How a solar battery works and what its numbers mean
A solar battery is a rechargeable energy store that takes in electricity from panels or the grid and gives it back later. The specification sheet is where buyers get lost, because capacity, power, depth of discharge, cycles and efficiency each describe something different.
How does a solar battery store and release energy?
Inside the case, chemical reactions in the cells move charge in or out. Charging happens when your panels make more than the house uses, or when the grid is cheap. Discharging happens when the house needs more than the panels give, for instance at night. Batteries store direct current, so an inverter converts it to the alternating current the home uses.
None of this is free. Each charge and discharge cycle loses a small share of the energy as heat. The purpose, benefits and limits of a battery for a home are set out in the solar battery guide, while this page decodes the numbers.
What is the difference between kWh and kW?
Kilowatt-hours measure the amount of energy stored, like the size of a fuel tank. Kilowatts measure power, the rate at which energy can flow out, like the width of the fuel line. A battery listed as 10 kWh and 5 kW can deliver 5 kW for two hours, or 1 kW for ten hours, in round numbers.
Both matter. A big tank with a narrow line cannot start a heavy appliance, and a wide line with a small tank runs dry quickly. When you compare quotes, check both figures and whether the power is quoted as continuous or as a short peak.
What are usable capacity and depth of discharge?
Nameplate capacity is the total energy in the cells. Usable capacity is what the system lets you draw, because running a battery fully empty shortens its life. Depth of discharge is the percentage of the capacity that can be used in a cycle. If a 10 kWh battery has 90 percent depth of discharge, 9 kWh is available.
Some quotes list nameplate numbers and some list usable ones, so a battery that looks larger on paper can deliver less. Always ask for usable capacity, and ask how much of it the installer will reserve for outages if the system is set to backup mode. The sizing logic is covered in how to size a solar battery.
What do cycles and round-trip efficiency mean?
A cycle is one full charge and discharge. Manufacturers state how many cycles a battery should deliver before its capacity falls to a given level, often with a warranty tied to that figure. The longer-term effects are described in battery degradation and warranty.
Round-trip efficiency is the share of stored energy you get back. If you put in 10 kWh and recover 9 kWh, the figure is 90 percent. The gap is cost, because it is electricity you paid for or generated and did not use. It is a good tiebreaker between systems that otherwise look alike.
Which battery chemistry should you expect?
Most new home batteries use lithium cells, with lithium iron phosphate common. Lead-acid batteries still appear in some off-grid systems because they cost less up front, though they usually last fewer cycles and want more care. The comparison is in lithium versus lead-acid solar batteries.
Temperature matters across chemistries; the cold weather guide explains why a heated location is usually needed in Canadian winters. A battery is also one of several ways to store energy, and energy storage types for homes shows the alternatives, including the heat storage used at Drake Landing, where summer heat was kept in boreholes.
Quick answers
What is a solar battery?
It is a rechargeable store that holds electricity from solar panels or the grid so it can be used later, for example at night or during an outage.
What is the difference between battery capacity and power?
Capacity, in kWh, is how much energy the battery holds. Power, in kW, is how fast it can deliver that energy at one time.
What is usable capacity on a solar battery?
It is the part of the total capacity the system allows you to use. It is lower than the nameplate figure because the battery keeps a reserve to protect the cells.
How do I compare two solar batteries?
Compare usable capacity, continuous power, round-trip efficiency, cycle life and warranty terms, plus the operating temperature range, rather than the headline capacity alone.