Two solar proposals both predict 5,000 kWh a year. That shared total can hide a different monthly pattern, and your household does not use the same amount of electricity every month. Ask for the monthly figures before treating the annual estimate as the whole story.
The PVWatts documentation describes a modelling tool used to estimate photovoltaic energy production. A model uses assumptions about the site, system and weather; its result is an estimate rather than a guaranteed meter reading for a particular future year.
Put generation beside demand
Make twelve rows, one per month. Add your past electricity use in kWh and the proposed system’s estimated production in kWh. Keep the periods aligned. A bill covering six weeks should not be dropped unchanged into a one-month row.
A fictional household might use most electricity during a cold season while the roof produces most energy during a brighter season. The annual numbers can look balanced even though substantial imports remain in particular months. The pattern is different for another climate or a household with heavy daytime cooling.
Ask what explains the curve
- Which weather dataset and location were used?
- Were roof orientation, shading and system losses included?
- Are the figures electricity produced by the system or energy expected to be used in the home?
- How are export limits, battery behaviour and tariffs treated in the savings calculation?
Monthly totals still cannot show whether production and demand happen at the same hour. A household may export at midday and import in the evening. Ask for a more detailed assessment when that timing is central to the proposal.
Keep energy and money in separate columns. Converting kWh into a bill saving requires the applicable import and export prices and the amount used directly. A clear monthly comparison will not predict every cloudy week, but it will expose assumptions that disappear inside one attractive annual headline.