Most people size a battery bank from daily energy use, buy one pack, and then find the inverter throttling or the BMS disconnecting on a bright afternoon. Energy is the wrong first number. Current is the right one.
An inverter has a maximum charge current it will push into the bank. A battery has a maximum charge current its BMS will accept. If the first number is larger than the second, the pack has three ways to respond, and two of them end your charge cycle.
Work it in the order that matters
Take the inverter charge figure first. An EG4 18KPV will deliver up to 250 amps. A single EG4 LifePower4 accepts 100. Two of them accept 200. Three accept 300, which is the first count that covers the inverter at full output without any software limiting.
Now check energy. Three LifePower4 packs give roughly 15 kilowatt hours, which happens to suit a house running a fridge, lights, a well pump, and a modest air conditioner overnight. The two numbers usually converge, and when they disagree, current wins, because a bank that cannot absorb the charge never gets full enough for capacity to matter.
The cheaper way out
Charge current is a setting on every hybrid inverter worth buying. Cap the charger at what your bank accepts and one battery will work fine with a large inverter. You give up charge speed, which on a short winter day is exactly what you cannot spare, and you keep the option to add packs later.
What you should not do is leave the default and hope the BMS handles it. It will handle it, by opening the contactor under load, which drops the whole system rather than tapering the charge.
The number nobody prints on the box
Parallel limits exist and they are lower than people expect. Fortress publishes ten units, most rack batteries land between four and sixteen, and a few bargain packs cap at four. Check that ceiling before planning a bank you intend to grow, because the fix for hitting it is a second inverter, not a fourteenth battery.
Sizing a bank
The packs that come up most often when charge current is the constraint.
48V rack batteries on Amazon280Ah wall mount packsAffiliate links. As an Amazon Associate this site earns from qualifying purchases, at no extra cost to you. Prices and stock change often, and a listing appearing here is not a statement that the pairing above is approved.
Related pages on this site
- EG4 18KPV, the 250 amp charger this note keeps referring to
- EG4 LifePower4, and what one pack will accept
- That pairing worked through in full
- Every inverter, sorted by charge current