Running costs comparison: electric oven 66L -vs- electric compact oven 47L

ANNA

I'm considering a change of built-under oven, as it's bigger than I need now. I do have a separate freestanding combination microwave oven as well.

 

Present electric fan oven/grill, capacity 66 litres, A rated energy effieciency.

New compact electric fan oven/grill, capacity 47 litres, A+ rated. 

As the capacity is 70% compared to my current oven, will I save a corresponding amount in electricity usage? How do I do the calculation?

DullGreyGuy

Not automatically, you also need to consider the energy efficiency of each oven. A big oven can be more efficient than a smaller oven depending on insulation etc. 

vacheron

Most of an ovens' energy goes to :

• Heating the oven itself (inside the oven walls, shelves, doors).
• Heating the food inside the oven and the dishes / trays they are on (the larger and heavier the food and trays are, the more energy is required).
• Losses from the oven to the outside world (the oven body itself getting hot, the air around the oven getting hot) which depends on the insulation methods used, as @DullGreyGuy mentioned above.

Only a small amount of the heat goes to heating the actual air inside the oven, so I would have thought that with similar efficiencies the energy savings of an oven with a different internal volume would be relatively small (though it may heat up a little faster initially).

ANNA

So the old oven is A-rated and the proposed new one is A+ rated for energy efficiency, meaning better insulation then? Is it worth it to make the change over?

Grumpy_chap

ANNA said:

So the old oven is A-rated and the proposed new one is A+ rated for energy efficiency, meaning better insulation then? 

Does the A+ rating mean better insulation than the A-rating?
There are multiple parameters that set the different ratings so that topic might be worth some research to better understand.

It is also worth considering how full each oven would be, or could be, when in use.  As an example, we have recently started using spare space in the oven when cooking the Sunday roast to cook-up a tray of chicken drumsticks so we have them available to enjoy with salad during the summer evenings and the oven does not need to get used during the week.  I think that makes for an energy efficiency that we could not realise if we had a smaller oven.

molerat

Only if you need to replace the oven. 

vacheron

ANNA said:

Present electric fan oven/grill, capacity 66 litres, A rated energy effieciency.

New compact electric fan oven/grill, capacity 47 litres, A+ rated. 

As the capacity is 70% compared to my current oven, will I save a corresponding amount in electricity usage? How do I do the calculation?

You asked!  

The method to calculate the cost of heating the extra air volume inside the larger oven (assuming no other additional losses) is as follows:

Mass of air = 0.0012 kg (1.2 grams) per litre

Lets say you want to heat the air in the oven from ambient (20C) to 180C = a difference of 160C

Heat capacity of air = 1.005 KJoules / KG / degree C = 1.005 x 0.0012kg x 160 degrees C = 0.193

So to heat 1 litre of air from 20C to 180C would require 0.193 kilojoules of energy. 

But you are heating more than 1 litre of air
Amount of extra air: 66 litres - 47 litres = 19 litres of extra air. 

So 0.193 kilojoules per litre x 19 litres = 3.666 kilojoules of energy. 

Lets say your oven has a 1KW  (1 kilojoules / second) heating element:

The 1KW heating element therefore deliver the required 3.66 kilojoules of energy in 3.66 seconds (about 1/1000th of an hour!

The cost of heating the extra 19L of air is about 1/1000th of a killowatt hour.
Lets say a kilowatt hour of electricity is 50p.
 
This equals a cost of £0.0005 or 0.05p per oven heatup cycle!    

Lorian

Thought about one of the larger air fryers. Potentially a greater saving in use and purchase.

QrizB

ANNA said:

So the old oven is A-rated and the proposed new one is A+ rated for energy efficiency, meaning better insulation then? Is it worth it to make the change over?

No-one can answer that categorically as it depends on how you use the oven.

However,  together with the rating (A , A+ etc) the energy efficiency label should tell you how many kWh the oven will use "per use" under standard test conditions (which are meant to be similar to typical oven use). If the new oven uses fewer kWh under test, there's a fair chance that it will use less electricity when you have it in your house.

