Solar · Battery banks
An array sized for the worst month still makes nothing after dark, and very little during a four-day overcast. Storage is what carries a property across those gaps, and it is usually the most expensive decision in the system.
Days of autonomy
Days of autonomy is the number of days the bank can carry the household with no useful solar input. Snow on the array, a stalled front, a week of flat grey sky. It is a design choice, not a property of the battery.
Two to three days is the common target where a generator exists as backup. Without one, the figure climbs, and so does the cost. Autonomy scales the bank linearly, so moving from two days to four doubles the battery budget while doing nothing for the other three hundred and sixty days.
Decide this deliberately. A generator that runs twelve hours a year is often cheaper than the battery capacity it replaces, and that tradeoff is the honest centre of the decision.
A grid-tied household with batteries sizes for a few hours of outage. An off-grid property sizes for the worst stretch of weather its climate produces. Those are different questions, and the second one is answered from local weather records rather than from a product page.
Usable capacity
Every battery has a rated capacity and a smaller figure you can actually use without wearing it out early. The gap between those two numbers is depth of discharge, and it is the difference between a bank that lasts a decade and one that does not.
The share of rated capacity you draw before recharging. Lead-acid banks are generally held near 50 percent. Lithium iron phosphate tolerates 80 percent or more.
A 50 percent limit means buying twice the nameplate capacity you intend to use. That is not a flaw in the chemistry, it is simply a cost that has to appear in the comparison.
Cycle-life ratings are quoted at a stated depth of discharge. A bank cycled shallower lasts longer than its rating, and one cycled deeper lasts markedly less.
Worked example
Carrying forward the property from sizing an array: 9,100 watt-hours a day, including standing draw. Two days of autonomy, on a 48 volt bus.
9,100 × 2 days = 18,200 watt-hours that must come out of the bank between charges.
Lithium at 80 percent: 18,200 ÷ 0.80 = 22,750 watt-hours nameplate. At 48 volts that is 474 amp-hours, so a 500 Ah bank, 24,000 watt-hours.
AGM lead-acid at 50 percent: 18,200 ÷ 0.50 = 36,400 watt-hours nameplate, or 758 amp-hours, so an 800 Ah bank, 38,400 watt-hours.
Each bank delivers 19,200 usable watt-hours, about 2.1 days at this load. The same useful storage from 24,000 watt-hours of lithium or 38,400 of lead-acid. That 14,400 watt-hour difference is what depth of discharge costs.
The same power at a higher bus voltage moves as less current. Delivering this bank's output at 12 volts instead of 48 would draw four times the current, which means far heavier conductors and higher losses in every cable run. Property-scale systems land on 48 volts for that reason.
Chemistry
Lithium iron phosphate dominates new off-grid installations, but lead-acid is not obsolete and the reasons to choose it are real. Verify every figure below against the datasheet for the specific battery you are considering, because these vary by manufacturer.
Deepest usable discharge, longest cycle life, highest round-trip efficiency, and no watering or equalisation. It carries a management system that will refuse to charge when cold.
Highest cost per nameplate watt-hour, lowest per usable watt-hour over its life.
No maintenance liquid, tolerant of cold charging, and available everywhere. Half the usable capacity per nameplate watt-hour and a much shorter cycle life.
Reasonable where cold is severe and a heated enclosure is not practical.
Lowest cost per nameplate watt-hour, and the only type that asks for routine work: topping with distilled water, checking specific gravity, and ventilation for the gas it produces.
Suits someone who will genuinely do the maintenance, and punishes someone who will not.
Cold weather
Charging lithium iron phosphate below roughly 0 degrees Celsius, 32 Fahrenheit, causes metallic lithium to deposit on the anode instead of moving into it normally. The result is permanent capacity loss and, over repeated events, internal short circuits.
The damage is cumulative. Each cold charge takes a little more, and none of it comes back.
Discharging in the cold is a different matter and is generally fine, with reduced capacity at low temperatures. The restriction is on charging.
Slow charging does not make it safe. Lower current reduces the severity but does not prevent plating. Temperature decides, not charge rate.
The operating temperature range on the spec sheet refers to discharging. A battery rated to well below freezing is rated to deliver power there, not to accept it. Reading that range as a charging window is a common and expensive mistake.
The battery management system is the protection. A quality unit blocks charge current below its threshold. Confirm that cutoff exists before buying, because a battery that silently accepts charge in the cold will damage itself without complaint.
Design around it rather than hoping. Put the bank inside conditioned space, or in an insulated enclosure, or buy cells with internal heating and count that heater's draw as part of the daily load on the sizing page.
Exact thresholds vary by manufacturer. The number that governs your system is the one on your battery's datasheet, not a figure from any article including this one.
Lifespan
An undersized bank gets pushed past its intended depth every cloudy week. The capacity you saved buying comes back out of the replacement interval.
Sustained high temperature degrades every chemistry. A bank in an uninsulated shed in summer ages faster than the same bank in a basement.
Lead-acid in particular suffers when it rarely reaches full. An array that cannot finish the job in winter shortens the bank as well as the evenings.
Adding new batteries to an aged bank drags the new ones down to the old ones. Banks are replaced as a set, which is worth knowing before the first one is bought.
Common mistakes
Buying 18,200 watt-hours to deliver 18,200 watt-hours. Depth of discharge means the rated figure always has to be larger than the usable one.
The honest comparison is price per usable watt-hour across the bank's expected life. That calculation looks very different from the one on the shelf tag.
A self-heating lithium bank draws from the system it is part of. That load belongs in the daily watt-hour figure, which means it belongs back on the array sizing page.
Three days gets picked because it sounds prudent. It is the largest single line in the budget and deserves an argument, not a habit.
Next
The 48 volt bus and the array figure together decide the conversion hardware. These guides are in progress and will link here as they publish.
Sizing conversion hardware from the array watts and the bank voltage established here.
Conductor sizing and overcurrent protection, where bank voltage and fault current decide the answer.
Where the cold rule above meets short days, snow cover, and a bank that has to last the week.
Back to Solar at Property Scale, or return to sizing an array for the load figure this page builds on.
Sources
Capacity, cycle life and temperature limits vary between manufacturers and product lines. Every figure on this page is a planning figure. The datasheet for the battery you buy is the one that governs your system.