Key Takeaways
What size solar battery do you need? Choosing the right solar battery comes down to three key factors: output, capacity, and expandability. Battery output determines which appliances and circuits can run during an outage, while battery capacity (measured in kWh) determines how long those loads can be powered.
Whole-home backup systems typically require more storage than critical-load backup systems, and many battery platforms allow homeowners to expand their storage over time by adding additional units. Understanding these factors can help you size a solar battery system that provides reliable backup power without overspending.
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Batteries have been the latest hot topic in the solar industry over the last few years, with no sign of slowing down. The Solar Energy Industries Association’s (SEIA) market insights reports show that 91% of all new electricity-generating capacity added is solar and storage, and batteries are being included on 45% of all new residential solar installations.
Driven by losses in net metering and rising energy prices, in part due to AI data center demand, batteries are a valuable addition to solar energy systems, giving homeowners better control over the energy produced by their solar panels.
But with the growth of the popularity of battery storage comes one of the same core questions that exists with solar: How much do I need?
And like solar, sizing battery storage systems is easier than most people think. The following guide will help break down three easy steps homeowners can take to properly size their battery storage system.
Table of Contents
Step #1: Battery Output
When sizing solar panel systems, solar installers always ask for 12 months of a homeowner’s electric usage. Why? To determine how much solar output is needed, installers need to know how much energy a home uses. If a home uses 10,000 kWh annually, a solar installer will want to install enough solar to produce roughly that amount of energy (or a little more in many cases).
The same concept applies to batteries. To determine how much storage a home needs, two questions need to be asked.
- What do you want to back up?
- How long do you want to back it up for?
So, for step #1, let’s start with that first question: what do you want to back up? What a homeowner wants to back up is critical when choosing a battery, as batteries have different output capacities. The battery datasheet will be a great starting point. See our guide if you aren’t sure how to read one.
First things first, you need to identify what items in the home you want to back up. The answer to this question has a lot of possibilities: maybe you only want to back up critical components like a refrigerator, lights, or a sump pump. Maybe you want to back up medical equipment or HVAC, or have your whole home available during an outage.
Head over to your home’s circuit breaker box. Each breaker has markings indicating the amperage (A) it is rated for. It’s common to see 10, 15, 20, 40, or 60. If you want to back up an appliance connected to a 20A breaker, you know your battery will need to be capable of discharging at least 20A continuously.
The very top of your breaker box has a “main breaker” that will usually say 100 or 200. If you wanted to back up the whole home, your battery system would need to be capable of continuously outputting at least that amount.
Now that you know what you want, head over to a battery datasheet and look for the discharge value. Here’s an example from Fortress Power’s eForce system:
Many battery data sheets will list a few values for “discharge,” so it can be a bit confusing. Generally, there are two things you want to know: continuous discharge and max discharge. Continuous is the primary number, and it is what the battery is generally capable of doing. Max discharge is what the battery is capable of doing for a short period of time, usually a few seconds.
Batteries have high maximum discharge current because of something called Locked Rotor Amperes (LRA). LRA is a measure of the amount of power it takes to start a motor from a total stop. Motors, like an air conditioner, require a higher amount of power to start up, then use less power to keep going. Have you ever had your lights flicker when the AC first starts up? That because of a high LRA.
Step #2: Battery Capacity
Now that you know what you want to back up and know what batteries can handle that demand, the next question is, how long do you want to back those items up for? Batteries don’t have unlimited energy, and when all the energy is used up, they must rely on the attached solar panels to recharge. That recharge isn’t immediate and will take several hours depending on available power.
Going to the data sheet again, you’ll want to look for capacity – this will always carry a unit of kilowatt-hours (kWh). Here’s an example from EG4’s WallMount All Weather Battery:
So you know what the battery is capable of, but how much energy does your appliance use? Most appliances have a sticker on them telling you what their power draw is, or you can look them up. Let’s do an example with the above EG4 battery.
Let’s say you wanted to back up a refrigerator. According to EnergySage, refrigerators actually have a lower running wattage than their rated average, because of the LRA we discussed above. Using their example, a fridge running at 167W ( 0.167 kW) over the course of a day would use 4 kWh.
With a battery capacity of 14.3 kWh, you could run that fridge for three and a half days, not counting any recharging the solar is able to do. Pretty good, right?
As you start to add more things to backup, that time starts to drop. Let’s say you wanted to back up your whole home. If you use 10,000 kWh per year, that’s roughly 27 kWh per day. With a 14.3 kWh battery, you could power your home for roughly 12 hours.
Once you know what you want to back up and how long you want it backed up for, that leads to step #3.
Step #3: Expandability
Step #3 is how many batteries you’re able to connect to give yourself more backup time or output capacity.
Every battery system on the market allows for multiple batteries to operate together in parallel or series to increase the total capacity and output potential. With the Fortress Power battery above, multiple batteries can be stacked together to produce a total capacity from 9.6 kWh to 28.8 kWh. The larger units can also output up to 250A.
Expandability is critical for two reasons. Number one, if you need to power a whole home or heavy energy use items for longer periods of time. And number 2, if your energy needs change in the future.
Even if you back up your whole house today, you may decide later down the road that you want to extend the time it can be backed up for. Knowing what your chosen technology is capable of can keep you prepared for whatever life might throw at you.
You’ll find a battery’s expandability either on the data sheet or the associated inverter data sheet. Here’s an example from Tesla’s Powerwall 3:
Protecting Your Battery
With those three measurements in hand, you can easily size a battery system to give you the desired reliability that comes with having a storage system. Of course, batteries are more than just backup; they also let you store and dynamically use energy throughout the day to avoid peak time rates or demand charges.
