Solar and battery systems offer homeowners an unprecedented opportunity to own and control the production, storage, and consumption of their essential electricity needs.
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While installing solar panels is relatively straightforward, pairing them with battery storage is a little more nuanced given the various types of batteries available and what they&#;re able to do.
So, in this article, we&#;ll explore which batteries pair best with solar panels to accomplish the three most common energy goals: Cost savings, essential backup, and whole-home backup.
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Let&#;s start with a quick recap of the different types of batteries on the market.
What types of solar batteries are there?
There are three distinguishing features that determine a battery&#;s &#;type.&#;
(what it&#;s made of)
(AC vs DC)
(Backup vs Consumption-only)
Battery chemistry: Lithium-ion versus Lithium Iron Phosphate (LFP)
There are no fewer than five types of battery chemistries that could be used (theoretically or practically) for residential energy storage. However, Lithium-ion (Li-ion) and Lithium Iron Phosphate (LFP) have emerged as the dominant chemistries today, as they provide an ideal balance of energy density and efficiency.
Side note: The &#;F&#; in LFP is for &#;ferro,&#; which is a synonym for iron.
Compared to Lithium-ion, LFP batteries generally last longer, allow greater depth of discharge, handle a wider range of temperatures, and have a virtually non-existent risk of thermal runaway.
On the other hand, Lithium-ion batteries tend to cost less and are less affected by maintaining a high state of charge, which is common for essential backup systems.
Feature
LFP
LI-ION
Lifecycles before degradation
1,000 to 10,000
500 to 1,000
Energy density
40-55 Wh per lb
45-120 Wh per lb
Depth of discharge
Up to 100%
80-95%
Operating temperature
-4 F to 140 F
32 F to 113 F
Risk of thermal runaway?
Near non-existent
Noteworthy if not properly installed
Ideal usage
Self-consumption & TOU modes
Backup mode
Cost
$$$
$$
AC vs DC batteries
Another distinguishing feature to consider is whether a battery is AC- or DC-coupled. Certain batteries can charge on Direct Current (DC) electricity while others charge on Alternating Current (AC) electricity.
In general, DC batteries are more efficient while AC batteries are much easier to configure into existing solar systems.
AC-coupled batteries
DC-coupled batteries
Round-trip efficiency
85-90%
Up to 97.5%
Compatability
Compatible with most existing solar and inverter configurations
Difficult to configure into existing solar systems, especially systems with microinverters
Whether or not you already have a home solar system &#; and how that system is configured &#; will determine whether an AC- or DC-coupled battery is best.
Consumption-only vs backup
The third distinction to consider is whether the battery is backup-enabled or configured for self-consumption only. While backup power is typically thought of as the primary benefit of battery storage, there is a growing demand for battery storage without the additional costs attributed to the equipment, labor, and programming required to enable backup power when the grid goes down. Given this smaller scope of work, consumption-only battery projects typically costs around two-thirds of comparable backup-enabled battery projects.
With that in mind, consumption-only batteries (aka &#;non-backup&#; batteries) are typically used by homeowners who:
Have the primary goal of energy cost savings
Are largely unaffected by power outages
For example, under California&#;s NEM 3.0 solar billing policy, it&#;s much more cost-effective to store and use your own solar production in a consumption-only battery than to trade electricity back and forth with the utility grid.
If your primary goal is energy cost savings and you have no need for backup power, then the best battery to pair with solar panels is a Lithium Iron Phosphate (LFP) consumption-only battery. Whether an AC- or DC-coupled battery is best depends on whether or not you already have solar panels.
Some of the best LFP batteries currently on the market include:
Enphase IQ (AC-coupled)
Panasonic EverVolt (AC or DC-coupled)
SunPower SunVault (AC-coupled)
Many of these batteries are offered as consumption-only packages.
Why is LFP battery chemistry best for cost savings?
Batteries used primarily for cost savings typically operate in solar self-consumption mode. This means that the battery will only charge on solar power and discharge as soon as the solar panels can&#;t meet household electricity demand.
In self-consumption mode, the battery is charged and discharged (aka &#;cycled&#;) on a daily basis and carries a very low charge overnight (known as a low &#;state of charge&#;).
Based on a study by the National Renewable Energy Laboratory (NREL), LFP batteries are more resilient to daily cycling and actually prefer a low state of charge, which makes them particularly suited for self-consumption mode.
Why consumption-only battery for cost savings?
