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How to Size Batteries for a Solar System

Grid-tied solar doesn’t work when the electricity goes out. While it is frustrating, having a grid-tied system gives you many other benefits. But we’re not here to talk about those. We’re here to talk about how you can get electricity when the grid goes down.

As battery technologies improve, price decreases and efficiency increases. That means grid-tied solar with battery backups are becoming a more prevalent - albeit still expensive - option. With the proper setup, you can install batteries along with your new solar system or add batteries to your existing system. However, these systems aren’t meant to cover 100% of your normal usage, and you should take time to really think about which devices and appliances need to be powered when the lights go out.

So where do you begin with determining how much battery storage you need? Like so many other answers you’ll find on the internet, we’re going to say “it depends.” But we’re here to help clear that up. We’ll show you what you need to think about when determining what size battery system you’ll need, and we’ll also give you some examples of how much you can power with batteries powered by solar. 

What Are Your Energy Goals

First and foremost, the size of your energy system will depend on your energy storage goals. Are you a homeowner looking to power a few essential appliances in case the electricity is knocked out for a few hours? Does your business or farm own a solar system and you’re looking to use batteries to shave off demand costs? Before reading on, give this some consideration. 

While solar-charged batteries are a sustainable method of backup power, they can be expensive. In most cases, batteries are not meant to back up 100% of your normal electricity usage. There are limitations when running on backed-up power. 

Battery backups are meant for powering essential devices and appliances only.

Here is a list of things people typically power with their battery backup systems:

  • Lighting
  • Ceiling fans and/or air conditioning (often window units for one or a few rooms only)
  • Refrigerator
  • Medical equipment
  • Personal electronics, modems, and routers
  • Sump pumps and well pumps

For an idea of how much you can back up with your budget, check out the chart at the end of this blog. 

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How Much You Want to Spend

Another big factor that probably influences your backup-energy goals is how much you want to spend on batteries for your solar system. Unless you pay high demand charges, solar storage battery banks probably won’t save you any money (by high demand charges, we mean around $15 per kW - according to most research).

You may be able to use the 30% Federal Solar Tax Incentive. Another incentive that may be available to businesses and commercial solar system owners is accelerated depreciation.

If your system is in Maryland, there’s an extra incentive you could take advantage of: the Maryland Energy Storage Income Tax Credit Program. This incentive offers 30% off the cost of an energy storage system installed during 2019 but is capped at $5,000 for homes and $75,000 for businesses. 

The Duration of Backup Power/Storage Capacity

Once you know how much you want to spend on batteries, we can start getting into the technical stuff. First, how long would you like your batteries to provide power (this is known as your battery system’s storage capacity)? The longer you’d like your system to operate, the more batteries you will need. 

Keep in mind how much space you have available to house the batteries. The more storage capacity you need and the more batteries you have, the larger the storage space will have to be. Depending on which type of battery you go with, they will either have to be stored inside or outside. If outside, they should be stored in a shaded and temperate area.

what battery is best for solar

Rate of Discharge and Depth of Discharge

Another thing to note when determining the duration of your battery system is the rate of discharge or the amount of time it takes for your batteries to go from fully charged to their appropriate depth of discharge amount. 

While it may seem unnecessary, it’s important that you don’t use 100% of your batteries’ stored energy. Discharging your lead-acid batteries (and some lithium-ion batteries) down to 0% will shorten their lifespan. Depending on what the battery manufacturers advise, you may only be able to use 70% or 80% of your batteries capacity. 

The Size of Your Backed-Up Appliances/Battery Power

The next thing to consider is how many watts it takes to power what you’d like to back up. Think about how much instantaneous power it takes to run different applications and appliances. An emergency light may only take 60 watts, but a refrigerator may take 700 watts. This will play a significant part in estimating the battery bank size you need.

Your Solar System’s Production

Consider how many volts your PV panels are producing. Batteries charge when your solar system is generating more instantaneous power than you use at that moment. For a grid-tied system without battery backup, this power would go to the grid, and you would receive a credit or some other form of payment through net metering or another program. If you add a battery, your extra energy will charge up your battery, and any excess after that will pass to the grid.

If you rarely send solar-produced electricity back to the grid, you may have a hard time charging your batteries unless you add on to your solar system or reduce your electric load. Another thing to keep in mind is the seasonality of your solar system. Your system likely produces much more electricity in the summer than the winter.

However, certain battery setups have the availability to charge batteries using grid power as well, meaning your battery backup won’t just be dependent on what your solar system produces.

Some Examples

Though many factors go into determining the price of your battery system and the amount of power it can provide to your home or business, here are some general estimates that may give you an idea of what kind of power is available from batteries. 

If you’re adding batteries to an existing system, you may need to purchase new inverters and other equipment on top of the battery itself.


Number of Batteries

Install Costs (Before Incentives) Install Costs (After 26% Tax Credit)* Estimated Total Usable kWh  What That Can Power**

1 x 9.8 kWh Battery



9.3 kWh

A load of clothes in the washing machine and drier, plus an hour of oven use

3 x 9.8 kWh Batteries



27.9 kWh

6 hours of air conditioning for the average house

6 x 9.8 kWh Batteries $90,000 $66,600

55.8 kWh

Fully charge the battery for some electric cars

12 x 9.8 kWh Batteries $180,000 $133,200 ($95,616 with accelerated depreciation [commercial customers only]) 111.6 kWh

Can power large-scale commercial operations. May be helpful for peak demand shaving.

**These are very general estimates. Battery capacity varies, as does how much electricity is needed to power devices and appliances. Estimates are taken from*Cost after incentive include 26% Federal Tax Credit only, which may or may not be available for tax-paying homeowners and businesses until the end of 2019. Consult your tax professional for more information. If you’re a Maryland resident, you’re also eligible for the Maryland Energy Storage Income Tax Credit Program. If you’re a business, you may be able to use accelerated depreciation. 

Ready to find out how much a battery system would cost for your household or business? Request a quote today!Request Pricing for Your Solar SystemIf you’re not sure you’d like to invest in batteries for solar energy, but having electricity while the grid goes down is important to you, consider a generator. While these don’t run on sustainable energy like your solar-powered systems, they are a much more affordable option. 

batteries vs generators