Solar Systems: Grid-Tied vs. Battery Backup

Grid-Tied Solar Systems

Ninety-five percent of the solar systems installed today are “grid-tied.” When the solar system produces more energy than what the building consumes, the excess energy is sent to the utility grid. The power company gives you credits for the extra production of energy. These energy credits are later used when you need electricity at night and during periods of no sun. Thus, the grid essentially acts as your battery for storing excess energy.

The downside of this setup is that there is no storage system for your excess solar energy when the grid goes down. In addition, there is no backup because National Electrical Code requires that solar systems tied to the grid stop producing when the utility power is no longer present.

The law ensures that electrical linemen don’t get electrocuted while working on your service. So your solar system can’t send its excess power to the built-in battery of the utility grid.

A solar system needs a cut-off switch that disconnects (or “islands”) the home from the power grid when it wants to self-consume solar energy. But even solar systems that isolate themselves from the grid still can’t produce electricity. Why, you might ask?

Battery Backup Solar Systems

It’s because you don’t have a constant supply of power. Your solar modules are producing amps only when the sun is shining on them. In the early morning and late afternoon, your solar may only be making enough electricity to supply minimal amps to your home or building. Your refrigerator compressor motor doesn’t like running on 1 amp when it needs five.

It is not suitable for your needs! Even in the middle of the day, when the sun is high and shining on your panels.  A sound amp output can quickly drop to zero when a cloud gets in the way. It would help if you had a constant source of voltage and amps for the appliances in your home or building to run correctly.

Batteries provide that constant, reliable source of energy for your solar system when the grid is down. Excess production from your solar modules flows into the batteries and is stored for use when the sun isn’t shining. The battery bank always provides a constant voltage and as many amps as the batteries can hold. When the battery charge gets too low to provide an adequate output, the system will shut down to not damage your equipment. Adding batteries to your solar system is the only way to have your solar system operate during a grid outage.

The alternative to the solar battery system is getting a whole-home generator that runs on gasoline, natural gas, or propane. As with any solar system, there a PROS and CONS to using batteries for your home backup needs, and here we’ve listed a few against the generator option:

Pros

As long as the sun is shining, you will always have power. In extended grid outage situations, you don’t have to worry about getting gasoline or propane for your generator. Even if you run a generator to help with heavy loads, the battery backup system is a fuel extender, allowing you to operate longer on less fuel.

Generators are noisy beasts. Battery backup systems are not. You will not drive yourself batty hearing a combustion engine running for multiple hours of the day when using batteries.

A generator augments most solar battery systems for multiple days of lousy sun. These systems work well with other types of energy generation, including wind turbines and geothermal energy.

There aren’t many moving parts in a battery backup system. It’s low maintenance compared to a generator running for hours on end. Furthermore, generators need service and oil changes. We install Lithium-Iron-Phosphate (LiFePO) batteries. LiFePo technology has a life of 5,000-6,000 cycles. Each would have a life expectancy of 13-16 years for batteries that discharge and charge daily!

If your power company only pays you a wholesale rate for the electricity you send back to the grid, having a battery store the energy you produce can help the return-on-investment of your system. Energy stored can be used for your consumption later (called Time-of-Use). Many Co-Op power companies in our area are currently buying back electricity at wholesale rates. Keeping your solar energy will allow you to essentially get retail rates for your excess solar generation because you are not buying that power from the power company but using it from your battery storage system.

Cons

Batteries are the most expensive component of solar systems. While the cost of batteries has fallen in recent years, they have not dropped as much as solar panels and inverters. As a result, it will be 3x more expensive to put a battery storage system in than a whole-home generator.

The size system that most people want to install does not provide a considerable power output. The inverter we usually install is 12 kW. That equates to a 12 kW generator being connected to your loads. It’s a reasonably good size but provides only 38 amps of continuous output. That’s not many amps for homes built with 200 amp service.

A single battery we install can provide 10 kWh of energy storage. Most homes in our area will use on average 50-70 kWh per day during the summer. Meaning that you need 5-7 batteries to power your whole house for a single day. Each battery costs roughly $5,000, so we need to work within a budget to make it work. To get around this, start by powering a Critical Load Panel (CLP).  Then making sure only the most critical loads are on battery backup.

Summary

Being prepared for an extended outage is probably the most compelling reason for a solar battery backup system. It is the reason cited by most of our customers who add a battery to their solar project. There is unrest in the world, and these folks want to be prepared should the worst happen. Many go entirely off-grid, too! Having a battery backup system ensures our customers will ALWAYS have access to power. Regardless of what is going on with the power company, the government, or world strife.

Battery backup will not increase your solar system ROI or payback. It does the opposite. You can’t sell electricity to the power company because the excess energy is stored. Having a battery bank store the solar energy you produce to be consumed later will be a benefit. Batteries offer a service of uninterrupted and unlimited power to your critical loads.

You’ve got to weigh the probability of an extended outage at your property with the cost of installing a battery backup system. If you are prone to have frequent and extended outages at your residence, and the cost and hassle of keeping up with generator fuel and noise is a burden, then a battery system may be right for you.

We’ve detailed some design concerns to consider when considering adding a battery bank to your solar system. We recommend reading that article to learn about the factors to heed when you want to power your house on a battery backup solar system.