Until recently the majority of solar powered homes with energy storage used lead-acid batteries—particularly those that were entirely off-grid, but in the past few years that's starting to change as an increasing number of companies are offering home energy storage systems using lithium-ion batteries. But which is really better? Here's a rundown on the pros and cons of both.
This is the grandaddy of energy storage. Lead-acid batteries have been used to provide backup power for solar homes since at least the 1970s. While they're similar to conventional car batteries, the batteries used for home energy storage are called deep-cycle batteries since they can be discharged and recharged to greater levels than the batteries in most cars and trucks.
Lead-acid batteries have traditionally cost less than lithium-ion batteries, which made them more attractive for homeowners. However, they have shorter lifespans than lithium-ion batteries.
Image source: 2012: White Paper: Comparison of Lead Acid vs. Lithium Ion for Stationary Energy Storage. Courtesy All Cell Technologies.
The cycle life of a lead-acid battery also is lower than lithium-ion batteries. While some last as long as 1,000 cycles, others will only last for about 200 cycles of full charge and discharge. Lithium-ion batteries on the other hand have cycles of between 1,000 and 4,000 cycles.
As such, most lead-acid batteries last about five years and have a warranty that reflects that. So, over the lifetime of a solar array, a homeowner will have to replace lead-acid batteries numerous times.
The energy storage efficiency of lead-acid batteries is lower than other energy storage technologies like lithium-ion. Since they're less efficient, they also can't charge or discharge as fast as other energy storage systems.
Lead-acid batteries have low discharge capabilities, meaning that draining them of too much energy will cause their ability to store energy to deteriorate quickly. National Renewable Energy Laboratory (NREL) study found that discharging 50 percent of the energy in a lead-acid battery would allow it to complete 1,800 cycles before its energy storage capacity fell significantly. If it was discharged to 80 percent of its capacity, it could only withstand 600 cycles before seeing significant reductions in storage capacity.
Since they're less efficient at storing energy and can't discharge fully, lead batteries require more cells and space than lithium-ion batteries do. They're also much heavier and a bank of lead batteries will require racking and more space than a lithium-ion battery pack will.
Lead also is a toxic, heavy metal and while it's recyclable, it can still be disposed of improperly. However, it is usually recycled into new batteries.
Lithium-ion batteries are quickly becoming the battery of choice for many applications, from cordless power tools to laptops and vehicles. They're prepackaged into solar batteries for homes, too. However, they still have some limitations. The first and foremost has been cost.
The up-front cost of lithium-ion batteries is higher than for lead-acid batteries. In May 2018, Tesla lists the cost of its Powerwall in the US at $5,900 or $6,600, including supporting hardware. That's for a 14 kilowatt-hour (kWh) battery that can deliver up to 7 kilowatts of power on peak demands. The cost doesn't include the cost of installation, which often runs between $600 and $2,000.
Image source: Lazard's Unbusidized Levelized Cost of Storage Comparison.
But the cost of lithium-ion is changing rapidly. For the last few years, Lazard has evaluated the costs of energy storage technologies in its Levelized Cost of Storage Analysis reports. In its latest report in Nov. 2017, it found that the installed capital cost of a residential lead-acid battery ranges from $598 to $635 per kilowatt hour. A lithium-ion battery has installed capital costs between $831 and $1,089 per kilowatt hour.
Using those figures a 14 kWh lead-acid battery would cost as low as $8,372 and a comparable lithium-ion battery as low as $11,634. But the lower costs of the lead-acid batteries hide a lot of other costs, like shorter lifespans and operating costs.
The cost of battery systems over time will be significantly different. Lazard shows that the cost per megawatt hour produced by an energy storage system is lower for lithium-ion batteries than lead-acid batteries. A lead-acid battery system produces a megawatt-hour at between $1,160 and $1,239. A lithium-ion battery system, on the other hand, produces a megawatt-hour at between $1,024 and $1,274.
Image source: BNEF's Lithium-ion battery price survey.
The costs of lithium-ion batteries continue to fall as well. In another report, Bloomberg New Energy Finance found that in 2010 lithium-ion batteries were selling for as high as $1,000 per kilowatt hour saw price drops of more than 20 percent over subsequent years. The average selling price of lithium-ion batteries fell $209 per kilowatt hour by the end of 2016.
However, Bloomberg's Mark Chediak noted those prices are chiefly for electric vehicle manufacturers. "Developers of stationary storage systems -- like the kind that back up rooftop solar panels -- can expect to pay 51 percent more than automakers because of much lower order volumes," he stated.
For all these reasons its important to talk with local solar and energy storage installers to learn what the current costs of lithium-ion batteries and lead-acid batteries are for a home energy storage system. They can either be used as a stand-alone system or in conjunction with a solar rooftop or ground installation to help offset some or all a home or business' energy needs. SolarReviews.com offers the largest review database of solar installers in the US and is an ideal resource for connecting people interested in solar and energy with qualified installers.
In terms of lifespan, lithium-ion batteries are expected to operate continually for roughly 10 years, they're capable of being charged and discharged to much higher levels without degrading significantly. NREL's research assumed that a Tesla Powerwall could operate for 15 years without significantly losing its ability to store and discharge energy. That's 5,475 cycles.
Lithium-ion batteries can also charge much faster at higher voltages. While it can take lead-acid batteries up to 16 hours to fully charge, even the slowest charging lithium-ion batteries can fully charge within about four hours.
Then there's weight issue. Lithium-ion batteries for home energy storage systems aren't exactly light, but compared to lead, they're like a feather. A 13.5 kWh Tesla Powerwall weighs 278 pounds. The 1.7 kWh lead-acid battery NREL tested weighed 132 pounds. It would take eight of those batteries to offer the same storage capacity as the Powerwall and together they'd weigh over a thousand pounds!
All in all, lithium-ion batteries offer enough advantages over lead-acid batteries and they're just getting better at this point.
The best way to learn how much a solar battery actually will cost you is to talk with neighbors and local installers. You can connect with quality installers through a tool like Solar-Estimate.org. A qualified installer will inform you about battery capacity, power, cycle life, depth of discharge (DoD), round-trip efficiency and warranty.