AC vs. DC coupling: Which is best for your application and why?

AC or DC coupling

AC or DC coupling refers to the way in which solar panels are coupled with and interact with a battery system. A hotly debated topic among solar installers today is whether AC or DC coupling is the best approach for solar+storage installations and retrofits. The truth is there really is no right or wrong answer. Both approaches have their merits, and the optimal approach depends entirely upon the application.

What is AC coupling?

AC coupled systems require two inverters: a common grid-tied solar inverter and a battery-based inverter. This means that the energy used by the batteries may be inverted as many as three times before being used in the home — i.e., from DC (PV array) to AC (load center) through the solar inverter, then back to DC (batteries) through the battery-based inverter, and then back to AC again (home loads). See Diagram 1 below.

Simpliphi Single line diagram AC coupling

Diagram 1

Small losses occur through each inverter, resulting in a reduction in overall system efficiency. Therefore, while AC coupled systems may be easier to install, using battery storage to cover AC loads is likely to result in a marginal decrease in efficiency.

However, AC coupled systems can be much more convenient for retrofits in which customers want to add batteries to existing residential grid-tied solar systems. One only needs to purchase an additional battery-based inverter to connect the batteries.

Because of the ease of installation, AC coupling can be ideal for grid-tied residential battery backup systems as well as large commercial systems, especially for retrofits where solar panels have already been installed.

What is DC coupling?

DC coupled systems use a charge controller to directly charge batteries with solar generation and a battery-based inverter to power home loads (AC). See Diagram 2 below.

Simpliphi Single line diagram DC coupling

Diagram 2

As a result, DC coupled systems are slightly more efficient than AC coupled systems because the power is not inverted multiple times. However, they often require more labor to install because the charge controllers tend to require smaller strings from the PV array.

When it comes to solar+storage retrofits, DC coupled systems present more challenges because the existing grid-tied solar inverter must be entirely removed and replaced with a battery-based inverter. In most cases, the existing PV array wiring will also need to be reconfigured.

DC coupling is ideal for new on- and off-grid solar+storage system installations in both residential and small commercial applications, but not retrofits with existing solar panels.

Sum it up: Pros and cons of AC and DC coupling

AC coupling pros:

  • Get to keep grid-tied inverter
  • Easier installation, especially for retrofits

AC coupling cons:

  • Less efficient
  • Less system functionality

DC coupling pros

  • More efficient
  • More regulated charging

DC coupling cons:

  • Not ideal for retrofits
  • Longer installation time

There are many factors that determine whether AC or DC coupling is best suited to a given application, so it’s important to understand the particular conditions and constraints of each project. Be sure consult with the battery manufacturer and/or the inverter manufacturer(s) to help you design the best energy system that optimizes the cost and performance of all the equipment for your particular project.

This post and the entire 12 Days of Storage was contributed by SimpliPhi.

— Solar Builder magazine

Here’s a crash course in battery system sizing

 

battery size calculation

Get your calculator ready.

There are various ways to determine the size of a battery bank when designing a system. The most efficient way to size a battery bank is to determine the electrical loads and load requirements for both power and energy. Proper system design involves a number of factors and requires analysis and calculations on the loads, PV or other generation sources, as well as the battery performance profile being used in the system.

Depth of discharge

As discussed a few days ago on the Fourth Day of Storage, depth of discharge plays an important role when sizing batteries because battery banks must be calculated according to the actual amount of usable energy storage. Check your battery’s warranty for the most accurate statement of its depth of discharge. For example:

  • 80% DoD = 3.5 kWh x .8 = 2.8 kWh
  • 90% DoD = 3.5 kWh x .9 = 3.1 kWh
  • 100% DoD = 3.5 kWh x .1 = 3.5 kWh

Sizing based on loads

The most important step when sizing a battery system is to determine the required or desired amount of energy storage — most often using a measure of kWh-per-day. The minimum kWh-per-day value can be calculated based on the wattage and runtime of all potential loads to be supported by the system. From there, the battery size may be adjusted depending on whether the system is intended for daily cycling or backup power. The load profile and desired duration of backup power should also be considered.

