Lifetime hourly and hourly results

For detailed simulations of solar PV systems, both with or without battery, the results now include 'Lifetime Hourly Data' under Data Tables. In SAM 2018 when retrieving the system generation in kW in pysam with the variable 'gen', it returned a single year. However, in SAM 2020, 'gen' returns the lifetime production (e.g. 8760 h x 25 years). My solution so far has been to retrieve 'gen' with the lifetime time-series and slice the series to the first year of data to perform further analysis. I wonder if there is an actual variable that returns the generation for the first year only.
 
The brief answer is no. Your approach to slice the first year's worth of data from the lifetime array is a good way to get that data.
Some of the performance models in SAM run the simulation over a single year, and others, specifically the Detailed PV and battery models run the simulation over the project life to more accurately account for battery and photovoltaic module degradation over time.
Earlier versions of SAM did not perform lifetime simulations at all, and as we transitioned to the longer simulations, some of the output variables were being reported for one year and others over the life in an inconsistent way. In the latest version of SAM, the outputs should show how the time series data is actually used in the simulation. For example, for lifetime simulations, SAM uses time series weather data for the year represented in the weather file for each year of the simulation, so the weather data outputs are shown for one year. Similarly, for the distributed or behind-the-meter financial models, the electricity bill savings calculations are based on the simulation results for Year 1 and adjusted for the annual electricity bill value in the cash flow, so those time series values are shown for one year.