Solar Thermal vs Photovoltaic: How Each Works

A solar thermal panel and a photovoltaic panel can look almost identical on a roof and behave nothing alike. One is a heat exchanger with a fluid running through it. The other is a semiconductor device with no moving parts and no working fluid. Both absorb sunlight, and past that point almost nothing they do is shared: the temperature that helps one hurts the other, the storage each needs is a different kind of equipment, and the failure modes have nothing in common. The term “solar panel” covers both, which is where most confusion starts.

What a photovoltaic cell does

A silicon photovoltaic cell is a thin slab of semiconductor with two differently doped layers forming a junction. Light absorbed in the silicon frees electrons, the junction separates them, and the cell delivers a voltage across its terminals. A single silicon cell produces roughly half a volt regardless of its area; the area sets the current, not the voltage. Cells are wired in series to build voltage and in parallel to build current, and the module is rated at standard test conditions: 1000 W/m² irradiance, 25 °C cell temperature, and a defined air mass spectrum.

Those test conditions are a laboratory reference, not a roof. A module on a hot afternoon runs well above 25 °C, and silicon loses voltage as it warms. The temperature coefficient, quoted as a percentage per degree, describes that loss. It is why a cool, clear spring day can outproduce a hazy hot one at the same irradiance, and why mounting with airflow behind the module matters.

Output is direct current, which an inverter converts to alternating current for the building. Nothing in the array stores energy; the grid or a battery does that.

What a solar thermal collector does

A flat-plate collector is an absorber, usually a metal sheet with tubes bonded to it, behind a glazing layer and in front of insulation. A heat-transfer fluid — water, a water-glycol mix, or a dedicated fluid — circulates through the tubes, picks up heat from the absorber, and carries it to a storage tank or a heat exchanger. Evacuated-tube collectors use the same principle with the absorber inside a vacuum, which reduces heat loss to the surrounding air and allows higher fluid temperatures.

The governing quantity is the temperature difference between the collector and its surroundings. A collector is efficient when the fluid entering it is cool and the day is bright; as tank temperature climbs, the collector runs hotter, losses rise, and useful gain falls. A thermal system therefore performs best when it is being drawn down — morning showers, a heating loop — and worst when the tank is already hot on a sunny afternoon.

That is the opposite of the photovoltaic case. Heat degrades a PV module’s voltage. Heat is the product a thermal collector is trying to deliver.

Storage: tank versus battery or grid

A thermal tank holds energy as sensible heat. Its capacity is finite and its losses are continuous, so a tank that is hot on Monday is warm on Wednesday. A battery holds electrical energy and can discharge into any circuit; the grid, where interconnection is permitted, does the same without a battery at all. This is the structural difference: photovoltaic output is fungible and thermal output is not.

Freeze and overheat protection

Water freezing in an absorber tube splits it. Thermal systems in freezing climates use one of several approaches: a closed loop charged with glycol, a drainback design that empties the collectors when the pump stops, or a controller that circulates warm tank water through the collectors on cold nights. Each has a failure mode that depends on the controller and the pump continuing to work.

Overheat is the mirror problem. In summer, with a hot tank and no draw, the collector stagnates: the fluid stops moving, temperature climbs, and the loop must be able to survive it. Glycol degrades at sustained high temperature and needs periodic testing. Stagnation is a design condition, not an accident, and a thermal system that is oversized relative to hot-water demand will stagnate routinely.

A photovoltaic array has neither problem. It produces less on a hot day and nothing at night, and it does not care. Its DC side, however, is energised whenever light falls on it, and that is a hazard of a different kind.

Why heat pumps now enter the comparison

A heat pump water heater uses electricity to move heat into the tank rather than to generate it resistively, so one unit of electricity delivers several units of heat. Paired with a photovoltaic array, it delivers the same service as a solar thermal system — hot water — using a component that is mass-produced, has no roof loop, no glycol, no freeze protection and no stagnation. The trade is that it depends on electricity being available, either from the array at the moment or from the grid or a battery.

The comparison is therefore no longer collector versus module in isolation. It is a thermal loop on the roof versus a PV array plus an efficient electric water heater. Which fits depends on hot-water demand, climate, roof area and whether the building already has somewhere to put a tank.

What people get wrong

A common misdiagnosis: a homeowner sees a thermal system underperform in summer and concludes the panels are failing, when the tank is simply already hot and the collector is running at low efficiency by design. The mistake is natural because the same observation on a PV array — lower output on a hot afternoon — does indicate a real temperature effect, and the two look like the same symptom. On a thermal system, the useful question is whether the tank is being drawn down, not whether the collector is receiving sun.

The reverse error is expecting a PV array to heat water directly. A module’s output is electricity; connecting it to a resistive element works, but it discards the efficiency advantage a heat pump would provide and produces hot water only when the sun is out.

A third: assuming a thermal collector’s efficiency rating applies at all temperatures. It is quoted at a specific temperature difference, and it falls as the tank warms.

Reading a controller or inverter log

Both system types report performance, and the numbers mean different things. A solar thermal controller typically displays collector temperature, tank temperature and pump state. A collector temperature far above the tank temperature with the pump off usually indicates stagnation - the tank has reached its limit and the controller has stopped circulation - rather than a fault.

A PV inverter or its monitoring portal reports AC power, DC voltage and, on many models, internal temperature. DC voltage that falls as the modules heat up is the temperature coefficient at work. A day’s energy total is best compared against a clear day in the same season, not against the module’s nameplate rating multiplied by daylight hours.

What the comparison does not settle

Net metering, interconnection rules, export compensation and any incentive that applies to either technology are set by state and by utility, and they change. Nothing here should be read as a statement about what is permitted or compensated where the reader lives. The items to check locally are: whether export is compensated and at what rate, whether a thermal system qualifies for any programme, what the interconnection process requires for a PV inverter, and whether a heat pump water heater is eligible for any rebate. Those answers differ by jurisdiction and by year.

Roof work, mains connection and any meter change are licensed trades. The DC side of a photovoltaic array remains energised in daylight and cannot be switched off at the module; it is not a place for anything but observation from a distance.