The schedule mismatch
A solar system produces from morning to afternoon, peaking around midday. A company's vehicle charging has two very different schedules depending on who charges. Employee and visitor cars plug in on arrival and stay parked all day. Fleets charge when they return from their route, in the afternoon or at night.
The first case matches the sun almost by design. The second never does. All the engineering of a station that aims to pay for its own energy lies in closing that gap, and the tool to do it is storage.
Before sizing anything, it is worth separating the two loads in the metering. If employee charging and fleet charging come from the same switchboard, the mixed profile hides exactly what you need to see: how much energy is drawn with sun and how much without it. An analyzer on each circuit for one week is enough to separate them, and that data is worth more than any assumption from the vehicle's spec sheet.
Three profiles, three answers
Daytime charging, employee cars. This is the most favorable case. Vehicles are plugged in during generation hours, charging can be slow and can be modulated to follow production. Solar feeds the charging directly, without going through batteries. This is the domain of employee mobility at the workplace.
Overnight fleet charging. The sun does not help directly. The plant's solar reduces the energy bought during the day, and the fleet charges at night during off-peak hours, which are usually the cheapest. Adding batteries to shift solar energy to the night only makes sense if there is a real surplus at midday.
Fast charging with peaks. Here the problem is not energy but power. The battery delivers the peak to the chargers and the grid only sees a moderate load. Solar helps recharge the battery, but what pays for the project is the peak shaving.
What the battery solves and what it does not
A battery shifts energy in time and limits the power the station draws from the grid. It does not create energy: if solar generation is not enough for the day's charging, the battery only changes the hour at which the difference is bought.
Capacity is sized with a simple calculation: the power to be shaved times the duration of the peak, divided by the usable depth of discharge. Shaving 100 kW for one hour with a 90 % depth of discharge requires about 111 kWh (illustrative: 100 × 1 ÷ 0.9). The figure for your case comes from metering, not from the charger's spec sheet.
The same battery can serve as backup for the plant's critical load. That dual use changes the math, and it is worth putting on the table from the start. We cover it in BESS (battery energy storage).
The framework: exempt generation and injection
A solar system under 0.7 MW is an Exempt Generator (Generadora Exenta) and does not require a generation permit (Electricity Sector Law, Ley del Sector Eléctrico, article 19). We explain it in the distributed generation threshold.
Above that threshold, in interconnected self-supply, article 32 of the Law sets a condition for anyone who wants to sell surplus energy: if the plant is intermittent and injects energy, it must have its own backup with storage or pay the state-owned public utility for it. A station that consumes all its solar generation on site, with the help of batteries, does not inject. We explain it in article 32 and backup.
When the station pays for its own energy
When a significant part of the charging happens during sunlight hours, or when there is a midday solar surplus that is not used on site today and the battery can carry it to the afternoon charging. In both cases the energy bought from the grid genuinely goes down.
And when the fast-charging peak is what sets the demand charge, the battery pays off through peak shaving even if the sun plays no part. The data to know this is the same: twelve months of bills, one week of site metering and the real usage schedule of the vehicles.
Sources
Electricity Sector Law (Ley del Sector Eléctrico), DOF March 18, 2025, articles 19 and 32.
BESS energy sizing formula: (peak − target) × duration ÷ depth of discharge, the same one used by our calculator.
If your vehicles charge only at night and your bill does not show a demand problem, adding batteries to the solar for the station almost never pays off: charge during off-peak hours and let solar reduce daytime consumption. First measure how much solar surplus you have at midday; with no surplus, there is nothing to store.
