Robco Electric, Inc.
Solar University


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 excellent reasons for that here in Las Vegas. To help you decide if a battery is a good fit for your project, you first need to understand what a solar storage battery does and how it works.Solar Batteries

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, your home will switch automatically to the solar batteries 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 sound useful, but let’s dig deeper and see if they make 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 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 three 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.”

It would be best if you understood that when the power goes out, a standard grid-tied solar PV system will shut off, too. This is required by the 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 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 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 energy produced 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 prevalent 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, you have zero financial benefits in load-shifting power. Overton Power District and Valley Electric Association do not offer Time of Use options for our Mesquite and Pahrump customers.

On the standard residential pricing plan for NV Energy, VEA and OPD, you pay the 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 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 common in California, but we don’t have this here (except for the City of Boulder City Power), so load shifting is no financial benefit 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 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, an internet modem/router, a 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 doesn’t pencil out in Las Vegas and surrounding areas right now.

Solar backup batteries in Las Vegas or Henderson 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 do not have power in your home, 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 solar batteries. Since you have no connection to the grid to supply backup power, this system is more challenging and quite expensive due to the number of batteries you’ll need to provide overnight power. Off-grid systems use different types of batteries depending on cost.

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

Deep Cycle Batteries: This solar battery looks like a boat or car battery. They do not store as much power as the newer lithium-ion chemistry types and must 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 determine if it will be cost-effective and how long the selected size would provide backup power.

Are there any rebates for installing solar backup batteries?

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

There is also an NV Energy Rebate available on batteries.

Can you run your central AC unit using solar backup batteries?
Yes, you can run a central air conditioner on a backup solar battery system, but it will cost much more money than you think.

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 considerable effort required to start pushing a heavily loaded shopping cart. You strain for a few seconds, then it moves, and the action needed to keep pushing it tapers away. That is how it works with electric motors.

The big surge current “jump starts” the engine and gets it going. After a few seconds, 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 familiar battery brand 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 of yours, let’s quickly look at the most significant power user in your home…and the one with the most considerable 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 outside the unit, usually near where the hoses are connected. You are looking for two numbers on the label – RMA/RLA and LRA. The RMA/RLA (running motor amps or running load amps) is the number of amps needed to keep the motor running once it gets started. The LRA (locked rotor amps) is the “jump start” current required 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. It takes over 6x of the running power to start this motor.

A single unit of the Tesla Powerwall, Enphase Encharge 10, LG Chem, or another backup solar battery generally supplies between 3 and 7kw of surge power. That is not enough to meet the surge on this HVAC system. It’s not enough to meet the surge on ANY HVAC system. Check your label and look for the LRA number to see your surge current.

If this HVAC were connected to a single unit of any of these batteries, the system would trip and go offline as soon as the compressor started up. We’d have to supply 35.04kw of surge current to operate this HVAC on backup. A solar backup battery that delivers 7kw of the surge would require five batteries 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 soft starter device 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 two units of most of the standard battery brands to supply enough surge current JUST to run the HVAC! For this setup, a correctly sized backup battery would require three 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. Pumps, electric water heaters, and electric vehicles have similar high inrush currents. You can’t run 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.