Battery storage with (and without) solar is becoming increasingly popular.
A number of factors are contributing to the rapid adoption of storage.
Utility costs are increasing as more utilities are switching to time of use rate plans.
Grid outages are common in some areas.
Decreasing Costs
The substantial growth in electric vehicles “EV” is spurring increased supply in batteries both in terms of manufacturing and R&D. Companies like Panasonic, whom are one of the largest suppliers to worlds EV manufacturers, are also making battery cells and modules for their own storage systems “EverVolt” and other manufactures including “Tesla”.
Other large, multi-national companies like LG, Samsung, BYD, CATL are ramping up production as well. This will continue to increase capacity and reduce costs. While battery costs may continue to decrease they are only a portion of the cost related to an installation. If a battery decreases by 10% in cost, the installation cost to a homeowner doesn’t decrease by 10%. The cost of engineering, permits, installation, miscellaneous parts, as well as the batteries mechanical enclosure and electrical system do not decrease.
Time of Use Rate Plans
Time of use rate plans are designed for utilities like PG&E and SMUD, to reflect demand on their grids. In the early evenings when the sun goes down, renewable energy from solar goes away and homeowners use more power coming home from work and school.
Using HVAC systems (air conditioning), ovens, washers, dryers, etc increase the demand on the grid. The utilities increase their rates during these increased demand times. Battery storage can help offset this cost by charging batteries during the day with less expensive electricity (whether through an attached solar system or from the grid) and then switch to a “self-consumption” mode when the rates increase.
The batteries systems include monitoring a home or businesses consumption. They are programmed to match the rate tariff schedule of the utility. For example, PG&E’s rate tariff, E-TOU option D, increases the rates for residential customers from the hours of 5-8pm (this is a common rate that new solar customers are forced to use). The battery is programmed in “self-consumption” mode during these hours.
During the day, the battery is charged (from existing solar on site or from the grid). When the rates increase the battery will supply the loads instead of the utility. One specific thing to note, most utilities including PG&E, do not allow exporting electricity from the battery to the grid. Customers are not allowed to charge at the low cost and then sell back to the utility at the high cost.
Grid Outages
The dependability of the grid varies throughout the USA. California is experiencing PSPS, “Public Safety Power Shutoffs” events. Utilities including Pacific Gas & Electric, Southern California Edison and San Diego Gas & Electric are shutting down sections of their grid during weather conditions and San Diego Gas & Electric are shutting down sections of their grid during weather conditions that have a high probability of bringing down power lines causing fires.
These shutdowns can range from hours to days. Battery storage systems can provide backup power to the home or business during these shutdowns.
Depending on what needs to be backed up, generators and manual transfer switches may be a desirable solution. Battery storage when coupled with a solar system can allow a home or business to run during extended outages. As long as the system is sized correctly, the owner understands the available power (how much can be used at the same time) and available energy (the amount or hours of storage available), and the amount of solar generated to be used during the day and charge the battery at night.
It is important to understand that unless both the solar system and battery storage are large, during an outage, the usage will need to be decreased proportionately. That is why for some customers, we recommend Generac automatic home backup generators
Types of Battery System Technologies
ACCOUPLED
VS
DCCOUPLED
DC Coupled
With a solar system installed with a DC coupled battery like the Generac PWRcell of the Panasonic EverVolt, the batteries are charged directly from the solar power. When the energy is needed, the inverter changes it from DC to AC one time.
This system is far more efficient than an AC coupled system. Additionally, if the solar system is producing more power than the batteries can take and the home is using, the inverter can adjust the power output accordingly. This allows the solar system to still supply power, but not exceed the required power.
This is the most efficient battery system and a great solution to consider when installing a new solar system with battery storage.
AC Coupled
When a home has an existing solar system, an AC Coupled storage system is often the most economical. Solar systems produce DC power. The inverter (whether a string inverter, like SMA or SolarEdge, or microinverters like Enphase) change that to AC power, match our utility grid.
