Values in a solar array
If they wrote the firmware right, a reading in kilowatts (kW) is the output of the solar array at that moment.  The total "work", taken over time, will be in kilowatt-hours (kWh).  The time period is arbitrary; it may be taken over the course of a day, or a month, or it may be a lifetime total.

If you want to work it out yourself, by recording the output each second, you still have to assume that the output over each second was constant.  But that will normally be close enough.  One second's worth of power, by the way, gives you watt-seconds, a unit not normally interesting unless you are looking at something like an impulse from a capacitor.  3600 consecutive 1-second readings will give you the output that was generated during that hour, and it will be in kWh.  Remember, it's a cumulative total, just like filling a swimming pool.