Another common term with batteries is round-trip efficiency. When you put energy into a battery and then take it out again, the battery system consumes some of that energy during the process. In a nutshell, round-trip efficiency refers to how much energy you can draw from the battery, compared to how much you put in.
As a quick example, if you put 5 kWh of energy into your battery, and can only draw out 4 kWh of usable energy, your battery has a round-trip efficiency of 80%.
As you may gather, the higher the percentage, the more efficient your system is – and the more usable energy you can access.
Lead acid batteries generally have a round-trip efficiency somewhere in the ballpark of 80%. This means that for every 10 kWh of energy you put into your battery, you can draw 8kWh back out.
Lithium batteries offer an even higher round-trip efficiency, generally around 90% (such as the Tesla Powerwall 2). Lithium batteries are very efficient at storing and discharging energy, consuming very little in the process.
In summary, both battery types are efficient, but lithium has the edge in terms of how much stored energy is returned as usable energy. It’s a significant factor to consider, as even a 5% difference can equal thousands of kilowatt-hours of electricity over the lifetime of your system.
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