The seasonality of solar is reasonably well matched to Idaho's consumption patterns. The 3 months with the highest demand for fossil generated electricity in Idaho are July, August, and September. This is in contrast to e.g. Germany where wintertime demand is highest.
Awkwardly, this excellent data is all in Excel files. I got the numbers from Monthly data from Electric Power Monthly - Fossil Fuel Consumption for Electricity Generation by Year, Industry Type and State, using sheet 2018_Preliminary inside the spreadsheet.
Solar on the WEIM market [1][2], which (given more and more utilities joining) is more and more of the actual interconnected infrastructure within the WECC [3] -- including plants down south in CA, AZ, NV, NM.
The scenario to envision is that there will be times when solar is cheap enough to buy on the market that a hydro-heavy utility can ''reduce'' hydro generation during the day, and timeshift that same generation into later hours when solar drops off to sell it for profit.
A solar- or natgas-heavy, hydro-poor utility (e.g. San Diego Gas & Electric [5], NV Energy [6][7]) can't do the same, they have to invest in storage, buy power from elsewhere (where the exact source is abstracted away), or spin up more natgas at peak times. The latter two are the most popular solutions now, but regulations in California are now forcing [4] the first issue in CA.
The problem with areas north of 42N is not that times of day.. its the times of year!
Jun 22 - 15:30 hrs of sunlight Dec 22 - 9:00 hrs of sunlight
there are 206 sunny days per year in Boise (could not find twin falls).