How do solar batteries work?

Solar batteries are an integral part of a home’s energy independence. Here, we’ll go through a step-by-step breakdown of exactly how solar batteries work. For this example, we’ll assume that the battery is paired with a solar system, rather than just a standalone battery.

Step 1: Solar Power

The process begins with the solar array on the roof producing power. Sunlight hits the panels, which convert the visible light into an electrical current. 

The DC electricity produced by the solar system can then either be converted to AC power or kept as DC power, depending on whether the system uses an AC or a DC battery. More on that a little later.

Step 2: Battery Charging

The home will take first priority for the solar power. The electricity produced by the panels will directly feed the home’s main electrical panel and power everything in the home, from TVs and lights to air conditioning and electric vehicle charging.

Often, solar systems can produce more power than the home needs at the moment. Picture a beautiful spring day, when the weather is temperate so the home isn’t using much electricity, but the panels are producing a lot of power. In these kinds of conditions, the output from the solar system can easily exceed the home’s demands.

Without a battery, this extra energy would flow back to the grid through the process known as net metering. In effect, this extra power would “spin the meter backwards” and provide the homeowner with bill credit that would help to offset power drawn from the grid when the system isn’t meeting all of the home’s needs (like at night).

With a battery, instead of going to the grid, the extra electricity the solar system is producing flows into the battery and charges it up. How quickly the battery charges depends on the amount of extra energy being produced, which itself depends on multiple factors like the size of the solar system and the current electricity demand in the home.

Step 3: DC vs AC Charging

While homes run on AC (alternating current), all batteries need DC (direct current) to charge. That’s why your laptop cable has that big box on it – it’s converting the AC power coming from the wall into DC power to charge the laptop’s battery.

If all batteries require DC power to charge, then what exactly is an AC battery? 

An AC battery is one that can accept incoming AC power, and it uses a built-in inverter to convert that to DC power, which then charges the battery. When the battery dispenses power to the home, the inverter then converts the DC power coming from the battery pack back into AC, which then feeds into the home.

Most modern solar batteries, including the Tesla Powerwall, are AC batteries. The biggest advantage of AC batteries is that they can be used with any solar system out there. Any solar inverter can be paired with them, including microinverters, since they can accept the AC output from any system. This makes them very flexible and easy to retrofit to existing solar systems.

By contrast, DC batteries like LG Chem do not have a built-in inverter. As a result, they must be charged directly from DC power. This requires a specialized inverter that can pass DC power produced by the solar panels directly into the battery without inverting it to AC first. When the battery needs to power the home, the DC power from the power is fed into the external solar inverter, where it’s converted into AC power that then powers the home.

The biggest advantage of DC batteries is that they have higher round-trip efficiency. Every time electricity is inverted from DC to AC (or vice versa), roughly 5% of the power is lost to heat. Therefore, the more times you invert the power, the more total energy you’ll lose. As a result, DC batteries are more efficient than AC batteries, since they’re not inverting the power as many times. DC batteries also tend to be cheaper, since they don’t have the additional inverter hardware included.

Step 4: Battery Discharging

Now that the battery has been charged up with extra solar power, whether AC or DC, that stored energy can then be used in the home at a later point. There are two primary reasons why a homeowner would want to have extra energy stored for use later on.

The first reason is if the home is subjected to a power outage from the grid. As soon as the power outage occurs, the battery system’s backup gateway will isolate the home from the grid and activate the battery to immediately provide power to the circuits it’s been connected to. In this respect, the battery will function like a typical generator, except that batteries are far better than generators in every important way.

The second reason to store energy for later use is to take advantage of time-of-use (TOU) rate plans from utility companies. Many utility companies are moving homeowners to TOU rates because those plans more accurately reflect the changes in wholesale power prices throughout the day. In California, homeowners in the major investor-owned utilities (PGE, SCE, SDGE) are required to move to a TOU rate plan when they install solar panels.

On a TOU rate plan, the extra energy that the panels produce in the middle of the day is less valuable than the power drawn in from the grid at night. As a result, using a solar battery can help homeowners save more money by avoiding the peak utility charges in the evening. You can read more about TOU rates and savings here.

Conclusion

It’s really that simple! Solar batteries capture excess solar power and allow for that power to be used when it’s most beneficial for the homeowner, like during an outage or to take advantage of additional electric bill savings. Speak with one of our Energy Advisors to see your customized solar and battery system!