There are a number of online resources available to calculate loads and determine the appropriate kWh-per-day value from utility billing information.

Consider this example:

  • 2.8 kWh at 80% DoD
  • Load calculations: 10 kWh per day
  • Customer requests: 1.5 days of backup power
  • 10 kWh x 1.5 days = 15 kWh of desired storage
  • 15 kWh/2.8 kWh (battery size) = 5.3 batteries

In this example, based on the actual usable amount of energy for 5.3 of the batteries selected, you may choose to size up to 6 batteries or round down to 5 batteries based on customer preference, either to create extra cushion for unforeseen electrical loads or to be cost conscious and design a more conservative system.

Sizing based on solar

Simpliphi Governor Brown off grid project

The battery bank at California Governor Jerry Brown’s off-grid residence.

When retrofitting an existing PV installation to add storage, battery bank size is most often computed based on the size of the solar array. It is important to consider peak sun hours, PV Watts data (realistic energy production based on location), and PV size (kW) as part of the calculation. In addition, it’s critical that the system cannot exceed the maximum continuous charge rate of the battery bank to prevent damage and ensure a long life. For example:

  • Battery for system: 3.5 kWh battery with maximum charge of 1.7 kW continuous
  • PV array size: 4 kW
  • Average PV daily production: 20 kWh per day
  • 4 kW (PV) / 1.7 kW (Max. battery charge) = 2.3 batteries

Round up the 2.3 battery units to determine that the minimum number of batteries would be three of the 3.5 kWh batteries.

Based on daily PV production: 20 kWh (PV per day) / 3.1 kWh (battery at 90% DoD) = 6.4 (3.5 kWh batteries)

From this calculation, you can round up to 7 or round down to 6 batteries based on customer preference (such as mode of operation).

Power Requirements

Every battery has a maximum charge and discharge capacity. Those rates must be adhered to for adequately charging the battery, like PV system size (charge rate) and the continuous load value to be supported by the battery (discharge rate).

Consider this example:

  • Battery for system: 3.5 kWh with a maximum continuous discharge of 1.7 kW
  • Home maximum continuous discharge: 6 kW
  • 6 kW (continuous load) / 1.7 kW (battery maximum discharge) = 3.5 batteries

When it comes to power requirements, you always round up to determine the minimum battery bank size. In this example, the system requires 4 of the 3.5 batteries.

For additional guidance, SimpliPhi Power offers a simple battery bank sizing estimator tool right here.

This post and the entire 12 Days of Storage was contributed by SimpliPhi.

— Solar Builder magazine

Pika Energy is cutting prices on its smart battery products

pika Energy

Pika Energy, maker of the Pika Energy Island smart solar-plus-storage platform, announced a price cut of $1,000 for its Coral smart battery, effective immediately. The price reduction follows the previous announcement of cost reductions to Pika’s Harbor smart battery line, lowering prices across all of Pika’s smart battery products.

Pika says the price reductions have been enabled through scale efficiencies achieved through rapid sales growth and are designed to deepen market share in areas where local demand is surging and other residential storage providers cannot effectively compete on performance.

“With the recent storms in Puerto Rico, Florida, Texas and other coastal regions, demand for powerful energy storage systems has rapidly escalated,” said Ben Polito, CEO of Pika Energy. “This demand is often going unmet. Our new, lower prices will enable more of the people who need it most to be able enjoy clean, reliable backup power.”

Get a new tip on battery buying each day during our 12 Days of Storage

What is the Coral?

The Coral smart battery is Pika’s entry level system but it packs serious power, up to 8 kW. This affordable and powerful system will help to meet the surging demand for whole-home backup power in regions recently impacted by severe weather events. Coral is a safe and affordable smart battery built on trusted AGM technology. Coral seamlessly integrates with solar for multi-day protection from extended grid outages.

“Pika’s systems have set the bar for excellence in the Puerto Rican energy storage market,” said Juan C. Díaz-Galarza, Vice President of Operations at Dynamic Solar in Puerto Rico. “By taking a platform approach, Pika enables homeowners to add more storage and access more power as their needs change. This is very important as storms in the Caribbean continue to intensify and people look for new technologies to keep their homes safe. Instead of living in fear the next hurricane, Pika system owners are ready for the next big storm.”

