Do You Need Solar Batteries?

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There is a lot of talk about solar + storage or solar batteries attached to solar systems, but most systems do not come with a battery. There are some very good reasons for that here in Las Vegas. To help you decide if a battery is a good fit for your project, first you need to understand what a solar storage battery does and how they work.

What is a solar storage battery good for?

A battery attached to your solar array is like the battery in your cell phone or laptop. It gets charged by sunlight hitting your panels or power from the grid and will discharge that power in one of two ways.

BACK UP POWER: If the power goes out for any reason at any time, your home will switch automatically to battery backup power. Depending on the type of solar and battery system you select, the solar may continue to provide power if the sun is up or it may be shut off until the battery drains past a certain point. 

LOAD SHIFTING or POWER STORAGE: The solar storage battery will store excess solar production in the battery to use later – during high energy demand times, at night, or during a power outage. Some batteries can also be charged with power from the grid. 

Both of those sound awfully useful, but let’s dig a bit deeper and see if they make both practical and financial sense in Las Vegas and the surrounding areas.

Batteries for backup power

Honest solar companies do not include solar batteries by default here in Las Vegas because they are still quite expensive compared to how frequently you will actually use them. What a battery system can power at what any reasonable person would call an “affordable price” does not include central air conditioning, pool pumps, electric water heaters, or charging electric vehicles. These high load appliances will require 3 or more battery units to power them. More on this in a minute. 

Most solar PV systems in Las Vegas and surrounding areas are connected to the power grid (grid-tied). The power company’s grid acts like a virtual battery “storing” excess power production as credits on your power bill. Those credits then pay for the power you need at night, on cloudy days or in the peak of summer heat. Unlike adding a physical battery which can cost upwards of $20,000 per unit when all the labor, permit, additional switch gear, and critical loads panel installation are added in, you will not pay any additional cost for this “virtual battery.” 

You should understand that when the power goes out, a standard grid-tied solar PV system will shut off, too. This is required by electrical code and is in place for the safety of the utility workers while they restore power. Unexpected power coming from your house would be a hazard to them. As soon as the grid is back up, the system will wake up on it’s own and start making juice again. Here in Las Vegas, with our very stable power grid, we rarely see outages of more than a few minutes, so spending an extra $10,000 – $20,000 to have backup power from a physical battery really doesn’t make economic sense for most people.

Solar Batteries for Load Shifting/Power Storage

Load shifting means storing power made by your solar array in your battery to use at a different time. Usually this means power made early in the morning is stored in the battery to be used in the late afternoon when some power companies charge a lot more for power. This is called Time of Use pricing.

NV Energy DOES NOT offer Time of Use pricing as the default. These pricing plans are very common in  California, Arizona and Hawaii but are only available by request from NV Energy. So, unless you ask NV Energy specifically to be moved to the Time of Use pricing plans, there is zero financial benefit to you in load shifting power. For our Mesquite and Pahrump customers, Overton Power District and Valley Electric Association do not offer Time of Use options at all.

On the standard residential pricing plan for NV Energy, VEA and OPD, you pay the exact same price for every kilowatt hour – no matter when you use it.  Here in Nevada load shifting power from morning hours to late afternoon/early evening has no financial benefit no matter which utility you buy your energy from. (If you elect to be on one of NV Energy’s TOU plans, you can only benefit from load shifting when summer peak pricing is in effect. That’s only 65 days a year. The math just doesn’t pencil out – even on the TOU plans.) 

Another way load shifting could help is if your power company has tiered rates. You pay so much per kwh up to 500 kwh on Tier 1, then more for 501-2,000 kilowatt hours on Tier 2 and maybe even more for 2,001 and above. This is really common in California, but again, we don’t have this here (except for City of Boulder City Power) so there is no financial benefit to load shifting to avoid tiered rates

What can I run on a single solar backup battery?

The battery most commonly talked about – the Tesla Powerwall –  holds 13 kilowatt hours of power and can deliver 5kw in a steady stream. It can surge to 7kw for a few seconds.  That’s not a whole lot of juice. Other brands such as Enphase, LG Chem and  Sonnen offer slightly different running and surge outputs but none of them surge over 7kw on a single unit. Since most central ACs require a surge current of over 20kw, it is IMPOSSIBLE to power a central HVAC, pool pump, electric water heater or charge an electric car on a single unit of any of them. (Keep reading to find out more about surge current.)

So, what can you power on a single solar backup battery?

You could power some lights, a phone charger, a laptop, internet modem/router, TV and your fridge and freezer, a microwave, a fan or other small appliances. You might even be able to power a window AC unit or a swamp cooler. 

Since these units do not provide nearly enough power to run a central AC (or kind of motor with a high surge current such as a pool pump) during an outage unless you get multiple units, the payback math on any brand of battery just doesn’t pencil out in Las Vegas and surrounding areas right now.

Solar backup batteries in Las Vegas or Henderson really only make sense if you have 24/7 medical equipment or some other very critical power need and you are willing to spend a LOT of money to avoid having that thing go offline for 5 minutes once a year.

