Whether you live in a remote location and can’t connect to the power grid, or you simply want to cut your ties to traditionally supplied electricity, going off the grid can be a great option. However, it’s not always as easy as it sounds. While you may already have solar panels, you’ll also need a battery bank that has enough storage capacity to supply your home with power for several days, in case there are prolonged periods of cloudy weather. Your battery bank will be your main form of power when the sun is hidden or goes down at night, so it’s essential you choose the best batteries for the job. Keep reading to learn what you should be looking for in an off-grid battery bank.
Battery Capacity (Ah or kWh)
Battery capacity is the first specification you should be considering when looking for batteries. Typically, this specification is measured in amp-hours (Ah) or kilowatt-hours (kWh), and represents the amount of energy a battery can store. The larger the capacity, the longer the battery will be able to supply you with power without needing to be recharged. For an off-grid system you’ll need a high enough capacity to meet your household’s typical energy consumption patterns for several days.
To calculate the required battery bank capacity, you’ll need to consider both your typical daily energy usage and how many days of power you want to keep stored. The amount of time your batteries can provide power without being recharged is also referred to as “autonomy.” An off-grid system should usually have anywhere between 2 and 5 days of autonomy.
Depth of Discharge (DoD)
Depth of discharge, or DoD, is the percentage of the battery’s capacity that can be used without damaging the battery. Most batteries need to keep a portion of their charge stored in the battery at all times to avoid damage to the battery and a shortened lifespan. Batteries with a higher DoD can be discharged more deeply, which means you can use more of their total capacity before recharging. For example, a battery with an 80% DoD and a capacity of 10 kWh can actually only provide you with 8 kWh of power, as it needs to maintain 20% of its charge at all times. A battery with 90% DoD and 10 kWh will offer you 9 kWh of usable power. You can see how quickly these little differences will add up, and those batteries with a higher DoD, also known as deep-cycle batteries, will be able to provide you with more power overall.
Cycle Life
A battery’s cycle life refers to the number of charge and discharge cycles a battery can complete before its capacity significantly declines. Batteries with a high cycle life last longer, which means they’re more cost-effective over time. For off-grid solar power systems, a long cycle life is crucial, since you’re likely to cycle your batteries on a daily basis.
Different battery chemistries will have different average life cycles. For example, lithium-ion batteries typically have a cycle life of 2,000 to 6,000 cycles, and sometimes more. Lead-acid batteries, on the other hand, only get between 500 and 1,500 cycles in their lifespan. While lithium-ion batteries will cost more upfront, their longer cycle life means they’re a more cost-effective choice in the long run.
Efficiency
All solar power equipment has an efficiency rating, which represents how much of the generated energy is available for use after energy losses. Solar inverters, for example, will lose some energy when converting the power from DC to AC. Similarly, batteries lose some energy while maintaining a charge as well as during the charging and discharging cycles. When choosing a battery, you want to choose something with a high efficiency rating to minimize energy losses.
This is usually expressed as a percentage. For example, lithium-ion batteries have a round-trip efficiency between 90% and 95%. This means that 90% to 95% of the energy used to charge the battery will be available for later use. Lead-acid batteries, on the other hand, usually have an efficiency between 80% and 85%. Higher efficiency means less energy is wasted, which is especially important in off-grid systems, where available energy may be limited.
Temperature Tolerance
Finally, make sure you consider how tolerant your battery bank is of changes in temperature. Temperature can greatly impact battery performance and lifespan. Some battery types, like lithium-ion, are sensitive to extreme temperatures. They should be installed in climate-controlled environments, or with battery heaters/coolers, to maintain peak performance. If you live in an area with extreme temperatures, it’s important to choose a battery with a wide operating temperature range.
Selecting the best batteries for solar off-grid takes time and consideration, but in the end, it will be time well spent. Picking the right batteries will provide you with a reliable source of backup power that will last for many years to come.
