There are three parts to a perfect solar power system - solar panels, solar battery, and grid power. Solar panels are obviously the star of the system, generating clean, free power and saving thousands of dollars as well as the environment. Solar batteries, on the other hand, help store excess solar power for use during the night.
Ideally, solar panels and batteries should be sufficient to power a house continuously. However, we Kiwis know that our weather can be anything but predictable! Even with a well-designed solar + battery system, a multi-day cloud cover could leave the system ineffective. This makes grid power an important part of the equation.
Not only can the grid power your home in poor weather, but also charge your batteries during off-peak hours, so you can use power from the batteries without relying on either the grid or solar. Let’s take a closer look at how a solar + battery system operates in conjunction with the grid.
Working of a Solar Battery
Here is a graphic representation of how a solar battery works with a solar power system.
Solar battery (AC-coupled) layout
Let’s trace the route of the power flow in the above layout. In step 1, power generated by solar panels goes through the inverter and converts from direct to alternating current (DC to AC). In the next step, if any appliances in your house are operational, the power is used to run those appliances.
If, at this stage, any power is left (which usually is), it goes to another inverter that converts the AC power back into DC power, which gets stored into the battery. The second inverter here is a bidirectional inverter, and is typically an integrated component of a battery.
As we mentioned earlier, your solar panels may not always produce enough juice to run your appliances and charge your battery. In such a scenario, the bidirectional inverter in/near the battery can draw power from the grid to charge the battery. In this case, only the right half of the layout is active, as shown here.
Solar batteries charging from the grid
Modern solar batteries come with more than just battery cells in them. There is a whole host of smart electronics that controls how the battery charges and discharges.
When a solar battery detects that there is still room for charging and that there is low or no power coming from solar panels, it can be programmed to switch to charging from the grid. Older batteries, on the other hand, needed manual transfer switches to redirect the energy flow.
AC-Coupled vs. DC-Coupled Batteries
The battery shown in the above images is an AC-coupled battery. Unlike more traditional, DC-coupled batteries that connect directly to the solar panels, AC-coupled units connect on the AC-side of the system layout. AC-coupled batteries are slightly more expensive, but they offer remarkable flexibility - they can be added to an existing system, and they can charge using grid power - an important advantage!
Here is a layout of DC-coupled batteries, which lie on the DC-side of the system, meaning they are positioned before the inverter. DC-coupled batteries have slightly higher efficiency and lower costs but very limited functionality as compared to AC-coupled units. Check out our article on AC- vs. DC-coupled batteries for more details.
DC-coupled solar battery layout
Benefits of Charging a Solar Battery With Grid Power
The most important benefit of charging your solar battery with solar + grid power is the excellent flexibility it provides. Like we discussed before, grid power can keep your battery topped up when solar power is insufficient. But that’s not all.
In New Zealand, most utility companies use time-of-use pricing. The cost of a kWh consumed by a home depends on the time when it was consumed. Off-peak power consumption is cheaper, such as at midnight, but use more power in the mornings and evenings and you will see a spike in your power bill.
Owners of solar batteries often protect themselves from such spikes by using the battery to shift the load profile in their favor.
For instance, a homeowner with solar panels and a solar battery can charge the battery using excess solar power during the day. At night, they will use this stored power. Once the battery is discharged, it does not have to wait until the next afternoon for solar power. The battery can be recharged at night using cheaper, off-peak grid power, which is then used in the mornings, offsetting on-peak power, thus reducing costs. As illustrated below.
Charging a solar battery with off-peak grid power
Aside from that, having a grid connection to rely on means you don’t need to oversize your solar panels to ensure a guaranteed amount of energy each time. This can save you on the cost of your solar power system.
Solar Battery Intelligence and Grid Charging
As we mentioned earlier, modern solar batteries are smart devices that can do a lot more than just store energy. Thanks to intelligent sensors and integrated auto-transfer switches, your battery charging will switch from solar to grid and vice versa without you even knowing.
Besides just seamless switching, some battery models have more tricks up their sleeve. For instance, the Tesla Powerwall is an AC-coupled battery which comes with a “Storm Watch” feature.
When Storm Watch is enabled, Powerwall will automatically activate Storm Watch mode when the National Weather Service sends a severe weather alert. This mode pushes the limits and charges Powerwall to maximum capacity so it can provide backup power. You will receive an app notification when Storm Watch mode is activated. This remains active until the weather event ends, and your system will return to its previously selected mode.
Similarly, the IQ battery by Enphase also uses a “Storm Guard” feature, which is almost entirely the same as the Powerwall’s Storm Watch feature. It checks regularly for storm predictions on the weather service and fully charges the batteries to prepare for a blackout. In addition, the IQ battery also provides generator support for those using a generator for additional backup during extended outages.
Lastly, it also uses a “Load Controller” feature to turn power-hungry appliances selectively on or off to save charge during an outage.
Final Words
No matter how much cheaper and better solar power gets, a combination of solar and battery will always be more reliable. And no matter how much more efficient solar and batteries together become, solar, batteries and grid power will be the most solid, at least for the next decade or so.
Batteries can help store solar power and provide it during the night, but charging the battery partially with the grid allows you to spend less on oversized solar power systems. Most homeowners use grid power to charge the batteries after the solar energy in them is fully utilised. This helps them store cheaper, off-peak energy and use it later, preferably during peak hours when grid power is more expensive.
In conclusion - yes, solar and batteries are amazing, but you might still want to hold on to some of that grid power to make the most of your solar battery.
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