When are solar backup batteries a good idea?

If you live in the more remote areas and you do not have power to your home at all, an off-grid system with batteries is your answer. The system is not connected to the power grid at all. When the sun is up, the panels power your home and charge the batteries. Once the batteries are full, any extra power is shunted into the ground rod. 

If it’s cloudy or dark, your power will come from the batteries. Since you have no connection to the grid to supply back up power, this type of system is more difficult to design and quite a bit more expensive due to the number of batteries you’ll need to supply overnight power. Off grid systems use different types of batteries depending on cost.

Lithium Ion types (Tesla Powerwall, LG Chem, Enphase Encharge, Sonnen): These type of solar batteries are very similar to the ones you’d find in your laptop or cell phone … only on a much bigger scale. They use various chemical makeups – mostly based on lithium. They can operate both on or off grid and can be used for either backup power, tier shaving or nighttime power. Batteries may require a separate inverter.

Deep Cycle Batteries: This type of solar batteries looks a lot like boat or car batteries. They do not store as much power as the newer lithium ion chemistry types and need to be replaced more frequently.

Robco can install either type of solar battery.

If you still want to explore batteries in more depth for your home, you can consult  this Enphase storage sizing calculator to figure out if it’s going to be cost effective for you and how long the selected size would provide backup power.

Are there any rebates for installing solar back up batteries?

Yes, the federal solar tax credit is applicable to solar battery installs either adding to an existing system or installing with solar. 

There is also a NV Energy Rebate available on batteries. 

Yes, you can run a central air conditioner on a backup solar battery system but it is going to cost a LOT more money than you think it will.

Understanding surge current.

When any electric motor starts up (like the one in the compressor of your HVAC), it draws a lot of power for a few seconds to get started. This surge power draw is called the inrush current. Inrush current is like the big effort required to start pushing a really heavily loaded shopping cart. You really strain for a few seconds, then it gets moving and the amount of effort needed to keep pushing it tapers way off.  That is how it works with electric motors. The big surge current “jump starts” the motor and gets it going. After a few seconds, the the power required to keep it going levels off.  When you size solar storage batteries, you have to size to meet that inrush current, not just the running power on the motor.

Electric vehicles also have very high sustained power requirements for charging. Most common EVs require at least 7kw of sustained power to charge. Because that draw exceeds what a single unit of any of the common battery brands can supply, you’d need more than 1 unit to charge the EV. 

FIND YOUR INRUSH CURRENT ON YOUR HVAC

To learn about inrush current and get an idea what yours is, let’s have a quick look at the biggest power user in your home…and the one with the biggest inrush current, your central AC. You can find the inrush current on your HVAC by checking the data tag. You’ll find the data tag on the outside of the unit, usually near where the hoses are connected. You are looking for two numbers on the tag –  RMA/RLA and LRA.  The RMA/RLA (running motor amps or running load amps) is the number of amps needs to keep the motor running once it gets started. The LRA (locked rotor amps) is the “jump start” current needed to get that motor going. The battery system has to deliver enough surge to meet the inrush of the AC motor AND power whatever other loads you have connected such as a fridge/freezer, internet router, lights, small appliances, TV and so on. 

On the nameplate above, the RMA for this unit is 25.3 Amps or 6,072 watts/6.0 kw. (Operating voltage of 240v x 25.3 amps = 6,072 watts or 6.072 kw).

The LRA (inrush current) is 146 Amps or 35,040 watts/35.04 kw. That means it takes over 6x the running power to start this motor. 

A single unit of the Tesla Powerwall, Enphase Encharge 10, LG Chem or whatever other brand of backup solar battery generally supplies between 3 and 7kw of surge power. Clearly, that is not enough to meet the surge on this HVAC system. It’s not enough to meet the surge on pretty much ANY HVAC system. Go check your own label and look for the LRA number to see what your surge current is.

If this HVAC was connected to a single unit of any of these batteries, as soon as the compressor started up, the system would trip and go off line. In order to operate this particular HVAC on backup, we’d have to supply 35.04kw of surge current. For a solar backup battery that delivers 7kw of surge, it would require 5 batteries JUST to start the AC – never mind any other loads you might want to power.

Can you reduce the surge current?

Yes. A qualified electrician can install a device called a soft starter to reduce the inrush current. With a soft starter, the inrush on this particular HVAC would drop to about 13kw. Better…but it would still require at least 2 units of most of the common battery brands to supply enough surge current JUST to run the HVAC! A correctly sized backup battery for this set up would require 3 units of all the common battery brands to run this central HVAC system and the essential loads (lights, fridge, cable modem etc.) on solar battery backup. 

A pool pump. well pump, electric water heater or electric vehicle all have similar high inrush currents. That’s why you can’t run any of those heavy loads on a single solar backup battery.

So, the short answer is that backing up your HVAC is in no way cost effective with current battery technology…even with a soft start installed.

Learn more about how batteries are sized for both power, energy, inrush current and hours of autonomy desired by watching this webinar from Enphase.

If you are still interested in a solar backup battery, we’ll be happy to work with you to find a solution. Click here for a quote.