A hybrid inverter is three machines in one box sharing a battery bus: MPPT solar chargers on the input side, a battery charger and discharger in the middle, and an AC inverter with a transfer switch on the output. Every confusing spec-sheet line belongs to one of the three. Once you know which machine a number describes, you can sanity-check a design, trace where a watt goes, and explain why the grid can push 90A through a box that only makes 66.7A.
Machine one: the MPPTs
An MPPT (maximum power point tracker) is a smart DC-to-DC converter that constantly adjusts the electrical load on a string of panels to hold it at its best operating point. Each MPPT tracks independently, which is why different roof faces get different MPPTs.
An MPPT has a voltage window and a current limit, and both come off the spec sheet, never from memory. The FlexBOSS21 numbers: 600 VDC absolute max input (above that, the warranty is void), a 550 VDC protection point where the unit faults to save itself, a full-power window of 250 to 440 VDC, and three MPPTs rated 26A, 26A, and 15A with 2, 2, and 1 string inputs.
The 18kPV reads differently: an operating window of 140 to 500 VDC, the same 550V protection and 600V max, and MPPTs rated 25, 15, and 15A. Sibling units, different windows. That is the whole reason the check is per model, per datasheet.
Reading a real string design
A 44-panel whole-home design we ran recently (Canadian Solar 395W panels on one FlexBOSS21) makes the best worked example we have.
Voltage first. Eleven panels in series: 11 x 36.6V is 402.6V open circuit at standard test conditions. Cold raises voltage, so corrected to -10 C at the datasheet's coefficient the string reaches 439.2V. That sits under the 550V protection point and the 600V max, and the string's operating voltage (11 x 30.6V is 336.6V) lands inside the 250 to 440V full-power window, which is the number that decides whether the array harvests at full strength. Now watch the logic run the other way. Fifteen panels would put the string at 549V before any cold morning. Fourteen crosses the protection point on a cold snap. Thirteen is the longest safe string, and three strings of 13 only carries 39 panels. Four strings of 11 carries all 44. String length is set by the inverter, never by the roof.
Current second. The FlexBOSS21 manual allows both inputs of one MPPT to be used, paralleling the strings internally, with each single string capped at 13A. This panel runs 12.91A at max power, so the paralleled pair puts 25.82A against the 26A rating, and the per-string margin against the 13A rule is 0.09A. That margin is why one panel substitution can quietly break a legal design. Swap in a higher-current panel and the whole check gets re-run.
Machine two: the charge paths
Power reaches the battery two ways, and the spec sheet rates them separately. On the FlexBOSS21, PV to battery converts at up to 94.5% efficiency, AC to battery at up to 94%, and the battery feeds loads back out at up to 94%. Battery-side capacity is its own set of lines: up to 250A on a nominal 48V-class bus, and the FlexBOSS21 wants a recommended minimum of 600Ah of battery per inverter. That last line is a design input. The 44-panel job pairs two 314Ah wallmount batteries (628Ah) against it, and one battery alone would sit under the recommendation.
Those efficiency numbers are also the honest answer to "why not just AC-couple everything." DC-coupled solar charges the battery in one conversion. An AC-coupled array makes AC on the roof, and any energy headed into the battery converts again on the way. Every conversion step costs a few percent. So solar that mostly charges a battery wants to be DC-coupled, and solar that mostly serves daytime loads can AC-couple happily.
Machine three: output and pass-through
The inverter side of a FlexBOSS21 makes at most 66.7A (16kW) continuous, 12kW from battery alone. Peak ratings are a sprint for motor starts: 24,000W for half a second, 18,000W for one second, 15,000W for six minutes.
Pass-through is the different, bigger number. With the grid up, the FlexBOSS21 lets up to 90A of grid power flow straight through to the loads, while only up to 50A actually enters the inverter electronics for charging. So a 90A load panel behind the inverter works fine on a sunny grid-up day, and the same panel in an outage lives inside 66.7A with PV, or 50A per leg on battery alone. Have that conversation before the first outage, never during it.
Transfer to backup takes 20 milliseconds by default, fast enough that lights blink and electronics ride through. That figure is a spec, never a promise about every load in the house.
What to do with this
If you are speccing a hybrid system yourself, run the same three checks this design got: string voltage at standard conditions and cold-corrected against the model's window, string and MPPT currents against the rated limits, and battery amp-hours against the recommended minimum. Ten minutes with the spec sheet either confirms a design or catches the exact mistake that cooks equipment. Or let the builder pick your parts and the checks come baked in.
Sources
- EG4 FlexBOSS21 Spec Sheet, v1.2.9
- EG4 FlexBOSS21 Manual (per-string current and paralleling guidance)
- EG4 18kPV Spec Sheet, v1.4.3
- Canadian Solar HiKu6 All-Black CS6R-MS-HL Datasheet, v1.1C25 (August 2022)