You are correct that the current ET implementation results in one update per day, and it only uses the weather data from the previous day instead of accumulated over many days.
Part of the challenge of including more data is that the weather service generally charges more for access to more history data. We have never charged our users any subscription fee, so a big increase in weather service bill will be difficult for us to cover.
One possibility to avoid querying the weather service for history data is to simply cache the data locally on the controller. That way, if we keep the past, say, 2 weeks of data, that should be sufficient for ET calculation and won’t trigger additional charges. This is definitely possible, we just haven’t got around time to implement that.