While getting the right equipment is important, protecting it and your energy security for the long term is also vital. As you’ll see when you poke around data sheets, most batteries have a warranty between 10 and 15 years. During that time, battery capacity degrades and typically about 3% per year.
That means that at the end of your warranty term, the battery still maintains about 80% of its original capacity, and has a long usable life ahead with no warranty.
One of the best things a homeowner can do when they install solar and storage is to get an independent warranty like SI-30 Battery to cover it. SI-30 Battery is a warranty from us here at Solar Insure that covers a battery for 30 years for parts and labor. A battery covered by SI-30 Battery also qualifies for one battery replacement after the manufacturer’s warranty period ends if the capacity drops below 50%. So if degradation is taking its toll, homeowners can receive a brand new battery at no additional cost. Learn more about SI-30 Battery here: https://www.solarinsure.com/si30battery
We’ve also created a battery sizing calculator that homeowners can use to help them size their battery systems: https://www.solarinsure.com/solar-panel-battery-sizing-calculator
The Best Batteries in 2026
There are a lot of battery brands out there, so which are the best options to choose from? We maintain our Approved Vendor List, or AVL, a list of companies we’ve thoroughly vetted for quality and reliability. Our AVL for batteries includes: Canadian Solar, EcoFlow, EG4, Enphase, Fortress Power, FranklinWh, Generac, Lunar Energy, Sigenergy, Qcells, SolarEdge, Sonnen, and Tesla.
While all the brands on our AVL are top quality, our work with thousands of installs shows us that there are a few very popular options in the battery storage market. Below, we’ll highlight some of the standout batteries from our Best Batteries list that we find most commonly enrolled without warranties.
EG4 WallMount All Weather Battery
- Capacity: 14.3 kWh
- Discharge Rate: 140A continuous (200A max for 30 min)
- Expandability: 6 batteries per inverter; 18 batteries on-grid, 108 off-grid
- Warranty: 10 years, over 8,000 cycles
EG4’s WallMount All Weather battery is a standout in terms of power output potential. A single unit has 14.3 kWh of capacity and a continuous discharge of 140A. Using max discharge on a single battery would make the unit dry up extremely quickly, but the potential is there to run anything inside a home in a power-down scenario.
The expandability is another place where this unit shines. Grid-connected setups require the use of a microgrid interconnection device, called the GridBOSS, to have a singular 200A connection point to interface with a utility grid. Three inverters can be connected to a GridBOSS unit, with each inverter allowing for 6 batteries to be connected to it. Off-grid setups don’t require a GridBOSS and can have 16 inverters tied together.
Enphase IQ10
- Capacity: 10.08 kWh
- Discharge Rate: 16A continuous, 23A max(3.84 kVA & 5.7 kVA)
- Expandability: 1 per 20A branch circuit, 4 total with combiner
- Warranty: 10 years, 4000 cycles
Enphase’s IQ10 battery, and all of their products for that matter, are known for their high levels of reliability. The batteries contain Enphase microinverters, making them easy to service.
The continuous power output isn’t as high as some others on this list, making it a more common choice for those who don’t want a lot of backup power but want to best utilize their energy use throughout the day.
Because this is an AC-coupled battery, the expandability isn’t as straightforward. Each unit requires a double-pole 20A breaker. An Enphase setup would require the use of an IQ combiner, which comes with 80A of space, so 4 batteries could be connected to it. Enphase is known
Tesla Powerwall 3
- Capacity: 13.5 kWh
- Discharge Rate: 48A
- Expandability: 7 total units
- Warranty: 10 years
Tesla’s Powerwall 3 has exploded in popularity in recent years. As a hybrid inverter/battery, the unit has a battery and an inverter all in one convenient box. However, the inverter and battery can’t be serviced separately, so if one fails the whole unit must be replaced.
The Powerwall 3 boasts a powerful 48A max output for a single unit, and up to seven units can be attached in parallel. Four of those units are full Powerwalls, and Tesla also makes a Powerwall expansion unit, three of which can be connected to one of the Powerwall units.
Fortress Power eForce Stackable Whole Home Energy Storage System
- Capacity: 9.6 kWh – 28.8 kWh
- Discharge Rate: 195A – 585A
- Expandability: 16 units
- Warranty: 10-15 years
Fortress Power’s eForce Stackable Whole Home Energy Storage System features the highest discharge rate on our list at 195A for a single unit. Fortress’ system is unique in that it was built to stack together and easily expand as needed. Their largest units (28.8 kWh) can discharge up to 250A or 585A, easily backing up an entire home during an outage.
Fortress built its system around ease of installation, with units that easily stack and click together to maximize space. Batteries come with a standard 10 year warranty, but if installed with an Envy inverter and Guardian gateway, the system is eligible for a 15 year warranty.
FranklinWh aPower 2
- Capacity: 15 kWh
- Discharge Rate: 48A at 10 kW
- Expandability: 15 units per aGate
- Warranty: 15 years
FranklinWh’s aPower 2 is a great all-around battery. It’s got the highest usable capacity for a single battery unit on the list, and a respectable 48A max discharge, capable of backing up just about anything in a home. It also comes standard with a 15-year warranty, leading most other competitors in the category.
Franklin setups utilize their aGate, which is a microgrid interconnection device. The aGate allows up to 15 units to be connected to it, making for a massive possible system. Franklin’s battery also has a neat LED on the outside that shows charge status.
Download our storage planning worksheet
Learn more about Solar Insure’s AVL and how we evaluate quality standards: https://www.solarinsure.com/solar-component-quality
Learn more about Solar Insure warranties and find a Certified Provider near you: https://www.solarinsure.com/our-solutions
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