An independent photovoltaic system is one that keeps working where there is no utility to lean on. The useful way to classify it is by the hour the load wants power. A pump that may stop when a cloud passes is a different machine from a clinic that needs light after sunset, and both are different from a mill that only runs while people are at work in the daytime.
Updated October 2026.
A load that can stop at dusk
Missouri Extension describes a PV-direct system as one with no battery and no grid connection. It generates while the sun shines. Water pumping and ventilation are the examples they give, because a pause in broken cloud is often acceptable. The parts list is short: modules, a controller matched to the motor, and the pump. There is nothing to maintain through the night and nothing to ventilate.
The moment someone adds “and it must run after 6pm”, this category is over. A battery, and a decision about how many dark hours matter, has entered the project. Pretending it is still a direct pump is how tanks run dry on the first cloudy afternoon the owner did not budget for.
A load that continues after sunset
Lights, a vaccine fridge, a household, a radio site: the energy has to be in a battery before the sun goes. VeraSol’s off-grid installation guidelines group the usual shapes. One is DC-coupled and DC-only: the array charges the battery through a controller, and the loads are DC. Another is DC-coupled with a battery inverter, so the same battery can feed AC appliances. Small systems of this kind often live at 12, 24 or 48 V, though that is a habit of the equipment, not a rule about what the site needs.
DC coupling puts the day’s harvest into the battery first. That suits a night load. It also means the battery has to be large enough for the night, and the array large enough to refill it on a short winter day. How to live with that arithmetic through the worst month is in living off grid through January.
A daytime machine does not have to live inside the battery
VeraSol also describes AC-coupled systems: the modules connect through a PV inverter to the AC side of a grid-forming battery inverter. The battery inverter is what makes the stand-alone grid. The PV inverter then behaves as if a grid were present, and drops off if that local grid disappears.
This shape fits a load that works while the sun is up. Milling, welding, a school office, a cold room that is opened in the afternoon. Those watts can come from the PV inverter in the same hour, and the battery only has to stabilise the local grid and cover the gaps. Size the battery for the night and for the clouds, and size the PV inverter for the machines. Size the battery for the machines as well and you will buy a bank that sits full all day and still cannot start a motor if the inverter’s surge rating was ignored.
Mixed sites exist. VeraSol allows a system that is partly DC-coupled and partly AC-coupled. A village that has both a night lighting circuit and a daytime workshop is a normal reason to mix them, not a failure to pick a category.
The generator is a component
The same guidelines treat a fuelled generator, on the DC bus or the AC bus, as a hybrid off-grid system, including systems that serve more than one building through a mini-grid. Missouri Extension says the same thing from the other direction: off-grid means not connected to the utility, and these systems typically have a generator or a small wind turbine beside the solar, because there are weeks when the array will not refill the battery.
A generator that runs every evening means the array or the battery is short. A generator that runs a few days a year, during the worst weather, is often cheaper than a battery sized for that week. Both outcomes are designs. Neither is a moral failure. Tracking the hours is how you tell them apart, which is why an off-grid monitor has to keep generator hours even when the radio link is down.
Domestic and everything else
IEA PVPS has long split off-grid into domestic systems and non-domestic ones such as pumping, telecom and signals. The split is about the load, which is the same test as above. A telecom hut that must stay up through four cloudy days is a night load with a hard autonomy target. A borehole that fills a tank by noon is a direct system until someone also wants the tank topped up at night.
A classification you can apply on a site visit
- Does the load accept a stop when the sun stops? If yes, start with PV-direct.
- If it runs at night, the battery is in the path. DC-coupled is the usual small-system shape.
- If large machines run in daylight, give them a PV inverter on the AC side and keep the battery for the night and the clouds.
- Write down the generator’s job: weekly crutch, or rare cover for the worst week.
The documents
PV-direct, grid-tied shutdown, and off-grid with a generator: University of Missouri Extension, solar system options (PDF). The five off-grid configurations, including mixed coupling and a fuel generator: VeraSol system installation guidelines (PDF). Distributed and off-grid as separate ideas in the global statistics: IEA PVPS Trends 2026.
