At what granularity? Using a theoretical horizon, or the actual local horizon even for valleys? Most days aren't the equinox, so how would this deal with second

At what granularity? Using a theoretical horizon, or the actual local horizon even for valleys? Most days aren't the equinox, so how would this deal with seconds being fixed but the 12 daytime hours being a different amount of actual time from the 12 nighttime hours? Would a kWh be a different amount of energy depending on what the date is and whether it's daytime or nighttime?