Calculate electric usage moving from gas

barker77

Hi im shortly getting air to air fitted to heat my home. Is there an easy ish way to estimate the kWh i will need in electric vs gas so I can compare tariffs? 

QrizB

Estimating the kWh is easy, if imprecise - divide your gas consumption by three. (You'll probably use less then that, but it's somewhere to start.)

Comparing tariffs is somewhat trickier, as the "best" ones have different prices at different times of day and you'll have to guess how well you can split your consumption between the rates.

barker77

QrizB said:

Estimating the kWh is easy, if imprecise - divide your gas consumption by three. (You'll probably use less then that, but it's somewhere to start.)

Comparing tariffs is somewhat trickier, as the "best" ones have different prices at different times of day and you'll have to guess how well you can split your consumption between the rates.

Ah ok and then just subtract my hot water which I think is approx 70kWh a month thanks!

Rodders53

barker77 said:

Ah ok and then just subtract my hot water which I think is approx 70kWh a month thanks!

70 kWh per month would hardly make up for the daily losses on my unvented HW cylinder (2 kWh per day x 30 = 60 kWh).

Air to air may be even more efficient than the COP of 3 suggested, above.  Check your Installer for the specs of the proposed install...  My nominal 5 kW cooling 6 kW multi-split heating outdoor unit has a COP figure of 5.17 for heating.

barker77

I can’t see thr specific heating cop but the scop is 4.3 which I think could be the same thing ? 

NedS

COP is the Coefficient of Performance, which is a real time value. SCOP is the Seasonally adjusted (or average) COP, so yes the SCOP is more useful for estimating annual performance, rather than specific performance on a particular Saturday afternoon in Aug. The COP will vary dramatically depending on outside temp - it's far easier to extract heat from warm air than very cold air.

As @QrizB suggests, I'd assume a value of 3 (a reasonable minimum assumption), and be pleasantly surprised if you get a better outcome, rather than expecting 4.3 and then being disappointed if you only get 3.5 or 4.0.

FreeBear

NedS said:The COP will vary dramatically depending on outside temp - it's far easier to extract heat from warm air than very cold air.

As @QrizB suggests, I'd assume a value of 3 (a reasonable minimum assumption), and be pleasantly surprised if you get a better outcome, rather than expecting 4.3 and then being disappointed if you only get 3.5 or 4.0.

During the summer months, you could well hit a COP of 5 or more. On an extremely cold winter day with the flow temperatures cranked up (on a wet system), COP could dip under 2. Overall, a figure of 3 is a reasonable number until you have some hard data.

matelodave

I guess that most people get a SCOP of around 3-3.5, with those who are prepared to tweak and optimise their system getting a bit more. However as said above, it depends on the how the system is installed, configured but most of all how its being used.

Air to air can be a bit more efficient than air to water but obviously wont be if you use it for cooling in the summer as well as for heating in the winter.

COP and SCOP are different. COP is the performance at a specific temperature whereas SCOP is the average over the year.

As @FreeBear says the COP is significantly lower (worse) in the depths of a cold winter compared  to the COP on a balmy autumn or spring day. If you include cooling into the mix then its likely that the overall "SCOP" will take a quite a big hit as you'll be using energy in the summer for air-con whereas with other systems there is usually little to no space heating requirement.

barker77

matelodave said:

I guess that most people get a SCOP of around 3-3.5, with those who are prepared to tweak and optimise their system getting a bit more. However as said above, it depends on the how the system is installed, configured but most of all how its being used.

Air to air can be a bit more efficient than air to water but obviously wont be if you use it for cooling in the summer as well as for heating in the winter.

COP and SCOP are different. COP is the performance at a specific temperature whereas SCOP is the average over the year.

As @FreeBear says the COP is significantly lower (worse) in the depths of a cold winter compared  to the COP on a balmy autumn or spring day. If you include cooling into the mix then its likely that the overall "SCOP" will take a quite a big hit as you'll be using energy in the summer for air-con whereas with other systems there is usually little to no space heating requirement.

Thanks - yeah clearly will be using energy if use for air con. But im making sure that don’t confuse efficiency for cooling and heating .