1) The season: increasing Tp in summer and decreasing in winter.
2) The hour: increasing Tp during morning (when is expected that
solar resource is growing) and conversely, decreasing Tp during
evenings.
3) The present solar resource: increasing Tp in a shinny day and
decreasing Tp in a cloudy day, remembering that this quality
could be inferred from historical data.
4) The water level of tank: increasing Tp when this level is low and
decreasing Tp when this level is high