For an off-grid house, the battery is sized to deliver about 3 to 5 days of power while being discharged up to 60% of capacity. Less than 3 days capacity means you’ll be cycling the battery heavily on a day to day basis, which isn’t great for life expectancy. More than 5 days capacity usually starts getting so seriously expensive that backup generator or other backup power source may become an alternative investment.

Why only take 60% of the battery capacity? The more deeply you cycle a battery, the fewer charge/discharge cycles you’ll get out of it. You can cycle your battery to 100% of capacity if you want but after a handful of cycles, you’ll be buying a new one. True deep-cycle batteries are rated for how many 80% depth of discharge cycles they’ll tolerate.  

To raise the voltage you connect cells in series. To raise the amp-hours you use bigger cells or you connect in parallel. Large household-sized battery packs can consist of many small batteries connected in series and parallel to deliver the voltage and amp-hour capacity needed. It can also consist of a few large cells in series. As a general rule, battery packs with a few large cells are going to last longer, be easier and less time-consuming to maintain, have fewer problems, but cost more initially.

In order the battery bank to run flawless batteries must all be the same age, same size, and same brand. Any deviation to those conditions will cause smaller batteries to work extra hard, and larger batteries to loaf and sulphate.

Deciphering Capacity Specifications

Deep-cycle battery capacity is rated in amp-hours (Ah). The power equation dictates the relationship between amps, volts, and watts: A × V = W. When you add in the time factor for figuring energy capacity, the same holds true for the relationship between Ah and Wh: Ah × V = Wh.

To compare the energy capacity of batteries of different voltages, it’s easiest to convert to Wh. For example, a 250 Ah,6 V battery has half the capacity of a 250 Ah, 12 V battery.

6 V × 250 Ah = 1,500 Wh
12 V × 250 Ah = 3,000 Wh

Battery capacity specifications also depend on how quickly the battery is charging or discharging. The faster the charge/ discharge rate, the less overall capacity a battery will have. For example, a battery may have a 100 Ah capacity when powering 2 A of LED lighting, but only 75 Ah with 8 A of compact fluorescents. To make matters worse, not all battery manufacturers clearly show what the charge/discharge rates are for the Ah ratings on their spec sheets. It’s common to see C/100 rates (quite slow) to make a battery look like it has more capacity. Most folks in the RE world like to use a C/20 rate, which is close to the typical daily (24-hour) cycling found in RE systems.