Can solar replace a water heater entirely?

No, and systems that claim to are describing an average rather than a worst case. A solar collector covers a large fraction of the annual load in a sunny climate, but it produces nothing at night and little during a cloudy week, so a conventional heater stays in the system as backup. The honest framing is pre-heating the water the main heater then finishes.

How much collector area do I need?

It depends entirely on your site insolation and your load, which is why this calculator asks for both rather than guessing. As an order of magnitude, 30 to 60 square feet covers a large share of a typical household load in a sunny region and considerably less in a cloudy northern one. Take the insolation figure from NREL data for your location.

What happens to a solar collector in a freeze?

An unprotected collector full of water can burst, which is why freeze protection is not optional outside warm climates. Systems handle it either by draining back when the pump stops or by circulating a glycol mix through a heat exchanger. Both add cost and complexity, and both are the part of a solar installation people underestimate.

Can I use a pool solar panel for domestic hot water?

No. An unglazed pool collector is designed to move a lot of water through a small temperature rise at low pressure, and it is not rated for potable water, for domestic hot water pressure or for the temperatures involved. They look similar and appear in the same search results, which is exactly why this site keeps them out of the catalog.

Efficiency calculator

Solar Water Heating Pre-Heat Calculator

Quick answer

A solar collector pre-heats water before the main heater, so the saving is the fraction of the load it covers. Forty square feet of collector at 1,500 BTU per square foot per day and 45 percent efficiency collects about 27,000 BTU, which covers most of a 55 gallon daily load at a 70 F rise.

Result

Collected per day 27,000 BTU Published standard

Fraction of the load covered 84% Published standard

Gallons pre-heated 46.3 gal Published standard

Daily load 32,071 BTU Published standard

This is an average day figure. A solar system still needs the conventional heater behind it for cloudy weeks and winter nights, which is why the honest framing is pre-heat rather than replacement. Freeze protection is the other requirement people underestimate in cold climates.

The formula

Collected BTU per day = collector area x site insolation x collector efficiency. Fraction covered = collected / (gallons x 8.33 x rise).

This calculator will not guess your insolation. A collector output depends on the solar resource at your site, the collector published efficiency curve and the inlet temperature, and none of those can be inferred from a postcode. Take the insolation figure from the NREL solar resource maps for your location and the efficiency from the collector specification sheet.

The figures above are computed with these exact inputs: Collector area 40 sq ft, Site insolation 1500 BTU/sq ft/day, Collector efficiency 45 %, Hot water per day 55 gal, Temperature rise 70 F.

What this calculator assumes

Some of these are published test procedures or code tables and some are planning conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Solar Efficiency: 0.45	Convention	Assumed average collector efficiency. Use the collector manufacturer published efficiency curve where you have it
Water Weight: 8.33	Published standard	Published density of water at 60 F, 8.33 lb per gallon

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Beginner

Frost King

Frost King SP57 R10 Blanket, 48 x 75 in

$32.54

R10 stated on the package, which is the only number worth comparing between blankets. Many listings publish none.

Best for: An older tank that is warm to the touch

Check price on Amazon

Intermediate

Frost King

Frost King SP60 R10 Blanket, 60 x 90 in

$64.93

The larger size for a 75 or 80 gallon tank, where a 48 by 75 blanket leaves a strip uncovered.

Best for: A 75 or 80 gallon tank

Check price on Amazon

Intermediate

SmartJacket

SmartJacket SJ-80 Blanket, 20 to 80 Gallon

$43.95

Polyethylene rather than fibreglass, so it can be fitted without the itch and can be cut around the controls cleanly.

Best for: Anyone who does not want fibreglass in a closet

Check price on Amazon

Expert

Trade Winds

Trade Winds R12 Foam Blanket, 40 to 80 Gallon

$52.88

R12 rather than R10, which is the highest stated value in this catalog. Leave the top of a gas tank and the draft hood clear regardless.

Best for: The coldest garages and basements

Check price on Amazon

Beginner

Duck Brand

Duck Brand Foam Pipe Covers, 3/4 in

$9.99

The first six feet of hot outlet pipe is where most of the standing heat loss happens, and this is the cheapest fix on the entire site.

Best for: The first six feet off the tank

Check price on Amazon

Beginner

CARGEN

CARGEN 3/4 in Foam Pipe Insulation

$8.69

Three quarter inch inside diameter, which is the size that fits 3/4 inch nominal copper. Measure the pipe, not the fitting.

Best for: A 3/4 in copper hot line

Check price on Amazon

Frequently asked questions

Can solar replace a water heater entirely?: No, and systems that claim to are describing an average rather than a worst case. A solar collector covers a large fraction of the annual load in a sunny climate, but it produces nothing at night and little during a cloudy week, so a conventional heater stays in the system as backup. The honest framing is pre-heating the water the main heater then finishes.
How much collector area do I need?: It depends entirely on your site insolation and your load, which is why this calculator asks for both rather than guessing. As an order of magnitude, 30 to 60 square feet covers a large share of a typical household load in a sunny region and considerably less in a cloudy northern one. Take the insolation figure from NREL data for your location.
What happens to a solar collector in a freeze?: An unprotected collector full of water can burst, which is why freeze protection is not optional outside warm climates. Systems handle it either by draining back when the pump stops or by circulating a glycol mix through a heat exchanger. Both add cost and complexity, and both are the part of a solar installation people underestimate.
Can I use a pool solar panel for domestic hot water?: No. An unglazed pool collector is designed to move a lot of water through a small temperature rise at low pressure, and it is not rated for potable water, for domestic hot water pressure or for the temperatures involved. They look similar and appear in the same search results, which is exactly why this site keeps them out of the catalog.

Researched guidance, not professional advice. These results are planning figures from published test procedures, code tables and manufacturer specifications, and they do not replace a sizing done by the person who will sign the permit. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve, and replace one that will not open when the test lever is lifted. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Water at 140 F scalds a child in seconds. Turn off the breaker before touching an element, and never energise an element that is not submerged.