An AC Coupled battery system, like a Tesla PowerWall or Enphase Encharge, take that AC power after the inverter, and change it back to DC electricity via a built in inverter (or microinverters in the case of Enphase) to store it on a DC battery. Then when the energy is needed, it is again, changed back to AC. Each time we change energy from AC to DC to AC we call that a round trip. The typical round trip loses about 10% of the stored energy. While that is not the most efficient way to do it, it costs the least. Changing an existing solar system to support a DC coupled battery requires replacing many components with extra cost.
One other disadvantage of an AC coupled system is that the battery cannot control the output of the solar system (it can only turn it on and off). If during a grid outage, the solar system is producing 5,000 watts. If the battery is full, and the home only needs 2,000 watts of power in real time. There is no place for the extra 3,000 watts to go. The AC coupled battery, shuts off the solar system and the energy is instead taking from the battery.
This is a less than ideal scenario where solar power is lost. DC coupled systems do not have this limitation. Also, Enphase’s Ensemble system, while AC coupled, doesn’t have this limitation when installed with their microinverters.
Comparison of Battery Storage Systems
Below is a simple comparison chart showing some of the most popular products available and their key attributes compared to each other. We should have the names of the batteries with hyperlinks to their Batteries Page 2
Generac PWRcell | Enphase Ensemble | Panasonic EverVolt | PowerWall 2 | LG Chem RESU 10H | |
|---|---|---|---|---|---|
AC Coupled | No | Yes | Yes | Yes | Yes |
DC Coupled | Yes | No | No | No | No |
Roundtrip Efficiency* | 96.50% | 89% | 84% | 90% | 94.5% |
Modular Battery Cabinet | Yes | No | Yes | No | No |
Usable Energy | 9 kWh -18 | 10.08 kWh | 11.4 kWh -17.1 kWh | 13.5 kWh’s | 9.3 kWh’s |
Load Shedding Integrated | Yes | No | No | No | No |
Peak Motor Starting | 25A -50A | 24A | 23A -31A | 29A | 18.9A |
Field | Yes | Yes | Yes | No | No |
Single Manufacturer^ | Yes | Yes | Yes | No | No |
Control Solar During Utility Outage | Yes | Yes | No | Yes | Yes |
Outdoor Rated Battery Cabinet | Yes | Yes | No | Yes | Yes |
Warranty | 10 Years, Throughput Warranty | 10 Years, >70% Capacity | 10 Years, >60% Capacity | 10 Years, >70% Capacity | 10 Years, >60% Capacity |
Weight | 276lbs – 441lbs | 341 lbs | 271 lbs -436 lbs | 251.3 lbs | 214 lbs |
Chemistry | Lithium Iron Phosphate (LFP) |
* Roundtrip efficiency on AC coupled storage systems don't include the loss of the solar system. Typical solar systems are 97.5%-99% efficient. These are additional losses not included in the number shown.
^ One manufacturer makes both the components for the solar system (independent of the solar panels) and the battery system. If there is an issue, there is one technical support company to diagnosis versus multiple manufacturers.
Design and Code Requirements
Best design practice and the National Electric Code “NEC” have a number of requirements. Some of the most important are highlighted below.
Automatic Transfer Switch "ATS" -a device that automatically changes the line side (also known as the supply side, where the power source comes from). For a battery storage system, during normal operation, the electrical panel (whether the entire panel or a protected loads panel) is fed by a utility like PG&E or SMUD. When the grid goes out, the "ATS" switches the source of the power from the grid to the energy storage system.
This is required by code to make sure that the grid is not back fed during an outage. For example, if PG&E is working on your neighbor's house they will shut off the local area. If a battery system started providing power without disconnecting from the grid, it would back feed the line and could potentially start a fire or electrocute a worker.
The battery storage system must be able to run the highest load in the panel that it is backing up (NEC 710.15(a)). This will limit what breakers are allowed to be backed up with a battery system. For example, a single PowerWall can run up to 29 continuous amps. That means 15-20A
Whole home backup versus a smaller protected loads panel
