A whole home battery backup system from Tesla.

Putting Battery Backup Capacity into Perspective

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Grid-interactive solar panels cease to operate when the sun goes down. They also shut down when there is a utility outage. That is unless you have batteries.

It’s a common misconception that solar panels can directly power your home during the day if the power company has a grid failure. Except in very limited circumstances, this is not the case. You need batteries to buffer the energy produced and consumed. That’s where battery backup comes in.

As batteries become more common in the United States, there is renewed interest in going off the grid. If not off-grid, becoming self-sufficient during a grid outage is attractive. All the talk about electric vehicles and fancy new battery systems has caused an explosion of interest in solar backup for homes. But before you get too excited, we need to put the capacity of current battery technology into perspective.

Brief Solar Battery History

Off-grid solar and solar with battery backup has been around for years. In fact, this is how the home solar power industry got started – before you were allowed to connect to the grid. Back in the day, we used lead-acid batteries, similar to a car battery, but built to be heavy-duty for deep discharge. These batteries are big, bulky, and require maintenance.

Maintenance-free gel and absorbed glass mat (AGM) batteries became more popular because they are slightly easier and safer. But there are downsides. The capacity didn’t increase and the cost did increase.

Recently, lithium-ion type batteries are all the rage. We are all familiar with this general technology from our portable devices. More importantly, electric vehicles have revolutionized battery technology, dramatically increasing capacity while reducing size and weight. The downside currently is the cost (arguably… we will get to that).

Battery Capacity Perspective

Let’s say your electric bill is $150, which is pretty typical for a modest home in Southwest Florida. That equates to roughly 1,300 kilowatt-hours (kWh) per month or 43 kWh per day.

When you are talking about solar with battery backup you need to consider how much energy you will use at night and during cloudy weather. You also need to consider how much energy you can produce. You can’t just look at one day of usage because what happens if you have a cloudy day with little solar production? The point is, 43kWh is a pretty small amount of energy relative to the amount of electricity we use here in Southwest Florida. And keep in mind, you usually need backup in the summer and fall when it is hot and you are using more electricity, perhaps 50% more.

To put this into perspective, the top-of-the-line Tesla Model S electric vehicle has a 100 kWh battery capacity. If the hypothetical house above uses 60kWh in the summer, the car has enough battery capacity for about 1-1/2 days of electricity for your home. That’s a $100,000 car (roughly). Let’s assume 50% of the car’s cost is the battery (this is a widely reported estimate, although battery costs are dropping quickly). That means the battery that can power your house for 1-1/2 days costs $50,000, and that doesn’t include any of the electronics to convert it to usable energy.

Currently Available Battery Options

A whole home battery backup system from Tesla.

A whole home battery backup system from Tesla.

For more perspective, some of the top players in the home battery field have recently come out with new batteries. For example, Enphase, Generac, and QCell all have new home battery options. You can stack them together for more capacity, but they top out at 40 kWh for Enphase and even less for Generac and QCell.

Tesla’s Powerwall 2 happens to be the current leader for the consumer-friendly battery options on the market. They can stack together to provide 140 kWh of capacity and ample instantaneous power for a typical home. But be prepared to pay. While they do have the lowest prices on the market right now, they are still expensive, and they are hard to get (supply is seriously constrained).

Aside from these players, you can get larger battery capacities from other manufacturers, but the systems have to be custom designed and cobbled together from different brands of equipment. You need inverters, combiners, and other electrical equipment to make it all work together.

Why All The Hype

Florida is a unique situation. We have very high electricity usage during times where backup power is typically needed. And most people consider the most power-hungry appliances critical (i.e. air conditioning). Many of the battery systems on the market today were conceived for markets with more modest electricity storage needs and for places where load shifting is desirable. This includes time of use billing markets and places where you cannot sell excess power back to the utility company.

So Florida is caught up in the hype of battery backup using equipment that is not highly suited to the market. Will it work? Absolutely! Is it ideal? No.

Matching Expectations and Reality

If you have modest needs and realistic expectations, battery backup is absolutely possible today. There are options for people who are early adopters or simply want the peace of mind and environmental friendliness of fuel-free utility backup.

But you should look at your utility bill and see what your usage is first. Match this up to what battery capacities are available today. And expect to pay a premium for this emerging technology.


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