The calculation that produces the nice number
The typical proposal takes your annual consumption in kilowatt-hours, divides it by the area's equivalent sun hours, applies a loss factor and arrives at a capacity in kilowatts-peak. Then it multiplies the estimated generation by your average price per kilowatt-hour, and out comes the annual savings.
The whole calculation is arithmetically correct. The problem is that it treats your consumption as a single thing that happens uniformly, and your bill does not work that way.
On the GDMTH tariff you pay three energy prices depending on the time of day, and you also pay for your maximum demand. A calculation based on annual consumption and the average price ignores the two things that determine whether the project performs.
Your peak happens when generation is worth zero
This is the uncomfortable part. In most industrial plants in Mexico, the billable demand peak does not happen at noon: it happens in the afternoon or at night, during peak hours, when the sun no longer contributes anything.
Practical consequence: the solar system lowers your energy consumption in off-peak and mid-peak hours —which is where energy is cheapest— and does not touch your billable demand or your peak consumption, which is where energy is most expensive.
That is where the disappointment comes from. The system produces exactly what the design calculations said it would, and yet the bill dropped much less than promised. There was no bad faith: there was a sizing that never looked at what time things happen.
What to measure before sizing
Your actual load curve, measured. What time the plant starts up, what time the peak occurs, how long it lasts, which load causes it and how a Saturday looks compared with a Tuesday.
The breakdown of your bill by time period. How much you pay in off-peak, mid-peak and peak, and what share of the total goes to demand charges. Without that breakdown you can estimate generation, but not savings.
The available area and its actual orientation, with the shading the plant itself casts at different times of year. And the remaining service life of the roof sheeting, because a twenty-five-year system on a five-year roof is a problem paid for at the wrong time.
What to do when the curve does not favor solar
There are three paths, and all three are better than installing more panels.
The first is to correct before generating. If your bill shows a power factor penalty or your installation has high harmonic distortion, correcting that costs a fraction of a solar project and pays back faster. Putting generation on top of a dirty installation is building on an unresolved problem.
The second is to go hybrid. Storage is what turns midday generation into peak savings: the battery stores what the sun produced when energy was cheap and discharges it when it is expensive, and it also shaves the billable peak. Solar and storage are sized together or they are not sized well.
The third is to accept a smaller system. A project half the size, sized on the curve, can have a better return per peso invested than double the capacity sized on annual consumption. It takes less capital and pays back sooner.
How to read a solar proposal you were just handed
Ask three things. First: where did my load curve come from? If the answer is "from your annual consumption", there is no curve. Second: what time does my peak occur according to that curve, and how much of my estimated savings comes from peak hours? Third: how much of my bill goes to demand charges, and how much of that does this project touch?
If all three answers exist and come with numbers, you have a sized proposal. If not, you have an estimate built on an average.
And you do not need to hire us to ask those questions. It is the kind of review worth doing with any proposal, including ours.
A well-sized solar system is not the largest one that fits on your roof: it is the one that matches your load curve. The difference between the two is the difference between promised savings and real savings.
