Solar Battery storage is a hot topic right now, so brush up on these common terms.

**Battery Capacity**

Think of a battery as a bucket that holds energy for later use. As such, “battery capacity” defines the size, usually in amphours (Ah) but sometimes watt-hours (Wh), of our bucket.

For example, a 200 Ah, 12-volt battery has twice the storage capacity as a 100 Ah, 12 V battery. We can calculate the energy storage in each battery by using the equation: volts x amphours= watt-hours. The 200 Ah battery can store 2,400 Wh, while the 100 Ah battery has 1,200 Wh of energy storage.

**Depth of Discharge**

Depth of discharge (DOD) describes the percentage of battery capacity that has been used. For example, if we discharge our 2,400 Wh battery by 20% (20% DOD), then practically, we have pulled 480 Wh from that battery.

**State of Charge**

Conversely, state of charge (SOC) describes the percentage of battery capacity remaining. So our 2,400 Wh battery that has been discharged 20% is now at an 80% SOC, and still has 1,920 Wh of energy stored.

**Charge & Discharge Rate**

Charge & discharge rate is the speed at which you charge (or discharge) a battery, stated in terms of capacity over time. For example, if you have a 200 Ah battery, and you are charging it at 10 A, your charge rate is C/20 (200 Ah ÷20 hrs = 10 A). Conversely, if you discharge the battery at a rate of 2 A, then your discharge rate is C/100 (200 Ah ÷ 100

hrs = 2 A).

Battery capacity is also dependent on the speed at which you discharge it, and batteries will actually offer a slightly higher capacity if discharged more slowly. For example, one type of 12 V battery has a 200 Ah specification if discharged over 20 hours (C/20), but offers 212 Ah if discharged over 100 hours. Home PV system batteries most often use the 20-hour rate since it closely relates to the 24-hour cycle common in those systems.

**Cycle Life**

Cycle life is the number of charge/discharge cycles a battery can complete during its “operational life,” which is commonly defined as ending when it has dropped below 80% of its original capacity. Cycle life is dependent on how deeply the battery is discharged—battery manufacturers provide cyclelife graphs showing this relationship. For example, a leadacid battery may provide 1,000 cycles if DOD is limited to 50% each time, but only 500 cycles if it is discharged by 80%. You can get by with a smaller battery bank and less initial cost by discharging it more deeply, but you will have the hassle of replacing the batteries more frequently.