverter’s self-consumption and efficiency losses. Inverters consume a small amount of power while they are operating. Reference the inverter spec sheet, and add the self-consumption to your daily total. Inverter self-consumption typically ranges from under 1 watt, to around 30 watts depending on the inverter.

Efficiency losses can be from 5% to 15% depending on the inverter and how much it’s loaded. This will be accounted for when sizing the batteries. It’s important to invest in a quality, high-efficiency inverter.

Step 2: Evaluate Site Location

Next, determine where the system will be installed to estimate available solar energy.

Use a solar insolation map (also called a ‘sun hours map’) to estimate available PV resources. The system should be sized based on the month with the highest power consumption and/or lowest solar resource, typically December or January.

The National Renewable Energy Laboratory (NREL) has an online resource for mapping available solar radiation. Here is a map of the minimum daily sun hours in January in the United States with a south-facing array:

Most of the US has fairly low solar insolation in January. Generally, 2.5 sun hours is a good estimate, but it could be lower or higher depending on your location. We will use 2.5 minimum sun hours for our example.

Solar panels are designed to be installed in full sun. Shade is going to impact performance. Even partial shade on one panel will have a large impact. Inspect the site to make sure your solar array will be exposed to full sun during daily peak sun hours. Keep in mind that the sun’s angle will change throughout the year.

Other Considerations for DIY Solar

There are a few other things to think about at this point:

System voltage: Determine what power requirements your equipment has. Off-grid PV systems typically output these common voltages: 12Vdc, 24Vdc, 48Vdc, or 120Vac.

Solar panels and batteries use DC power, and some equipment can be wired directly to the batteries provided it can handle real-world battery voltages. These can range from 10-15 volts for a 12-volt system, 20-30 volts for a 24-volt system, and 40-60 volts for a 48-volt system.

Days of autonomy: The number of days the equipment must operate on battery power with limited solar power. Between 5-20 days is typical, depending on the area and expectations for operating performance. You need enough autonomy to keep the equipment operating through extended periods of overcast weather.

Step 3: Calculate Battery Bank Size for a DIY Solar System

Now we should have enough information to size the battery bank. After the battery bank is sized, we can determine how much solar power is required to keep it charged.

Here is how to calculate battery bank size in our example of 240Wh/day based on lead-acid batteries:

First, we need to account for the inefficiency of the inverter (if you are using an inverter). Depending on the equipment, 5-15% is usually reasonable. Check the spec sheet for the inverter to determine the efficiency. We’ll use a 10% inefficiency for this example:

240 Wh x 1.1 efficiency compensation = 264 watt hours

This is the amount of energy drawn from the battery to run the load through the inverter.

Next, we need to account for the effects of temperature on a battery’s capacity to deliver energy. Lead-acid batteries lose capacity as temps go down and we can use the following chart to increase battery capacity, based on the expected battery temperature:

For our example, we’ll add a 1.59 multiplier to our battery bank size to compensate for a battery temperature of 20°F in the winter:

240 Wh x 1.1 x 1.59 = 419.76 watt hours

Next, account for the efficiency loss that occurs when charging and discharging batteries. Typically we use 20% inefficiency for lead-acid batteries, and 5% for Lithium-ion.

240 Wh x 1.1 x 1.59 x 1.2 = 503.71 watt-hours minimum energy storage requirement

This is for a single day of autonomy, so we need to then multiply it by the number of days of required autonomy. For 5 days of autonomy, it would be:

504 wh x 5 days = 2,520 watt hours of energy storage

As you can see, the battery bank size is quickly increasing due to factors including temperature and required days of autonomy. All of these things affect your battery bank size significantly and need to be carefully considered.

Lead-acid batteries are commonly rated in amp hours (Ah) rather than watt-hours (Wh). To convert watt-hours to amp hours, divide by the system’s battery voltage. In our example:

2,625 Wh ÷ 12v = 220 Ah 12V battery bank
2,625 Wh ÷ 24v = 110 Ah 24V battery bank
2,625 Wh ÷ 48v = 55 Ah 48V battery bank

When sizing a battery bank, always consider the discharge depth, or how much capacity is discharged from the battery. Sizing a lead acid battery for a maximum 50% depth of discharge will extend the battery’s life. Lithium batteries are not as affected by deep discharges, and can typically handle deeper discharges without substantially affecting battery life.

Total required minimum battery capacity: 2.52 kilowatt hours

Note that this is the minimum amount of battery capacity needed, and increasing the battery size can make the system more reliable, especially in areas prone to extended overcast weather.

Step 4: Figure Out How Many Solar Panels You Need

Now that we’ve determined battery capacity, we can size the charging system. Normally we use solar panels, but a combination of wind and solar might make sense for areas with good wind resource, or for systems requiring more autonomy. The charging system needs to produce enough to fully replace the energy drawn out of the battery while accounting for all efficiency losses.

