Continuous performance improvements in solar energy batteries are improving the cost and energy efficiency equation of home solar power installations. Solar power that is generated but not currently required - called excess capacity - can be sold to the grid or placed in an energy storage device.
Lithium-ion (Li-ion) batteries are the storage technology of choice for most home solar energy systems. By storing energy when the sun is shining, producing a surplus and using it during night or overcast conditions, solar energy batteries improve energy efficiency and quality by stabilizing energy supply.
A solar energy battery system consists of the lithium ion battery, an energy management system and an inverter to connect the DC battery to the AC solar power system. Though the energy management system is often treated like the dashboard of a car - a necessary control interface but the high-performance engine is more interesting - connected to real time energy usage. It’s becoming smarter and playing a bigger role in improving the performance of the overall solar energy system.
Solar Battery Energy Efficiency Metrics
When evaluating different brands of solar batteries, you should be familiar with the following measures of battery performance:
Battery capacity - the amount of energy the battery can store measured in kilowatt hours (kWh). For the average home using 28 kWh per day, you would require two Tesla 14 kWh batteries to power the home on a cloudy day
Power output – also known as wattage, the amount of power the solar battery can output at a given time. Current systems range from 2 kW–5+ kW, though batteries comprised of smaller modules are often lower. An Enphase battery module starts at 0.26 kW. Peak power output adds up to 2 kW more on an average
Depth of discharge (DoD) - the percentage of capacity (in kWh) used before the battery should be recharged. For the standard lithium ion battery, the DoD is 80 to 100%
Round trip efficiency - the amount of energy stored that can be retrieved. For an efficiency of 90%, of 100 kWh stored, 90 kWh could be used
Cycles – it refers to the number of times a battery is charged and discharged and may vary from 6,000 – 10,000
Lifetime expectancy - like your laptop or cellphone battery, over time the charge of your storage battery will degrade. The DoD, discharge rate, and number of cycles will affect the battery lifespan. While newer batteries have a 10-year warranty, your solar panels have a life expectancy of 15 to 30-years, so your solar system cost analysis should include at least one replacement
Stackable - Solar batteries can be added to expand your storage capacity. You want to ensure your battery solution can meet any future increases in solar energy generation capacity. Lack of stack-ability and a high price killed Mercedes’ home energy storage
Your battery storage needs for your on-grid home solar energy system will be different from that of the off-grid solar system powering your cottage or summer home. Lithium-ion batteries provide many advantages over the incumbent lead-acid battery.
The depth of discharge of a Li-on battery is around 90% vs 50 to 60% for the lead-acid battery. The Li-ion will provide around 1,000 to 10,000 cycles, and the battery does not need a full recharge to recycle.
If it’s a cloudy winter day, when the battery only has enough excess solar energy to charge to 40%, it can still be used, whereas the lead acid battery will provide 100–1,000 cycles and if not charged to 100%, it’ll degrade quicker. The Li-ion battery capacity can be expanded by stacking.
The lead-acid battery is a more cumbersome device in several ways. With a lower energy capacity, lead-acid batteries are heavier and take up more room. Since they create hydrogen gas, they require proper ventilation. Sealed lead acid batteries are suitable for on-grid solar systems.
The lead-acid battery is about four times cheaper than the lithium ion battery and, for many remote off-grid applications, an economical and perfectly suitable choice.
On-grid Home Energy Solar Storage Battery Comparison
On-grid solar storage batteries are improving in performance but not necessarily coming down in price. As more competitors enter the market, including more car manufacturers such as BMW and Nissan, costs will come down. In addition to the battery price, plan on additional costs for installation (between $1,500 to 3,000) and the AC-DC inverter.
The Enphase Energy and Sonnen systems come with AC lithium batteries. Otherwise the inverter (if not inbuilt, as is the case with the Tesla Powerwall), can add $3,000–$5,000 to the cost of the battery.
Here is a review of how a few popular brands stack up on performance and price and the smart controls further improving energy efficiency and costs.
Like its cars, the Tesla energy battery is sleek and slim. The Powerwall has an output of 14 kWh and provides at least seven days of continuous power if the grid goes out. Tesla recommends two batteries to support 30 kWh of energy usage for 24-hours. Smart meter function, enabling the setting of storage to peak and of-peak usage, is managed through a mobile app.
Tesla guarantees 70% storage capacity over 10-years. Tesla upped its battery price twice in 2018. The current price is $6,700, an $800 increase. The Gateway hardware price has increased $400 to $1,100. The battery inverter is built in. Powerwall automatically detects grid outages and maintains power supply to the home.
LG Chem RESU battery
The LG Chem comes in sizes from 2.9 kilowatt-hours (kWh) to 12.4 kWh. The 9.8 kWh battery is currently selling for $7,200, which doesn’t include the inverter. Energy efficiency is greater than 90% over a 10-year life. Smart energy management and remote system monitoring are provided.
This 4-kWh battery, currently priced at $9,950, guarantees 70% capacity for 10-years. The inverter is integrated. The smart ecoLinx energy system monitors real-time and historical energy usage patterns, severe weather and grid outages, and automatically adjusts storage capacity and energy usage accordingly.
Integration with the home automation system allows smart functions such as blinds to be closed on a sunny afternoon, and thus power flow to the home lowered and battery increased.
Nissan Energy Solar
Nissan Energy Solar has recently entered the market with a complete system - six solar panels, the xStorage battery and the energy management system. A compelling feature of the battery is the ability to also charge your electric car.
Power options are 4.2 kW and 6 kW, with a 5 and 10-year warranty, respectively. The smaller battery costs under $4,000. Smart controls and a smart app allow the homeowner to track and control real time energy usage.
The SOLARWATT MyReserve Matrix is a 2.4 kWh battery module that can be stacked up to five units to provide 12 kWh of energy capacity. Any solar inverter can be added. SOLARWATT claims its intelligent charging function provides a longer lifespan than other batteries. Batteries that charge to 100% early in the day suffer high chemical stress. By charging to 100% before sundown, 3,000 kWh are added to the lifespan of a 7.2 kWh battery.
All these systems can be operated with a mobile app to monitor and control the solar power production, storage and balance in your home.