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Is a battery necessary with a solar power system?  This is a question that often pops up when consumers are interested in a solar power system.  And the answer to this depends on a few factors.  For Southern California residents and for us at Solar Power Supply, there are a few questions we often pose in order to properly answer this specific question.  The most important to address is, “are there any medical devices in your home that require them to be ON at all times?”  If that answer is “yes”, then a battery is a definite need, especially with more occurrences lately in unexpected power failures or scheduled grid maintenance.  There’s also the factor of storing medicine like insulin at a proper temperature making sure appliances like refrigerators are always working.  

The Grid-Tied System
For the most part, because Southern California weather is sunnier than most areas and the electrical infrastructure is in fairly good shape, a battery isn’t critical with a solar energy system.  That’s because the grid acts like a backup battery from which your home can draw energy from when your solar system isn’t producing enough electricity.  During the day a typical solar power system produces more energy than what’s being used.  That excess generation of electricity is sent out to the grid where your utility company gives you credit through a process called net metering. 

You’ll use that credit when the sun goes down or in bad weather conditions when your system simply isn’t producing enough energy to match your consumption.  You then settle the exchange of credit with the utility company at the end of a one-year cycle. However, when a power outage does occur, electricity produced by your solar power system won’t flow into your house.  This happens because this automated fail-safe system protects those working on the grid to repair or maintain the infrastructure.

Grid-Tied with a Backup Battery System
A backup battery allows you to store the unused electricity produced by your solar system for later use.  When the sun goes down and the solar system isn’t generating any more electricity, power from the battery kicks in to keep the lights on and appliances running.  You’ll draw stored energy from the battery first until it’s empty, and then from the grid as your next back up.  With a typical solar system plus battery set up, the stored battery power is ample to last you through the night under normal conditions and will be recharged by your solar system the next day as it simultaneously powers your home.
When you experience a power outage, the backup battery becomes the only source for energy.  In this scenario, the battery is set up to smartly kick-in as your primary device for electricity.  Although the solar system is producing energy, it will only function to energize the battery, not to provide electricity for your entire home.  Again, this is to protect anyone working on the grid.

So, the golden question is, “how long will the battery last?”  This is quite a difficult question to answer in relation to “a period of time” due to many different factors.  The complexity to answer this is due to variables that not only include the number of appliances/devices that are tied into the backup battery system but also the efficiency of these appliances and the battery’s (ies) capacity.     

There are multiple brands and sizes of backup batteries, but let’s take a typical modern Lithium-Ion battery today (whether it’s an AC battery or DC battery) that has a rating of 9.8kWh such as an LG.  With an energy star refrigerator that uses about 1-2 kWh in a day, a battery will last a few days assuming minimal usage during the power outage.  When you start tying in more appliances to that battery, then you simply need to find out the consumption ratings of those appliances and devices to really figure out the current draw from the battery to provide a better indication of how long it will last during an outage.  Also keep in mind that batteries do need to store a minimum charge of around 10-20% so in our example with a 9.8 kWh battery, your power capacity is more likely to be around 7.8-8.8 kWh.  So, the more appliances connected, the more power drawn from that battery.

Thankfully during the day, the solar panels will recharge the battery at a certain rate.  Again, based on the size of the solar system, regeneration speed will vary.  Take for instance, a 9.8 kWh solar power system can recharge the battery within an hour.  The larger the system, the faster the recharge and vice versa.

Back Up Battery Advantages and Disadvantages
There are certainly advantages to having a backup battery with a solar system.  In very simple terms, the battery significantly lowers your reliance for power from the grid.  There’s usually enough power stored in a battery to get you through your normal routine at night, unless you have the air conditioner cranked up, the washer and dryer on, lights blaring, and so on, all at the same time.  And, on occasional emergencies for power such as a black out, your essentials will operate for a period of time.

The biggest hurdle to a battery backup system is the price.  Today’s Lithium-Ion batteries such as Tesla Powerwall and LG Chem are still fairly expensive and will run in the thousands of dollars.  You’ll have to consider convenience and necessity against the cost for each battery.  You can stack batteries together for longer backup time, but you’re also looking at a cost premium.  There are rebates available for the purchase of a battery(ies) which does help offset the high price.  The other good news is that the current 26% federal tax credit also applies to the purchase of the battery with a solar system.  *Note, the federal tax credit is only available for a limited time so take advantage of it while it lasts.

One other consideration to a battery is its life-span.  The warranty for batteries is typically 10 years.  That doesn’t mean it will suddenly not work at the end of the 10th year, it just means it will not perform to that same capacity as it did in the first couple of years.  Note that batteries are constantly working, charging and discharging so it is a work horse within a hybrid solar power system.

What’s it worth?
In conclusion, is a battery worth it?  For many it is.  The peace of mind that you’ll have power in times of emergency as well as less reliance to purchase power from the grid is a great feeling.  Is a battery necessary with a solar system?  If you’re in So Cal, we would say not really.  The outage downtime, although could last for a good period of time, may not merit the price of a battery.  But if you can afford a hybrid system (solar plus battery) we say, “Go for it!”