In our example, based on 2.5 peak sun hours and 240 Wh per day energy requirement:

240 Wh / 2.5 hours = 96 Watts PV array size

However, we need to account for real-world losses caused by inefficiencies, module soiling, aging, and voltage drop, which are generally estimated to be around 15%:

96 array watts / .85 = 112.94 W minimum size for the PV array

Note that this is the minimum size for the PV array. A larger array will make the system more reliable, especially if no other backup source of energy, such as a generator, is available.

These calculations also assume that the solar array will receive unobstructed direct sunlight from 8 AM to 4 PM during all seasons. If all or part of the solar array is shaded during the day, an adjustment to the PV array size needs to be made.

One other consideration needs to be addressed: lead-acid batteries need to be fully charged on a regular basis. They require a minimum of around 10 amps of charge current per 100 amp hours of battery capacity for optimal battery life. If lead-acid batteries aren’t recharged regularly, they will likely fail, usually within the first year of operation.

The maximum charge current for lead acid batteries is typically around 20 amps per 100 Ah (C/5 charge rate, or battery capacity in amp hours divided by 5) and somewhere between this range is ideal (10-20 amps of charge current per 100ah).

Refer to the battery specs and user manual to confirm the minimum and maximum charging guidelines. Failure to meet these guidelines will typically void your battery warranty and risk premature battery failure.

Here are standard configurations of PV arrays with battery banks. The battery capacity calculated in the previous step can be compared against this table to find a suitably sized system:

Array Size: PV Watts (STC)	Battery Bank Size: Watt Hours (@ C20 rate)	Battery Bank Ah Capacity
100-175	600	50Ah @ 12Vdc
200-350	1,200	100Ah @ 12Vdc 50Ah @ 24Vdc
400-700	2,400	200Ah @12Vdc 100Ah @ 24Vdc
800-1,400	4,800	400Ah @ 12Vdc 200Ah @ 24Vdc 100Ah @ 48Vdc
2,000-3,000	9,600	800Ah @ 12Vdc 400Ah @ 24Vdc 200Ah @ 48Vdc
4,000-6,000	19,200	800Ah @ 24Vdc 400Ah @ 48Vdc
8,000-12,000	38,400	800Ah @ 48Vdc

This information is intended to be a general guide and there are a lot of factors that can influence system size. There are also alternative options such as incorporating a backup gas generator or wind generator(s) to reduce the minimum battery requirement.

If the equipment is critical and in a remote location, it pays to oversize it because the cost of maintenance can quickly exceed the price of a few extra solar panels. On the other hand, for certain applications, you may be able to start small and expand later depending on how it performs. System size will ultimately be determined by your energy consumption, the site location and also the expectations for performance based on days of autonomy.

If you need help with this process, feel free to schedule a free consultation with us and we can design a system for your needs based on the location and energy requirements.

DIY Solar Financing: Buy, Loan, or Lease?

If financing your solar system is your best option, three main financing solutions are available to help alleviate some of the costs, all with their own advantages and some disadvantages.

Why Buy Solar?

There are many reasons to buy; most importantly, when you buy it, you own it. Of course, solar panel financing can be expensive, so the money you put upfront will be a substantial investment. In most cases, you can claim the interest on your loan to purchase the system as a deduction on your taxes, something you cannot do with the solar lease program.

Systems are very reliable; they hardly ever need maintenance aside from a scheduled inverter replacement a decade or two later. In Europe most of the solar inverters come with a 5 year guarantee as standard while in the USA the warranty is 10 years; which in both cases is upgradeable to 10, 20 or 25 years).

Why Lease Solar?

Solar panel leasing and PPAs (power purchasing agreements) are options if you’re more concerned about offsetting your power bill and using renewable energy sources instead. Solar PPAs vs. leases vary to some degree, but they both allow you to have solar power installed without having to pay for a system. However, you’ll discover that you paid the leasing company more than twice as much as it would have cost you to purchase the system yourself with solar financing.

One of the main advantages of leasing of solar system is that you are not responsible for the maintenance, upkeep, and operation of it. That falls under the responsibility of the lenders, giving you some added peace of mind, especially if your solar system is on a vacation home or summer getaway.

Why Solar Loans?

For USA customers FHA PowerSaver Loans are available to qualified applicants in many states. These loans help cover the cost of solar panel financing and installation (among other green energy improvements) and come with a reasonable interest rate.

With a loan, you’re paying back both the solar system’s costs and anything you owe on your mortgage, property taxes, etc, making any ROI or utility bill offset negligible until the loan is paid.

Loans can often be paid off in as many as 10 to 20 years, which means you may be paying them off for the entire life of the solar system. By the time your loan is paid off, you may need to replace vital components to keep your solar system functioning.

Deep Cycle Batteries – Solar 101