Heating DHW with oil vs night rate electric

YFlyer

https://www.boards.ie/discussion/comment/119630746#Comment_119630746

Thank you.

The landlady is on the regular package for both Bord Gais and Electric Ireland. So currently, heating the water by gas is the better option.

Fantana2

https://www.boards.ie/discussion/comment/119630780#Comment_119630780

Don’t tank spec sheets use kw/24hr and not degrees to show losses in a practical manner across different volumes?

Maybe it was 1kw/24 hrs (still extremely good for 360ltr tank).

Using degrees per/24hrs is not really practical as 120ltr at 1degree loss would be completely different to a 300ltr tank losing a degree.

bullit_dodger

https://www.boards.ie/discussion/comment/119632112#Comment_119632112

Don’t tank spec sheets use kw/24hr and not degrees to show losses in a practical manner across different volumes?

Could do - I'm not really familiar with the space. In practical terms though (at least for me) degrees loss / time unit, is helpful to figure out how long the tank will realistically stay hot when you heat it.

WattsUp

https://www.boards.ie/discussion/comment/119630746#Comment_119630746

I was looking into this. While boilers might get to 90% efficiency in winter when heating rads + DHW they would rarely achieve this is summer doing just DHW. In summer energy is used to heat up the boiler and flu, maybe 15l of water in the boiler and maybe another 10l in pipes. This water just goes cold or even puts unwanted heat into the house in summer. So if you were to heat 100l of DHW in summer an additional 25l of it might just go cold in boiler and pipe. This is not issue in winter as that heat warms the house space and the the amount of water going cold in the boiler is a much smaller % of the total water for rads + hot water.

In winter the boiler efficiency of 90% transfers to DHW. However in summer even if boiler was 90% efficient you might be losing 25% in pipes etc so in reality only 67.5% (.75*.9) of your input power (oil or gas) is actually going into the water tank.

On the other hand electricity is probably 99%+ efficient at DHW even allowing for tiny losses in wiring etc.

Summer Oil for DHW : Oil is approx €1.05/l and for that you get 10.18kWh of energy. That would deliver 6.87kWh of DHW at 67.5% efficiency. The delivered cost using oil for DHW would be approx 15.2c/kWh (1.05/(10.18 * 0.675))

Winter Oil DHW: Oil is approx €1.05/l and for that you get 10.18kWh of energy. That would deliver 9.162kWh of DHW at 90% efficiency. The delivered cost using oil for DHW would be approx 11.4c/kWh (1.05/(10.18 * 0.90))

Electricity delivered cost for DHW is effectively the kWh rate which can be as low as 5-8c/kWh on smart night rate plans

Typical costs for domestic hot water based on oil at €1.05/l and 90% boiler efficiency.

Oil in winter for domestic hot water= 11.4c/kWh

Oil in summer for domestic hot water = 15.2c/kWh

Electricity for domestic hot water= 8c/kWh ( cheap middle of night smart rates) , 28c/kWh day rates

graememk

Interesting to come back to this a year and a half later.

Oil has come down a lot since 2022, and a good point about the summer and winter differences too.

Can I get my 9 hrs of 7c night rate back too?😂

unkel

https://www.boards.ie/discussion/comment/122065650#Comment_122065650

Night rate 5c now. Only a fraction above the cheapest I ever paid, which was about 4 years ago.