Using every spare unit of solar electricity at home sounds sensible. Financially, however, a unit exported can have more value than one used to replace cheap water heating. The comparison depends on what would have heated that water otherwise, when the water is needed and whether extra equipment must be purchased.
This guide uses hypothetical prices to explain the decision. It does not rank current suppliers or recommend changes to hot-water safety settings. SEAI describes both export and storage in a hot-water tank as possible destinations for surplus solar production.
First identify genuine surplus
Solar generation is not all surplus. Subtract electricity being used in the home and avoid counting energy already allocated to charging a battery. Look at the same time interval: an annual generation total cannot tell you how much is available on a particular afternoon.
The tank must also need heat. Warming water that will not be used, or reheating a tank unnecessarily, does not avoid an equivalent later purchase. Storage and standing losses reduce the useful value.
Compare against the heat you would otherwise buy
Use this worksheet with VAT-inclusive purchase prices and the actual export payment applicable to your account. These examples ignore tax, storage losses, maintenance and equipment costs initially so the comparison is visible.
| Alternative water heating | Hypothetical input | Cost per useful kWh of heat |
|---|---|---|
| Electric immersion | Electricity at 30c/kWh, roughly one unit of heat per unit of electricity before tank losses | About 30c |
| Cheaper-window immersion | Electricity at 12c/kWh, same assumption | About 12c |
| Heat pump | Electricity at 30c/kWh, assumed hot-water performance ratio of 3 | About 10c |
The heat-pump ratio here is an illustrative input, not a promised hot-water efficiency. Use an appropriate measured or installer-supported value for your system rather than its best advertised space-heating figure.
A 5 kWh afternoon, worked three ways
Suppose 5 kWh would otherwise be exported at 18c/kWh, earning €0.90. Diverting it to an immersion gives approximately 5 kWh of heat before storage losses.
If it replaces immersion heating at 30c, it avoids €1.50, a €0.60 advantage over export. If it replaces immersion heating at 12c, it avoids only €0.60: exporting is €0.30 better under these assumptions.
If the same useful heat would instead have come from a heat pump at an effective 10c/kWh of heat, the avoided purchase is €0.50. Diverting solar to the immersion would then give up €0.90 to avoid €0.50. That is different from scheduling the heat pump itself to use solar electricity, which needs a separate comparison of its electrical input and operating conditions.
Include the diverter before declaring a saving
An existing device and a new purchase have different economics. Imagine a fictional €600 installed cost and a net advantage over export of 8c per diverted kWh. At 1,000 useful diverted kWh each year, the benefit is €80 and simple payback is 7.5 years, before maintenance and changes in prices. If the advantage falls to 2c, that becomes €20 a year and 30 years.
Do not multiply the benefit by all annual solar generation. Only count surplus that the tank can use and that displaces water heating you would otherwise have paid for. Also do not add export income for those same diverted units.
Your decision record
Write down: current export rate; alternative heating method; its marginal energy cost; useful surplus; when hot water is needed; installation cost; and the assumptions you are least sure about. Run a lower-benefit scenario before buying equipment.
Keep the hot-water system’s required temperature cycles and controls intact. Ask a qualified installer about compatibility and safe scheduling. The right answer may be export, direct household use, timed water heating or a mix across the year. The important comparison is the value of the next unit, not the highest possible self-consumption percentage.