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By leaving out the equipment, labor, and programming required to enable backup power, consumption-only batteries typically come at around two-thirds of the cost of a traditional backup battery. And if your primary goal is to save money, then a battery with a lower upfront cost provides a quicker return on investment.
While consumption-only batteries can provide greater energy cost savings, it&#;s worth emphasizing that they do not provide backup power during grid outages.
Connect with an Energy Advisor to design a custom solar & battery system for your goals.
Is AC or DC battery better for cost savings?
While DC batteries are more efficient &#; and thus provide more energy bill offset per kWh of solar production &#; they typically come with a greater upfront cost, especially if when adding a DC battery to an existing solar system.
If your primary goal is cost savings, it&#;s worth crunching the numbers for your specific situation to see whether an AC or DC battery provides the greatest return on investment.
If the primary goal is powering essential systems (lights, Wi-Fi, refrigeration, etc) during grid outages, the best battery to pair with solar panels is a backup-enabled Lithium-ion battery. Again, whether an AC- or DC-coupled battery is best depends on whether or not you already have solar panels.
Some popular batteries that fit this criteria include:
Tesla Powerwall (AC-coupled)
Generac PWRcell (DC-coupled)
LG RESU 10H Prime (DC-coupled)
LG ESS Home 8 (AC-coupled)
Obviously, if you want to provide backup power, then a backup-enabled battery is required and consumption-only configurations are not an option.
Why is Lithium-ion (NMC) chemistry better for backup mode?
Unlike LFP batteries, which prefer a low average state of charge, Lithium-ion batteries (particularly the NMC chemistry used by Tesla, LG, and Generac) tend to prefer maintaining a high state of charge &#; which aligns well with backup batteries.
In backup mode, batteries go through fewer charge/discharge cycles and maintain a high state of charge until the grid goes down. According to the NREL, Lithium-ion (NMC) batteries maintain their original capacity longer under these conditions than LFP batteries, as shown in the figure below.
Is an AC or DC battery better for essential backup?
The choice between AC and DC batteries is typically dictated by whether you already have solar panels installed.
If you have an existing system, then AC-coupled batteries will be easier (and less expensive) to add to the mix. If you don&#;t have solar panels, then DC-coupled batteries becoming a much more attractive option.
In an essential backup scenario, having a more efficient DC battery allows you to squeeze more power out of every kWh of solar production during the outage. That means more time with the lights on and, possibly, more systems to power with your backup battery!
If the primary goal is to power every system in your home &#; during outages or when the grid is online &#; then the best batteries to pair with solar panels are the ones that can be stacked together to provide enough peak and continuous power output for large loads like air conditioning and EV charger.
Based on that criteria, here are a few of the best batteries for whole-home backup:
Battery
Chemistry
Number of battery units stacked together
Usable capacity
Continuous output
Round-Trip efficiency
Franklin Home Power (AC)
LFP
3
40.8 kWh
15 kW
89%
Tesla Powerwall 2 (AC)
NMC
3
40.5 kWh
15 kW
90%
LG ESS Home 8 (AC)
NMC
3
43.2 kWh
22.5 kW
90%
Enphase IQ 5P (AC)
LFP
8
40 kWh
30.72 kW
90%
Generac PWRcell (DC)
NMC
12
36 kWh
11 kW
96.5%
You&#;ll also want to consider battery chemistry. For example, if your goal is simply to store enough energy to power your entire home during a grid outage, the NMC chemistry is more suitable. However, if you plan on using charging and discharging on a daily basis, LFP chemistry is more suitable.
Additionally, round-trip efficiency plays an important role in battery systems of this scale. With a 10 kWh battery, 90% efficiency means losing 1 kWh per cycle. But with a 40 kWh system, 90% efficiency means losing 4 kWh per cycle.
So, if you don&#;t already have solar panels, it&#;s worth exploring more efficient DC-battery &#; as long as they&#;re stackable. If you already have solar panels, then keep an eye on the most efficient AC-coupled batteries.
Find the best battery for your solar system
With power outages increasing and net metering policies eroding, home batteries are becoming more mainstream and beneficial by the day. And while every battery company claims to have the best product, the best battery for your solar system is the one that empowers you to achieve your energy goals.
Connect with an Energy Advisor to set goals and find the best battery for your new or existing solar system.
Are you interested in learning more about battery storage solutions? Contact us today to secure an expert consultation!