A solar water heater: what it is, how the types work, and what one datasheet says

A solar water heater turns sunlight into hot water. It is a collector on the roof and a tank, and nothing in it makes electricity, so it has no inverter, no meter and nothing to do with net metering. The words on its datasheet are different from a panel’s too.

This page reads one certified listing and one government explainer. SolarPampanga installs photovoltaic systems, which is the other kind of solar, and the question people bring us is which of the two deserves the roof. The last section is about that.

The types, in ENERGY STAR’s words

A solar water heater is described by its collector and by how the water moves. The US ENERGY STAR programme’s explainer, read 8 October 2026, sets out both.

Flat plate collector: Copper tubes on a dark absorber plate, in an insulated box under tempered glass. ENERGY STAR says a flat plate collector is typically sized to hold about 40 gallons, and that two of them supply roughly half the hot water a family of four uses. This is the collector on the listing below.
Evacuated tube collector: Each tube is a glass vacuum flask around a tube of water or heat transfer fluid, so little heat escapes. ENERGY STAR calls them the most efficient collectors and notes they work in overcast conditions. Tubes are replaced one at a time.
Batch collector: A dark tank or tubes inside an insulated box, heating the water where it sits. Water left in it on a quiet day gets very hot, so a tempering valve mixes in cold before the tap.
Thermosiphon, or passive: No pump. Heated water rises from the collector into a tank mounted above it, and cooler water sinks back. This is the unit with the tank on the roof, and it is what the listing below is.
Active, or split: A pump moves water or fluid between a collector on the roof and a tank elsewhere, often at ground level, with a controller deciding when. The roof carries only the collector.
Direct and indirect: Direct systems run the household water through the collector. Indirect systems run a separate fluid through it and pass the heat to the tank through an exchanger, which ENERGY STAR describes as the arrangement for climates that freeze. The listing below is indirect.

One certified listing, line by line

The Solahart Thermosiphon Series ASE 302J (ENERGY STAR ID 2472708) is a rooftop thermosiphon unit with an electric backup, and its ENERGY STAR listing, captured 7 October 2026, prints the figures below. Solahart’s own dealer page lists the Philippines among the countries it sells in, with rooftop, split and heat pump ranges. The listing is a US-market document and the page says so; it is here as an example of what the lines mean, not as a recommendation.

Collector type and system type: Flat plate, Indirect thermosiphon; The tank sits above the collector and water circulates by heat alone. Nothing moves except the water.
Backup: Electric element in the tank; A heating element in the tank. The listing prints a solar auxiliary tank input of 9.0 kW. On a grey week the house pays for hot water the way it did before, from the meter.
Tank volume: 50 gallons, about 189 litres; The hot water available before the element has to help.
Collector area: 43.1 square feet, about 4.0 m²; The glass that faces the sun. For comparison, the Jinko module on the mono vs poly page is 2,278 by 1,134 mm, about 2.6 m², so this is a little over one and a half panels of roof.
Solar uniform energy factor: 9.0; The rating ENERGY STAR certifies solar water heaters on. It comes from a US test with a set draw pattern, so it compares one certified unit with another. It does not predict a bill in Pampanga.
Draw pattern: Medium usage; The test household the factor was measured against. A house that showers more or less than the test pattern gets a different result.
Certified, and market: 19 January 2023, United States; A US listing. Solahart’s own dealer page lists the Philippines, and the model sold here may carry different figures. Ask for the sheet of the unit you are quoted.

What it asks of a roof here

Weight first. A thermosiphon unit puts the water on the roof: the 50 gallon tank on the listing holds about 189 kg of water when full, before the tank itself, the collector and the frame, and it sits on one spot rather than spreading along rails the way panels do. A GI sheet roof on light purlins and a concrete deck are different answers to that load, and what a roof survey checks applies here with the weight as the first item. A split system takes the tank off the roof.

Sun second. The collector wants the same thing a panel does, a face that gets the middle of the day, and the same atlas figure describes it: the peak sun hours at five points around here. Nothing freezes in Pampanga, so the closed-loop arrangements ENERGY STAR describes for cold climates are not needed for that reason, though the listed unit is indirect anyway.

Wiring third. The backup element on the listing is rated 9.0 kW, and whenever the sun has not done the job it is an electric water heater on the house supply. How it is wired is set by the sheet of the unit you are quoted, which may differ from this US listing. On a grey week the bill carries it.

Or panels that run the heater you have

The other way to heat water with the sun is to make electricity and run the electric heater the house already has. The two cannot be compared on one line: a water heater’s listing prints a solar uniform energy factor, and a panel’s datasheet prints a module efficiency, which the mono vs poly page reads at 22 to 23 percent for a current module, and they measure different things. What can be compared is the roof area each takes and what else the roof could be doing. Panels run the whole house and export the surplus under net metering; a water heater runs the hot tap.

If the question is how many panels the electric shower heater takes, read its plate watts and the minutes it runs, and the solar panel calculator does the rest with the formula shown.

What people ask about solar water heaters

What is a solar water heater?

A collector on the roof that turns sunlight into heat, and a tank that stores the hot water. It makes no electricity, so it has no inverter, no meter and nothing to do with net metering. The unit most people picture, with a horizontal tank above a flat panel, is a thermosiphon system: the sun heats the collector, the hot water rises into the tank by itself, and an electric element in the tank covers grey weather.

Is a solar water heater the same as solar panels?

No. Solar panels, properly photovoltaic panels, make electricity for the whole house, which can include an electric water heater. A solar water heater makes hot water and nothing else. They share a roof and a word. If the question is how many panels would run the electric shower heater, that is a panel question and the solar panel calculator answers it from the heater’s watts and hours.

Does a solar water heater work on cloudy days?

Partly. ENERGY STAR notes that evacuated tube collectors work in overcast conditions, and a flat plate collector still gathers diffuse light, but the water is cooler than on a clear day. That is what the electric backup in the tank is for, and on a week of rain the backup element does the work and the bill shows it.

How heavy is a solar water heater on the roof?

Heavier than panels, because the water is up there. The 50 gallon tank on the listing here holds about 189 kg of water when full, before the weight of the tank, the collector and the frame, concentrated on one spot of the roof rather than spread along rails. A split system moves the tank to the ground and leaves the collector on the roof.

Which is better for my roof, a water heater or panels?

It depends on what the house heats water with now and for how many showers. A house that heats water with electricity for a family every morning has a daily load that either technology can take; a house that rarely uses hot water has nothing for a heater to do. An energy audit answers it from the bill rather than from the roof.

Tell us how the house heats water now

Say what heats the water today, how many people shower in the morning, and your monthly kilowatt-hours. You will get a written answer on whether the roof is better spent on panels, with the heater counted in the load.

Ask for a quotation

Tell us the property and what you are trying to cut, and you will get a written answer with a size and a next step. No visit is booked and nothing is charged for asking.

What an energy audit looks at

Sources: ENERGY STAR, “How It Works, Solar Water Heaters”, read 8 October 2026; ENERGY STAR certified product listing for the Solahart Thermosiphon Series ASE 302J (ENERGY STAR ID 2472708), captured 7 October 2026; Solahart, “Where to buy”, read 8 October 2026. The litre, square metre and ampere figures are this page’s arithmetic on the listing’s gallons and square feet. SolarPampanga does not install water heaters, and no figure here is a claim about what one would save.