Describe an off-grid solar setup, and someone 20 years ago would imagine a remote cabin in the woods, with lead-acid batteries and diesel generators used as backup power. But in the 2020s, off-grid homes might be closer than you think — like, right next door. Many neighborhoods, either because of their geography or restrictions on the existing grid, are flipping the script and using the grid solely as emergency backup.
“The newest thing is grid defection,” said David Norman, director of solar product and business development at lead-acid and lithium battery provider Discover Battery.
For example, in Hawaii, where utility prices average more than 30-cents/kWh and no new solar can be added to the grid, people are taking their power into their own hands. Often called total self-consumption or zero-export power, Hawaiian households are essentially working as off-grid homes with the grid as a backup.
In California where “public safety power shutoff” events have limited utility power for the last few years, homeowners are planning ahead for extended outages. In the best-case scenario, homes should now be designed to function off-grid for at least a month, while the other 11 months they can use the grid.
Doomsday bunkers aren’t present in either of these situations — off-grid setups already exist in many urban neighborhoods. Today’s power needs require battery technologies to keep up.
Lead vs. lithium in off-grid
An electric battery, by definition, is a device that stores energy that can be converted into electrical power. In that sense, all battery types are equipped to handle off-grid storage needs, but some are better than others at satisfying today’s electricity demands and cycling schedules.
“Off-grid is less about the battery and more about the use-case,” Norman said. “If you’re only doing backup power, lead-acid works. It’s not cycling regularly, and it’s primarily just sitting in reserve for a power outage or failure. But for demand charge applications, any lithium battery is better.”
Lead-acid batteries do work well for occasional, short-term backup needs. But if someone wants to switch power sources to take advantage of utility time-of-use rates or avoid the grid for an extended period of time, more frequent and deeper cycles are needed than what lead-acid can provide.
“Lithium is changing off-grid,” Norman said. “You still can live off-grid on lead-acid, but lithium is more efficient.”
This all boils down to the number of cycles a battery has and its depth of discharge — how many times the battery can be drained, and how much power can actually be used. KiloVault also offers both lead-acid and lithium-based batteries, and VP of sales and marketing Jay Galasso often explains the two chemistry-types’ charging specifications.
“Off-grid solar applications require batteries that can be discharged and charged every day,” Galasso said. “One cycle might be charging the batteries during the day, then discharging the stored energy for use during the evening. The more a battery is discharged, the ‘deeper’ the cycle.”
Lead-acid batteries degrade more with every cycle. Where a lithium battery may come with a 10,000-cycle guarantee, a lead-acid battery may peak at 2,500 cycles when discharged to 50%. Lithium batteries can be discharged to near-zero, or basically, all the juice in a lithium battery can be used in one cycle, where a lead-based battery can only use half of its juice before degrading even faster.
“Lead-acid batteries are lower in cost for the same voltage and capacity but do not last for many cycles,” Galasso said. “[Lithium-based] designs can use fewer batteries for a given application because of the higher charge/discharge rates, resulting in lower initial costs.”
SimpliPhi Power was founded in the early 2000s and has always been a lithium battery provider. The company knows the value of lithium batteries in off-grid applications since its original market was remote power scenarios.
“Lead-acid is a legacy product used in off-grid. It’s where the solar industry originated. You couldn’t have a PV system without lead batteries in those early days,” said Sequoya Cross, director of global sales and business development for SimpliPhi. “Why we see more people scaling to lithium: you have more usable capacity in a smaller space, you can pull them to 100% depth of discharge.”
Physical space is another reason lead-acid is falling out of favor in today’s grid-detached setups in urban environments.
“Most people who build off-grid don’t have a separate mechanical room off their house where the batteries and inverter can go. Most people don’t have that kind of space,” Norman said. “In order to have enough energy storage to run your house as an off-grid home, just by volumetric space alone, you have to go lithium because you could never put that much lead in your house.”
When someone wants to take their active household off the grid today, that usually means they want the refrigerator to work, along with computers, internet, televisions, lights and everything else, all at the same time. Just backing up one emergency load can’t cut it anymore, especially in places like California where life must go on when the utility turns off the power. This requires larger battery capacity, faster charging rates and deeper depth of discharge — things lead-acid just can’t provide.
“Lead is dying in off-grid. You can get long lives out of lead-acid, but they require a lot of maintenance,” Norman said. “There’s no sense in selling a premium lead when I can sell lithium for only 10% more. Lead still works in automotive markets, but on the solar and stationary side, lead is dead.”
The off-grid lithium battle: NMC vs. LFP
The two dominant lithium battery types used in residential applications are lithium nickel manganese cobalt oxide (NMC) and lithium iron phosphate (LFP). The Tesla Powerwall, Generac PWRcell and LG RESU batteries all use NMC chemistry, while many more suppliers use LFP (including Discover, Eguana, Enphase, KiloVault, SimpliPhi, sonnen). Although both battery types work great in demand arbitrage situations, LFP has an edge when it comes to off-grid. That may be why more companies are offering the cobalt-free chemistry.
“For on-grid applications where you don’t necessarily care about rapid recharge because you’ve got the grid there, NMC batteries are great,” Discover Battery’s Norman said. “The limitation with them for an off-grid scenario is that they can’t handle the high-current charge or discharge of demanding off-grid loads.”
Aric Saunders, EVP of sales and marketing for NMC and LFP battery provider Electriq Power, explained the differing charge-rates for the two lithium chemistries. NMC batteries have a charge-rate of 0.5C while LFP batteries are 1C. If it takes an LFP battery one hour to charge, the NMC battery takes twice as long because it accepts current more slowly. LFP batteries can be charged in half the time as NMC.
“Because of [the faster charging rate], LFP batteries can provide a higher power output to the grid or home. This feature allows the battery to back-up larger loads with a smaller battery than would be needed with NMC,” Saunders said.
“Time is money. If I’m charging on solar and I’ve only got a six-hour solar day, I want to get as much into those batteries as I can,” Norman said. An off-grid solar + storage system would benefit most from the fast-charging properties of LFP.
While “off-grid” might be a loose term to describe using the main utility grid as backup instead of a primary power source, it may be an essential way-of-life for many more people in the near future. Homeowners should then choose the right battery chemistry for their situation.
“Off-grid is around us all the time. It’s not just people in the woods anymore,” SimpliPhi’s Cross said. “It isn’t necessarily possible to completely disconnect or pull your meter, but it is possible to design around an off-grid lifestyle.”