Homeowners are increasingly looking for systems with more power to keep their homes operating normally when the grid goes down. Pika’s solar-plus-storage systems deliver up to two times the current and the ability to run major home appliances, like air conditioning. When needs change due to weather events or new utility rates, Pika Energy Island system-owners can uniquely upgrade and expand their system with more or larger batteries, rather than requiring an entire new system.

— Solar Builder magazine

Know your battery specs: Nameplate capacity (10 kWh) vs. Usable capacity (7 kWh)

lead acid battery bank

How many of these will you need to size a job properly for its lifetime?

A common misconception is that lead acid batteries cost less than lithium-iron phosphate batteries. However, what most fail to consider is that you have to buy many more lead acid batteries — sometimes double, triple or quadruple as many — just to reach the same usable capacity as you would achieve with far fewer lithium-based batteries.

The misconception is largely due to battery manufacturers touting their total rated or nameplate capacity, which is the kWh the battery is theoretically able to store. You have to dig deeper to find a battery’s actual, usable capacity — which is the kWh the battery is able to store after factoring in depth of discharge, efficiency and charge/discharge rate restrictions.

In layman’s terms, nameplate versus useable capacity is not unlike your annual salary versus your actual take home pay after taxes, social security, health care and other expenses have been deducted. In theory, your offer letter says you will earn $50,000 annually, but your paychecks only deliver a usable $32,500.

Depth of discharge, efficiency and charge/discharge rate not only directly impact the number of batteries that must be purchased up front, they also significantly impact the Levelized Cost of Energy, or the overall cost per kWh associated with a battery over the course of its lifetime.

Depth of discharge

DOD vs SOC v2

When trying to determine which battery is most cost effective, review the allowable or (more important) warrantied depth of the discharge. You can often find this information listed as part of a battery manufacturer’s warranty or product data sheets.

Let’s say you are trying to decide whether to go with 10 kWh total storage capacity of lead acid batteries vs. 10 kWh of total storage capacity of lithium batteries. Since lead acid batteries often can’t be discharged (used) more than 30% to 50% of their total rated capacity at a time (i.e., their state of charge cannot go below 50%) and lithium batteries can often be discharged 80% to 100%, this results in significantly more available energy for the lithium battery and much less usable capacity for the lead acid battery bank.

simpliphi battery bank calculation

Before factoring in efficiency or charge/discharge rates, it’s already necessary to purchase double the lead acid batteries to approach the usable capacity of the lithium battery bank, despite both banks having the same rated 10 kWh capacity.

Roundtrip efficiency

lower roundtrip efficiency image_60 percent_12 days of stroage_nameplate

higher roundtrip efficiency image_98 percent_12 days of stroage_nameplate

Roundtrip efficiency is the ratio of energy put into a battery versus the energy that comes out of a battery. No battery is 100% efficient because there are always some inefficiencies between the amount of energy sent into the batteries vs. how much energy can actually be used (i.e., is not consumed by the battery during the charge and discharge process). Lithium batteries generally have much higher efficiency than lead acid batteries, which again directly impacts the amount of usable capacity, as demonstrated below.

simpliphi usable capacity

Therefore, in addition to depth of discharge reducing usable capacity, roundtrip efficiency rates further reduce usable capacity to varying degrees. After factoring in both depth of discharge and roundtrip efficiency in the above example, the lithium batteries have almost four times higher usable capacity than the lead acid batteries, despite having the exact same 10 kWh nameplate capacity. In other words, you would need to purchase 4x as many lead acid batteries as lithium batteries to reach the same usable capacity.

Levelized Cost of Energy: The true cost of a battery over time

Beyond depth of discharge and roundtrip efficiency, be sure to consider cycle life, or the number of charge/discharge cycles you can get out of a battery over the course of its life. Consider this calculation of LCOE.

simpliphi depth of discharge

When evaluating which energy storage solution is best suited for your next project, it’s important to consider the full range of data specifications needed to determine the overall performance and cost of the battery over time, not just the often misleading upfront price point and assumed performance of the published nameplate capacity.

