Is solar battery storage with it for home

Falling solar battery cost and better solar battery storage options such as the Tesla PowerWall and the RESU energy storage solution from LG Chem have made more people think about energy storage for their homes.  Rising utility rates will only increase motivation for home and business owners to become independent from the grid. However, all of these people will ask if solar battery storage is worth it given current solar battery cost.

How much does a solar battery cost for my home in 2019?

Interest in solar batteries has further been pushed by the introduction of time-of-use electricity billing with expensive peak and inexpensive off-peak times. This has lead people to look at the possibility of using solar batteries to store power in off-peak times to use in peak times. With some states opting to end net metering, this will create further interest in using solar batteries to store excess solar power. 

The table below provides solar battery cost and capacity for the leading  Solar Battery brands:

Solar BatteryCapacity (kWh)CostDoes it include a battery inverter?Peak constant draw
Tesla Powerwall 14 (13.5 usable) $5,900 Yes 7kW, 5kW constant
LG Chem RESU 6.5 or 9.8 $5,000 or $10,000 No 5kW
Sonnen EcoLinx 10-20 $16,000 (min) Yes 3kW

What size of solar battery is needed to become independent from the grid?

The battery size needed to make your home capable of operating completely off-grid will vary with the amount of power your home uses, the times it uses power and the peak instantaneous draw of electricity required to meet your maximum load. Most home battery systems in the US are one or a few batteries designed to keep a small amount of backup power during an outage. Going completely off-grid takes a bank full of batteries.

The majority of solar installations are grid-tied systems which allow excess solar energy to be sent to the grid and used to offset the cost of your usage at night. Perhaps the biggest drawback to home battery solutions is that they can be rather unreliable at times as seasons change. Since your panels will produce less in the winter, there is no guarantee that an off-grid solution will be able to keep your power on.

If you want an off-grid system that will keep your power on 100% of the time, you’ll need to purchase a lot of batteries. To minimize the cost of your battery system, it is best to reduce your overall power usage and install energy efficient appliances in your home. Many utilities and local governments offer rebates for energy efficiency improvements.

What are the different types of solar batteries?

Traditionally a solar storage battery was a lead-acid battery bank hooked up to an off-grid solar battery charger and battery inverter. The batteries, battery charger, and battery inverter were separate. There were wet cell lead acid solar batteries and there were sealed gel lead-acid solar batteries. These were a stable technology but they are big and heavy, and only last for a long period when the depth of discharge is limited to 40-50% of the batteries capacity.

The newer solar battery technology that is now taking over is Lithium-Ion solar batteries, the same battery chemistry as exists in cell phone batteries and computer batteries. The advantages of the Lithium-Ion batteries are that they can be discharged to almost 100% of their total storage capacity and they have greater energy density per square foot and per pound of weight.

How do solar battery systems work?

Solar batteries

Solar battery systems can either be AC-coupled or DC-coupled. With AC coupling, the power from your solar panels is converted through a standard grid connect inverter from DC to AC, before then being converted by the battery inverter and stored in the solar battery as DC power. Then when the power is needed, the battery inverter converts the DC power from the battery back into AC power and supplies it to the loads.

With a DC-coupled solar battery, power from the solar panels is directed into the battery through a solar battery charger that regulates how the batteries are charged. It isn't necessary to convert the power because solar panels natively produce DC power and batteries store DC power.

AC-coupled solar batteries vs DC-coupled solar batteries?

Theoretically, a DC-coupled solar energy storage systems are probably architecturally more straightforward because the solar power from the panels is not converted from DC to AC back to DC as it's an AC-coupled system. The first version of the Tesla Powerwall was DC-coupled. Most of the old school off grid lead-acid battery systems were also DC-coupled.

However, the introduction of Lithium-Ion batteries threw a spanner in the works for DC-coupled solar battery storage systems because Lithium-Ion batteries required their own battery management systems to control their operation, and the traditional battery chargers used in DC-coupled off-grid systems didn’t work well. This is why there have been a lot of problems between 2015 and 2018 regarding the electrics of solar systems working with custom battery management system that get used with different brands of Lithium-Ion batteries. These issues are why the development of integrated Lithium-Ion home energy storage systems has been slow and also a key reason the original DC-coupled version of the Tesla Powerwall has been scrapped.

In time better battery chargers and battery management system integrations would have developed (and still might) for DC-coupled Lithium-Ion solar energy storage solutions, but two other issues have arisen that has pushed the whole industry towards AC-coupled solar batteries. The first of these issues is that a big part of the market already have grid connect solar systems and so their solar power is already being converted from DC to AC by their existing grid connect inverter. This means a system has to be AC-coupled to avoid having to change their existing system.

Secondly, and more importantly, there is a growing realization that where time of use electricity billing is being mandated, as it is for the three big investor-owned utilities in California, that a significant function of the energy storage system is to store power drawn for the grid at low cost off-peak times for use in peak times.  Given grid power is AC power then an AC-coupled system allows the battery to be seamlessly charged by either the solar panels or the grid. This is the main reason that all of the main Lithium-Ion solar energy storage solutions being developed now, including the three above, are AC-coupled.

How many solar batteries will I have to install to have enough energy storage?

The size of the battery bank required to take your home 100% off-grid depends on your average daily power usage and how many days of power you want to store (also known as days of autonomy).

How do I work out my average daily usage and what size of?

Your utility is required to send this to you by law, or you can visit CutMyBill to view your rate, daily usage and the best rate plan for your usage, as well as the cost and savings from installing solar. Once you know what size battery system you want, and your desired days of autonomy you’re ready to approach an installer. Our site SolarReviews has thousands of reviews for installers across the entire country many of which offer home battery solutions.

Are there any available rebates for installing solar batteries?

California offers the SGIP rebate for battery systems, rates vary for each utility and are as follows.

UtilityPer Watt incentive (small residential <10kWh)
PG&E $350/kWh
SCE $350/kWh
CSE (on behalf of Pacific Gas and Electric) $250/kWh

Rebates are based on the amount of energy your battery system can hold in kWh multiplied by the incentive amount, for example, a 10kWh battery system in PG&E’s territory would be eligible for a $3,500 rebate.

Can I claim the 30% federal solar tax credit and the SGIP rebate on a solar battery system?

If you install a battery system at the same time as your solar system, your batteries are eligible for the 30% federal solar tax credit. In order for batteries to be eligible, there are certain restrictions, such as your batteries must be charged 75% or more by your solar. When calculating your battery tax credit, you multiply the percentage of battery charge supplied by solar, eg. 80% by the 30% from the tax credit (0.8 x 0.3). This example would allow 0.24 (24%) of the cost of your batteries to be claimed as a deduction to your federal tax liability.

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