A revision 0 draft of the heat-rejection basis contained the line 642 kW [MODELED] − 233 kW [MODELED] = 409 kW [MODELED] and read the result as unallocated capacity. The controlled errata register classifies that subtraction as critical, because the 642 kW already has the 233 kW of battery recharge inside it. Deducting it again fabricates a spare that does not exist. 409 kW has exactly one legitimate meaning, the component excluding battery charging, and is not a remainder or a load in any operating case.
Revision 0 Deducted Recharge Already Inside 642 kW
Two remainders exist against the 15,000 kW [TARGET] service cap. Steady facility power of 14,125 kW [MODELED] leaves 875 kW [MODELED]. The charging-inclusive peak of 14,358 kW [MODELED] leaves 642 kW. The two remainders differ by exactly 233 kW, which is the recharge component, and the revision 1 text states the identity plainly: 875 − 642 = 233 kW, therefore 642 kW has already had charging deducted once.
The powers are 14,125 kW and 14,358 kW. The remainders versus 15,000 kW are 875 kW and 642 kW. The errata register then writes that facility, generation and heat-rejection calculations always take 642 kW as the branch load, because that figure already has charging inside it.
Read as an equation rather than as a deduction, the same three numbers are consistent — 642 = 409 + 233 is true, and it defines 409 kW as the charging-free component of a remainder. The failure was one of direction. A term that appears inside a total was treated as an external item still to be removed from it, which the errata register names a mirror-image double-deduction.
409 kW Is a Remainder Component, Not a Branch Load
Direction is what separates a rounding problem from a design problem. The errata register is explicit that this is a directional underestimate rather than a conservative bias. Every downstream user of a branch figure built on 409 kW would have been sized against a load 233 kW smaller than the one the plant presents. Expressed against 642 kW, that is a 36.3 % [MODELED] understatement of the branch load.
Worse, the two conditions coincide. Recharge is drawn after a utility interruption, at the moment the battery is recovering and the information-technology load is back at full power. A 36.3 % understatement therefore does not sit quietly in a spreadsheet through normal operation and reveal itself gradually. It is at maximum precisely when the equipment is closest to its rating, which is the case an underestimate is least able to survive.
Correcting 409 kW to 642 kW as the branch load follows the errata register. The envelope still has to be stated: 14,358 kW with recharge already inside, 642 kW remaining to the cap. 409 kW is not a third envelope.
The Error Hits Four Disciplines and Must Not Be Netted
The errata register lists the propagation path rather than leaving it to inference, and the four destinations are not in the same discipline. Distribution branch ampacity and protection settings come first, where a low load figure produces low trip settings and undersized conductors. Generator capacity and the derating check follow. Heat rejection duty comes third, since charger losses are rejected as heat. The power usage effectiveness numerator is fourth, which makes the error a reported-performance problem as well as a plant problem.
Correcting it also has to be done alone. Three numerical errata items are open against the same document — this one at +233 kW, a charger-efficiency omission at +46.8 kW [MODELED] and a third at −50 kW [MODELED] — and the register states that because their directions differ they must be propagated item by item and may not be netted against one another. A single combined adjustment would conceal the fact that this item and one other both understate.
The 609 kW [MODELED] figure on the earlier facility total of 14,391 kW [MODELED] pairs with the 875 kW steady remainder and not with the 642 kW charging-inclusive remainder. Different envelopes. Different columns.
Limits and open items
Confirmed: the 642 kW figure contains the 233 kW recharge component; the subtraction that produced 409 kW is a double deduction; 409 kW is prohibited as a load or margin value for capacity checking, generator sizing, heat rejection, power usage effectiveness or investment estimating. Facility, generation and heat-rejection calculations take 642 kW, and the envelope must be stated whenever either margin is quoted. [FACT]
Modelled: the 14,125 kW steady and 14,358 kW charging-inclusive figures and the 233 kW recharge value, which rests on a 0.92 [MODELED] charger efficiency and a state-of-charge recovery profile that carry no supplier documentation. [MODELED]
Held: closure itself. The item is dispositioned for correction in the controlling authority's editing window, and the register defines closure as the corrected document no longer carrying 409 kW as a load value, the prohibition statement being present, and written confirmation being given. Until then the correct reading exists in the errata register and not yet in the document it corrects. [HOLD]
One boundary belongs beside these figures. The shared engineering register separately declares that this document's weather chain was developed against the wrong location, so its weather-dependent outputs remain formula templates without site effect until a site-correct recomputation is completed. That does not affect the arithmetic above, which is an internal identity.
State the Envelope Beside Every Remainder
The most portable lesson here is not about recharge. It is that 642 and 409 are both arithmetically defensible readings of the same three numbers, and only the envelope label distinguishes them — which is why the register's instruction to state the envelope alongside every remainder is the actual control, and the prohibition on 409 kW is only the local remedy.
The prohibited value had already reached the shared engineering register's rejected list before the errata register reached the text that produced it. So the question for the next document review is whether any other figure now in circulation is a remainder quoted without the envelope it was computed on.
© 2026 K&K Data Service Inc. All rights reserved. Reproduction or republication is permitted only with clear attribution to K&K Data Service Inc. and a working hyperlink to the canonical URL of this article. Excerpts must preserve the technical context, maturity labels, assumptions, and limitations. No excerpt may imply project approval, field validation, certification, or endorsement that the original article does not state.
Source: K&K Data Service Inc., “Battery Recharge Margin Cannot Be Counted Twice Against the Service Cap,” https://www.kkdatasvc.com/lab/ups-battery-and-generation/642-minus-233-equals-a-forbidden-spare/.
Discuss This Constraint. If your project record shows a different result, or the same failure domain under another name, we want to see it. Email inquiry@kkdatasvc.com or use the contact page.