Why store energy
Solar panels make power by day. You use a lot of it at night. A battery bank, a group of batteries wired together, stores daytime energy for later.
Off the grid, the battery is what keeps the lights and fridge on after dark and through cloudy days. On the grid, a battery is optional. It can give backup during outages.
A battery’s size is given in kWh. That is its nameplate capacity, the energy it holds when full. You cannot use all of it.
Depth of discharge
State of charge is how full a battery is right now, as a percent. Depth of discharge (DoD) is how much of its capacity you use before charging again. Using half a battery is 50 percent DoD.
Batteries wear out faster the deeper you discharge them. Each full charge and discharge is a cycle. Cycle life is how many cycles a battery gives before it holds much less than when new.
So the usable size of a battery is less than its nameplate. A 10 kWh bank used to 50 percent DoD gives about 5 kWh.
Text description
A battery drawn as a long bar with a nameplate capacity of 10 kilowatt-hours. A scale below shows state of charge from 0 percent at the left to 100 percent at the right. The right half, from full down to 50 percent, is shaded as the part you use: a 50 percent depth of discharge, about 5 kilowatt-hours. The left half is kept in reserve and not used. A note says that deeper use gives more usable energy each cycle but wears the battery out sooner, so cycle life is shorter.
Battery types
Most home systems use one of two chemistries:
- Lead-acid is the older, cheaper type. It comes as flooded batteries, which need topping up with water, or sealed types such as AGM and gel. Lead-acid lasts longest when used shallowly, so many designs keep it above about half charge. Deep-cycle types can go deeper, up to about 80 percent, but they wear out sooner.
- Lithium iron phosphate (LFP) is a type of lithium-ion battery common in home storage. It can be used more deeply, often to about 80 percent DoD or more, and usually lasts many more cycles. It costs more up front.
LFP includes a battery management system (BMS), electronics that protect the cells. Ranges for cycle life differ widely between makers and test conditions, so compare warranties, not marketing.
Days of autonomy
Days of autonomy is how many days a battery can carry your loads with little or no sun. Off-grid designs often plan for about 2 to 5 days. Cloudy climates and homes with no generator plan for more.
Text description
A chart of battery state of charge over seven days, with illustrative values. On days 1 and 2, which are sunny, the battery drops at night and refills to full each afternoon. Days 3 to 6 are cloudy: each day it charges only a little, and the charge steps down day by day from about 90 percent toward a dashed floor near 20 percent, the lowest charge planned for, set by the depth of discharge. Early on day 6 a generator tops it up to about 70 percent before it reaches the floor. On day 7 the sun returns and it refills. A bracket from the start of the cloudy spell to the generator start is labeled days of autonomy, how long the battery carries the loads with little sun.
More autonomy means a bigger, more costly battery. Many homesteads pick fewer days and use a generator for long cloudy spells (lesson 27).
Set a battery size, your daily use and a usable depth of discharge. How many sunless days does it cover?
Usable energy: about 7.4 kWh.
With no sun at all, that lasts about 1.8 days.
Assumptions: Includes about 8% loss through the inverter. Cold batteries hold less; lithium iron phosphate cannot be charged below freezing without a heater.
Temperature matters
Batteries are chemical. Temperature changes how they behave.
- Cold lowers the energy a battery can deliver. Lead-acid can lose a large share of its capacity in a freezing shed.
- LFP batteries must not be charged below about 32 °F (0 °C) unless they have a built-in heater or are kept warm. Their BMS may simply refuse to charge.
- Heat shortens battery life for all types.
Battery ratings are usually given at about 77 °F (25 °C). Most batteries do best in a space that stays near room temperature, roughly 50 to 80 °F (10 to 27 °C). Check the maker’s range for yours. In cold climate zones, where the battery lives is a real design choice.
Text description
A temperature scale from about minus 10 to 110 degrees Fahrenheit, split into zones. Below 32 °F (0 °C), lithium iron phosphate batteries must not be charged unless they have a built-in heater or are kept warm, and lead-acid batteries lose a large share of their capacity. From 32 to 50 °F, it is still cold and capacity is lower. From about 50 to 80 °F (10 to 27 °C) is the range where most batteries do best; ratings are usually given at about 77 °F (25 °C). Above about 80 °F, heat shortens battery life for all types. A note says to check the maker's range for your battery.
Diagnose it
“The batteries are flat every morning in January.”
Clues
- The system worked well from April to October.
What is the most likely cause?
Battery safety
A battery bank stores a lot of energy. It can deliver very high current in a short circuit, enough to melt tools, cause burns, and start a fire.
- Flooded lead-acid batteries release hydrogen, a flammable gas, while charging. They also hold acid. They need ventilation and eye protection.
- Lithium batteries can overheat and burn if damaged, overcharged or badly installed. Their BMS and correct installation guard against this.
- The National Electrical Code and fire codes set rules for where batteries go, how they are protected and how far they sit from exits.
Building or wiring a battery bank is not a do-it-yourself job. Plug-in solar generators with sealed, built-in batteries and charge control are the low-risk way to learn.
Pro and permit
Check
Two banks both say 10 kWh. One is lead-acid kept above 50 percent charge. The other is LFP used to 80 percent DoD. Which gives more usable energy?
Summary
- Usable energy = nameplate × depth of discharge. Deeper use shortens cycle life.
- Lead-acid is cheaper and wants shallow use. LFP costs more, goes deeper and lasts longer.
- Days of autonomy set battery size. Many homes pair fewer days with a generator.
- Cold cuts capacity, and LFP cannot charge below freezing without heat. Battery banks are a job for professionals.
Go deeper: round-trip efficiency
Not all energy that goes into a battery comes back out. Round-trip efficiency is the share you get back. NREL studies use about 85 to 90 percent for a lithium battery system, counting the inverter. Lead-acid is lower, around 80 percent (DOE, NREL). Makers often quote higher numbers for the battery alone. That is one reason the battery dial in this lesson subtracts about 8 percent. It is also why it is better to run big daytime loads straight from the panels when the sun is out.