TradesHow this works
A battery bank is sized from two numbers: how much energy you use in a day, and how many days you want to ride through without recharging. Multiply them and you have the energy the bank must deliver. But you cannot use all of a battery, so that number gets grossed up for the usable depth of discharge and the round-trip losses to get the nameplate size.
The chemistry is the decision. Lead-acid is cheap per kWh but you only use half of it, so the bank is nearly double and heavy. Lithium costs more but uses most of its capacity and lasts far longer. The tool sizes both in kWh and amp-hours at your system voltage, and higher voltage cuts the amp-hours for the same energy. It sizes the bank only, so pair it with the solar array engine.
Common questions
How big a battery bank do I need?
Start with your daily energy use in kWh and how many days you want the batteries to carry the load without sun. Multiply them for the energy the bank has to deliver, then gross it up for the usable depth of discharge and round-trip losses. Ten kWh a day for two days is 20 kWh to deliver, which is about 26 kWh of lithium or 47 kWh of lead-acid nameplate.
Why is a lead-acid bank so much bigger than lithium?
You can only cycle a lead-acid battery to about half its nameplate without wearing it out fast, so a bank has to be nearly twice the energy you actually draw. Lithium can be cycled to about 80 percent and is more efficient, so a lithium bank is smaller, lighter, and lasts far longer for the same usable energy, at a higher price per kWh.
What does system voltage change?
The amp-hours. The bank stores the same energy in kWh, but amp-hours are the energy divided by the voltage, so a 48 volt bank needs a quarter of the amp-hours a 12 volt bank does. Higher voltage means smaller amp-hours and thinner wire for the same power, which is why larger off-grid systems run at 48 volts.
Sources: Usable depth of discharge: ~50% lead-acid, ~80% lithium (LiFePO4) · Round-trip efficiency: ~85% lead-acid, ~95% lithium (industry values). Engine version 1.0.0. Risk tier 1.