Michael L. Kelly v. Pamela Wagner, 393 P.3d 566

Wagner purchased a foreclosed property and hired Kelly (licensed electrical contractor, CHS) to perform numerous discrete remodeling/repair jobs over ~18 months; Kelly submitted roughly 40 proposals and invoiced after each job.
For most jobs, Kelly and Wagner reviewed invoices together and Wagner paid; relationship soured after Kelly failed to pay a subcontractor and a lien was filed and later released.
Kelly sued in 2009 for unpaid invoices exceeding $10,000; Wagner answered admitting she had refused to pay certain invoices and counterclaimed for defective work and incomplete work.
At a bench trial, Kelly introduced six invoices totaling $10,635.32 and testified they were unpaid; Wagner testified she paid all invoices but could not match checks to the specific invoices.
The district court found Kelly proved $9,429.64 in unpaid invoices, awarded Wagner $4,735 on her counterclaim (including $4,285 for back entrance defects and $450 lien payment), and awarded prejudgment interest on the unpaid invoices; net judgment to Kelly $13,762.54.
Wagner appealed, arguing (1) the unpaid invoices were not proven, (2) the parties had an "open account," and (3) prejudgment interest was improper; she also sought fees on appeal.

Issue	Plaintiff's Argument (Kelly)	Defendant's Argument (Wagner)	Held
Did Kelly prove unpaid invoices totaling $9,429.64?	Invoices + testimony that they were unpaid; invoices admitted and work undisputed.	Wagner: she paid all invoices; produced check records showing payments.	Court: Affirmed. Trial court credibility finding for Kelly supported by substantial competent evidence.
Were the transactions an "open account"?	Transactions were separate contracts for discrete jobs invoiced and paid individually.	Wagner: argued series of continuing transactions constituting an open account.	Court: Not an open account; evidence showed individual bids, performance, and (usually) immediate payment per job.
Was prejudgment interest proper on the liquidated invoice amounts?	Prejudgment interest proper because invoice amounts were definite and mathematically ascertainable.	Wagner: Amount not liquidated/readily ascertainable; offsets from counterclaim should defeat interest.	Court: Affirmed. Prejudgment interest allowed on liquidated invoice sums; counterclaim did not make sums unascertainable.
Is Wagner entitled to attorney’s fees on appeal?	N/A (Kelly sought fees below)	Wagner sought fees on appeal as prevailing party.	Court: Denied; Wagner not prevailing party on appeal.

Oregon Mut. Ins. Co. v. Farm Bureau Mut. Ins. Co. of Idaho, 148 Idaho 47, 218 P.3d 391 (2009) (bench-trial standard; appellate review of factual findings).
Panike & Sons Farms, Inc. v. Smith, 147 Idaho 562, 212 P.3d 992 (2009) (appellate deference to trial-court findings supported by substantial evidence).
Lamar Corp. v. City of Twin Falls, 133 Idaho 36, 981 P.2d 1146 (1999) (definition of substantial and competent evidence).
Credit Suisse AG v. Teufel Nursery, Inc., 156 Idaho 189, 321 P.3d 739 (2014) (discussion of open-account concept and continuous transactions).
Seubert Excavators, Inc. v. Eucon Corp., 125 Idaho 409, 871 P.2d 826 (1994) (open account definition and treatment).
Beco Const. Co., Inc. v. Harper Contracting, Inc., 130 Idaho 4, 936 P.2d 202 (Ct. App. 1997) (open-account as a particularized contract claim).
Ervin Const. Co. v. Van Orden, 125 Idaho 695, 874 P.2d 506 (1993) (prejudgment interest awarded when amount is mathematically ascertainable).
Pocatello Auto Color, Inc. v. Akzo Coatings, Inc., 127 Idaho 41, 896 P.2d 949 (1995) (prejudgment interest may be awarded on liquidated counterclaim despite unliquidated award to the other party).