This post and the entire 12 Days of Storage was contributed by SimpliPhi.

— Solar Builder magazine

Know your battery specs: Nameplate capacity (10 kWh) vs. Usable capacity (7 kWh)

lead acid battery bank

How many of these will you need to size a job properly for its lifetime?

A common misconception is that lead acid batteries cost less than lithium-iron phosphate batteries. However, what most fail to consider is that you have to buy many more lead acid batteries — sometimes double, triple or quadruple as many — just to reach the same usable capacity as you would achieve with far fewer lithium-based batteries.

The misconception is largely due to battery manufacturers touting their total rated or nameplate capacity, which is the kWh the battery is theoretically able to store. You have to dig deeper to find a battery’s actual, usable capacity — which is the kWh the battery is able to store after factoring in depth of discharge, efficiency and charge/discharge rate restrictions.

In layman’s terms, nameplate versus useable capacity is not unlike your annual salary versus your actual take home pay after taxes, social security, health care and other expenses have been deducted. In theory, your offer letter says you will earn $50,000 annually, but your paychecks only deliver a usable $32,500.

Depth of discharge, efficiency and charge/discharge rate not only directly impact the number of batteries that must be purchased up front, they also significantly impact the Levelized Cost of Energy, or the overall cost per kWh associated with a battery over the course of its lifetime.

Depth of discharge

DOD vs SOC v2

When trying to determine which battery is most cost effective, review the allowable or (more important) warrantied depth of the discharge. You can often find this information listed as part of a battery manufacturer’s warranty or product data sheets.

Let’s say you are trying to decide whether to go with 10 kWh total storage capacity of lead acid batteries vs. 10 kWh of total storage capacity of lithium batteries. Since lead acid batteries often can’t be discharged (used) more than 30% to 50% of their total rated capacity at a time (i.e., their state of charge cannot go below 50%) and lithium batteries can often be discharged 80% to 100%, this results in significantly more available energy for the lithium battery and much less usable capacity for the lead acid battery bank.

simpliphi battery bank calculation

Before factoring in efficiency or charge/discharge rates, it’s already necessary to purchase double the lead acid batteries to approach the usable capacity of the lithium battery bank, despite both banks having the same rated 10 kWh capacity.

Roundtrip efficiency

lower roundtrip efficiency image_60 percent_12 days of stroage_nameplate

higher roundtrip efficiency image_98 percent_12 days of stroage_nameplate

Roundtrip efficiency is the ratio of energy put into a battery versus the energy that comes out of a battery. No battery is 100% efficient because there are always some inefficiencies between the amount of energy sent into the batteries vs. how much energy can actually be used (i.e., is not consumed by the battery during the charge and discharge process). Lithium batteries generally have much higher efficiency than lead acid batteries, which again directly impacts the amount of usable capacity, as demonstrated below.

simpliphi usable capacity

Therefore, in addition to depth of discharge reducing usable capacity, roundtrip efficiency rates further reduce usable capacity to varying degrees. After factoring in both depth of discharge and roundtrip efficiency in the above example, the lithium batteries have almost four times higher usable capacity than the lead acid batteries, despite having the exact same 10 kWh nameplate capacity. In other words, you would need to purchase 4x as many lead acid batteries as lithium batteries to reach the same usable capacity.

Levelized Cost of Energy: The true cost of a battery over time

Beyond depth of discharge and roundtrip efficiency, be sure to consider cycle life, or the number of charge/discharge cycles you can get out of a battery over the course of its life. Consider this calculation of LCOE.

simpliphi depth of discharge

When evaluating which energy storage solution is best suited for your next project, it’s important to consider the full range of data specifications needed to determine the overall performance and cost of the battery over time, not just the often misleading upfront price point and assumed performance of the published nameplate capacity.

This post and the entire 12 Days of Storage was contributed by SimpliPhi.

— Solar Builder magazine