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See the below example on sizing Lead Acids vs a quality LiFe LFP lithium such as Power Plus

You previously had Lead Acid 1660ah Wet Cell batteries on this job which with a high level of maintenance can be a reliable battery. While they store up to 40kwh of energy, to achieve around 9.5 years of service life they can only be discharged 20% each day or 8kwh. If you discharge 50% or 20kwh they will last around 4 years.

A new set of Lead Acids 1600ah batteries in 24v bank would cost around $14K and freight is more expensive than a quality lithium product.

The other important factor is in the top 20% of the battery they are only 50 to 60% efficient. This means to recharge the 8kwh back in to the battery could require up to 16kwh of PV input.

If you purchased 4 x 3.3kwh (13.2kwh) of Lithium Ferro Phosphate PowerPlus batteries with the price on the web site as $11,704. If you discharge these fully (13.2kwh) the service life is expected to be around 7 years. At 75% discharge (9.9kwh) they could achieve 13-20 years.

An important factor is that the Lithium batteries are 96% efficient during charging. It would only take 8.5kwh to store 8kwh in the battery.

To determine what battery storage you need to know what your average daily energy usage is and how much energy you produce on a good day and a bad day. Based  on the answers to these questions you would typically install two to three days storage capacity in an off grid system when using LFP lithium batteries depending the generator setup. With Lead Acid batteries typically it would be 4 to 5 days capacity.

With reference to the PowerPlus batteries they will last 3 to 4 times as long at the same depth of discharge as the Lead Acids.

These will behave as lead acid in that it’s an easy install replacement, each battery has a BMS board inside so it behaves as a lead acid battery with all the advantages of lithium, as an installer all you need to do is program the charge voltage, float voltage, charge current and the battery does the rest, so if you are installing the inverter you don’t need to learn new protocols or maintain communication between the battery and inverter.