Calculating the cost of running various appliances - kw vs kWh

northlondonlynn

Inspired by Martin's 10 October tweet (the system won't let me include a hyperlink to it) I want to compare how much it costs to run various appliances. Fell at the first hurdle - finding the wattage of each appliance - because the user guide for my Zanussi oven doesn't seem to have that info! Instead, it sets out kwh. It says that the main fan oven is 0.89 kWh/cycle, and the smaller top oven is 0.75 kWh/cycle. I am bamboozled by this!

How do I use those figures to compare the running costs for the Zanussi oven against my combi microwave (1300w) and my air fryer (1500w)?

molerat

The biggest problem is that a 1kW oven heater element if running for 1 hour will use 1 kWh of electricity.  But the element will not run continuously for 1 hour, it will be clicking on and off so that is likely their estimate of the average consumption per hour.  My microwave again does not run at full power all the time, it pulses so is not running at 1200w all the time.

[Deleted User]

Watts are an indication of instant power. kWh is a calculation based on instant power versus time.

A simple example. I have a 1kW single bar electric fire and I leave it switched on for a 10 hours. During this time, the fire will consume 10kWh (or units) of electricity.

The problem with washing machines and Airfryers etc is that they don’t use a constant amount of power. The washing machine has a heater that might be rated at 2.3kW which will only be at the power for a short period of time. The machine will then just tumble using, say, 150W before it goes into its spin cycle. Similarly, for a fan over, it heats up and then switches on and off to maintain the set temperature. Figures of 0.89kW are an indication of what the device might use (a fruit cake will take longer to cook than a sponge cake: the firmer will consume more ekectricity).

You are charged for kWh not kW.

DE_612183

I would also "guess" that what zanussi call a cycle is the average kwh over the period of an hour - so a 1.5KW oven will use 0.75kwh on a cycle - where the cycle is based on only running 50% of the time.

Your air-fryer of 1500 watts is running at 1.5kwh - if it runs for a full hour - so if your chips cook in the main oven for 30 minutes, your cycle uses about 450 watts, your air fryer doing the same job on full max power for 10 minutes is only using 250 watts...

[Deleted User]

DE_612183 said:

I would also "guess" that what zanussi call a cycle is the average kwh over the period of an hour - so a 1.5KW oven will use 0.75kwh on a cycle - where the cycle is based on only running 50% of the time.

Your air-fryer of 1500 watts is running at 1.5kwh - if it runs for a full hour - so if your chips cook in the main oven for 30 minutes, your cycle uses about 450 watts, your air fryer doing the same job on full max power for 10 minutes is only using 150 watts...

Not quite. If the cycle takes 3 hours, then the usage figure is the total amount of energy consumed during the cycle. For example, if Zanussi is quoting 0.9kWh for the cycle that lasts 3 hours, then the average power (kW) during that time is 300W.

If an Airfryer rated at 1500W is on constantly for 30 minutes the total usage would 750Wh or 0.75kWh. (PS it is unlikely that any heating appliance is on at 1500W for 30 minutes. The chips would be charcoal.

DullGreyGuy

DE_612183 said:

Your air-fryer of 1500 watts is running at 1.5kwh - if it runs for a full hour - so if your chips cook in the main oven for 30 minutes, your cycle uses about 450 watts, your air fryer doing the same job on full max power for 10 minutes is only using 250 watts...

The air-fryer will use a peak of 1.5kw but will be able to cook at temps from 120c to 210c (depending on model) so is unlikely to use the peak power for the full cycle if you are setting it at below maximum temp 

DE_612183

DullGreyGuy said:

DE_612183 said:

Your air-fryer of 1500 watts is running at 1.5kwh - if it runs for a full hour - so if your chips cook in the main oven for 30 minutes, your cycle uses about 450 watts, your air fryer doing the same job on full max power for 10 minutes is only using 250 watts...

The air-fryer will use a peak of 1.5kw but will be able to cook at temps from 120c to 210c (depending on model) so is unlikely to use the peak power for the full cycle if you are setting it at below maximum temp 

Correct - I was just trying to say - worst case scenario the air fryer will still be cheaper

[Deleted User]

DE_612183 said:

DullGreyGuy said:

DE_612183 said:

Your air-fryer of 1500 watts is running at 1.5kwh - if it runs for a full hour - so if your chips cook in the main oven for 30 minutes, your cycle uses about 450 watts, your air fryer doing the same job on full max power for 10 minutes is only using 250 watts...

The air-fryer will use a peak of 1.5kw but will be able to cook at temps from 120c to 210c (depending on model) so is unlikely to use the peak power for the full cycle if you are setting it at below maximum temp 

Correct - I was just trying to say - worst case scenario the air fryer will still be cheaper

You might be right, but nothing that you said is a correct way to explain it.  Cycles don’t “use watts” in the way that you tried to describe things.

The only thing that matters is the kWh consumed during whatever activity you are measuring. Everything else can be ignored.

The only way to know is to measure the kWh used by an air fryer over the cooking cycle and compare it to the kWh used by the alternative appliance doing the same job.

[Deleted User]

DE_612183 said:

DullGreyGuy said:

DE_612183 said:

Your air-fryer of 1500 watts is running at 1.5kwh - if it runs for a full hour - so if your chips cook in the main oven for 30 minutes, your cycle uses about 450 watts, your air fryer doing the same job on full max power for 10 minutes is only using 250 watts...

The air-fryer will use a peak of 1.5kw but will be able to cook at temps from 120c to 210c (depending on model) so is unlikely to use the peak power for the full cycle if you are setting it at below maximum temp 

Correct - I was just trying to say - worst case scenario the air fryer will still be cheaper

I agree. It is important that people get to know the difference between kW and kWh as we are likely to see the introduction of kW limited tariffs in the not too distant future. Power-limited tariffs are a way of reducing demand at peak times. The consumer pays less if a power limitation is accepted. 

A 1.5kW Airfryer might be a better cooking option not only because it accrues less usage (kWh) but it also uses less power (kW). For example, if a consumer accepted a 5kW power limit from 4 to 7pm, a kettle with Airfryer combination would be fine whereas a standard cooker and kettle would exceed the power limit.

DullGreyGuy

[Deleted User] said:
A 1.5kW Airfryer might be a better cooking option not only because it accrues less usage (kWh) but it also uses less power (kW). For example, if a consumer accepted a 5kW power limit from 4 to 7pm, a kettle with Airfryer combination would be fine whereas a standard cooker and kettle would exceed the power limit.

But we still need to consider the difference between peak power and actual power.

My hob is rated at 7.5kw and has 4 rings that each go 1-12. It cannot actually support all four rings being on 12 at the same time, a bit annoying and also makes the maths a bit harder but can support all 4 at level 10. Clearly having 1 ring on level 5 (sufficient to simmer water) is not going to be anywhere near 7.5kw use... unfortunately as its induction it has an off/on approach and so probably is higher than 5/40ths of 7.5kw (assuming a linear scale)

Xbigman

This is what energy monitor plugs are for. There's nothing to calculate, you just measure the actual usage. With things like ovens that are hardwired you have to make a bit more effort and read your actual meter before and after use. If you turn everything else off you can get a very close idea of usage.



Darren