Pull up the spec sheet for an EG4 FlexBOSS21 and find the row marked peak power. It does not hand you a number. It hands you four: 24kW for half a second, 18kW for one second, 15kW for six minutes, 13.2kW for twelve minutes. Higher up the same page sits the number everybody quotes, 16kW continuous.
All five are real. They answer different questions, and the question you are actually asking when you pick an inverter is almost never "what can it do all day."
The number that starts your air conditioner
A motor does not sip when it wakes up. A compressor, a well pump, a table saw, every one of them pulls a hard slug of current for a fraction of a second before it settles into a running draw. The spec sheet calls that locked rotor amps, and the FlexBOSS21 is rated for 195A of it. EG4 also says on the same sheet that the unit can start a 5-ton air conditioner. Both of those lines are about the moment of startup. The 16kW continuous rating covers the load the AC settles into once it is running. It says nothing about that first half second.
Undersize here and the failure is confusing. The house runs an easy load all evening, the AC cycles on, the inverter faults, and you go hunting for a problem in the load list that was never in the load list. The average was fine. The first half second was not.
So when someone asks how big an inverter they need, the honest first question back is what motors are on the system, and how many of them can start at the same moment.
The number nobody reads: what one leg can carry
Split-phase gear has a second ceiling hiding under the headline. The FlexBOSS21 does 16kW total, but its per-leg limit is 50A, 6kW. The EG4 12000XP is blunter about it: 15kW across L1 to L2, and 7.5kW on either leg by itself.
Every 120V thing in your house sits on one leg or the other. A true 240V load, a well pump say, pulls across both. Plenty of big appliances are mixed, drawing 240V for the heavy part and 120V for controls, lights, and small motors, so the whole appliance does not land evenly on the two legs. Stack the microwave, the fridge, the freezer, the office, and a window unit onto the same side and you can hit a per-leg ceiling while the inverter's total sits half used.
That is a design question, not a shopping question, and the only honest way to answer it is circuit by circuit against your actual panel. Worth raising before you buy, because it is far cheaper to settle on paper than after everything is energized.
The number you actually get in an outage
This is the one that surprises people. The FlexBOSS21's 16kW continuous assumes PV and the grid are behind it. On battery alone, the same box is rated 12kW. The 12000XP tells the same story from the other side: 15kW with PV, 12kW without it.
Read that again with a hurricane in mind. The rating that matters at two in the morning on the third night of an outage is the battery-only one, because the sun is down and the grid is the reason you bought the thing in the first place. Size to the headline number and you sized to a condition you will not have when you care most.
Size to the battery-only number. If the sunny-day rating turns out to be a bonus, good.
Why we sometimes buy more inverter than the loads need
Undersizing has one failure mode and it is loud. Going bigger costs money up front and buys three things worth having.
Array room. The FlexBOSS21 takes up to 21kW of PV at up to 600VDC across three MPPTs. The 12000XP will use 24kW of panel behind a 15kW output, with a 500VDC ceiling. Panels are the cheapest part of a build and the part people add to first, so an inverter that can already see more of them saves you a second purchase.
Headroom for the next thing. A mini-split, a shop, a well, a charger in the garage. Almost nobody's load list gets shorter.
Margin on the surge. Running a box at its ceiling every August afternoon is how you find the ceiling the hard way.
One honest caveat about model numbers. EG4's own footnote on the 12000XP says the 15kW output applies to one specific model number and earlier units are capped at 12kW. Two boxes wearing the same name are not always the same box. Read the model line on the sheet you were sent, not the headline on the page.
The short version
Surge is the check people skip, so start there, but it is one of four. Look at the worst half second, the steady load, the per-leg split, and the battery-only rating, then let whichever of the four comes up tightest set the size. Buy a little past that, because arrays grow and load lists grow. And none of it replaces the electrician who signs off on your design running the real numbers for your house and your gear.
If you want to put real parts to it, the system builder asks these questions in order and keeps the pieces compatible. Or send us your panel photo and your load list, and we will read the spec sheets with you.