Note that saving 100kWh a year will only reduce your energy bill by £25 a year, and it might take a lot of £25s to recoup the cot of a new oven.

Example: if I go to the Currys website and look at ovens, just underneath the energy performance letter is a link to the product fiche. Here's an example:

https://currysprod.a.bigcontent.io/v1/static/244603_as_1.0_01062018_111404_20180602-18624.pdf

That particular A-rated Bosch oven uses 0.79kWh per use.

This (larger) Samsung oven is A+ rated and uses 0.71kWh:

https://currysprod.a.bigcontent.io/v1/static/energyfiche10249551-58340.pdf

Replacing the first oven with the second one will save 0.08kWh per use; if you use it once per day, that's 29.2kWh per year. At 25p/kWh you'll save £7.30 a year.

born_again

ANNA said:

So the old oven is A-rated and the proposed new one is A+ rated for energy efficiency, meaning better insulation then? Is it worth it to make the change over?

How old?

As energy efficiency keep getting updated.

Scot_39

The cost isn't just in the air though - its in the body of the oven - the metal and the air  - and then the food when its being raised to temperature - and then the losses to outside (wasted in summer). 

But the air difference will of course repeat - as likely to heat that extra air every time you open the door to load or check progress.

Even simple things like leaving unused trays - as an example - of costs for the metal - leave an unused heavy duty baking tray in the oven - you could be heating another 0.5 kg+ for larger pro grade ones (I got tired of using the cheaper ones that bend and warp with use - so invested in heavy duty ones - typical aluminium - say 150 degrees+.  Mines easily 0.5kg+ - would require a much higher 65-70 kJ a clear order of magnitude different.  But even thats only 0.02 kWh - or 0.5p at c25p/kWh for electric - less for gas.

The mass of surfaces determined in part by the volume and part by the construction - but remember your taking litres - so 70% in volume terms - is only around 20% in dimensional terms - 1.2*1.2*1.2 = 1.73 - so each side will only be around 40% larger.  And a bigger surface with thinner metal and better insulation behind it - could cost less than a less efficient old design.

But you shouldn't need to know any of the physics stuff.

The retailer should hopefully actually give a typical cycle energy usage - as well as the ratings - Currys for instance try to do so - see the product fiche links below the ratings flags - for many - but sadly not all models. 

They only do one A+ electric 47l model - and its just over 0.6kWh for typical fan assist cycle - checked couple of the A rated 65-70l - different brands - around 0.7-0.8kWh fan assist ratings.  There isn't a lot of savings there - even 0.2kWh will soon cost about 5p again - to cover the cost of a new oven. £18 per year if used on ave once per day.

Youd have to look at gas yourself - if you had it in the property - but you asked re electric.

But older ovens can be quite bad - in other respects - it takes around 20 mins for my old small Bosch oven to reach higher cooking temps (like 200) - and thats using around 2.2-2.3 kW combined upper and lower elements. Thats 0.7kWh before any food is het.  But it tends to then cycle less.

Other than special occasions with guests - or cooking large dishes e.g. 12" pizzas - its never used.

For most things I heat (lazy singleton) - its the microwave and / or air fryer.  

For oven type dishes - it takes my air fryer - a mid size single drawer (6l) - less than 3 mins to preheat when / if do so - at 1.7kW.  And it cooks things at either lower temperatures and/or in a lot less time - as little as half sometimes. Often much less time than it takes my main oven to heat. 

But then as above it doesn't do 12" pizzas or family roasts with the trimmings.

I'm sure the combi microwave offers potential savings over the conventional oven as well - and probably for larger dishes than many air fryers.

So IMO if it isn't broke - and already has fan assist to reduce energy use - and it isn't to fit in with a new kitchen upgrade for aesthetics - I am not sure I'd bother.  Especially when the shiny new 47l Bosch example they have is well over £1000.   

I'd treat myself to something else - or if bills really your primary concern - look at better insulation somewhere else - (loft insulation, thermal curtains, even old external doors like mines etc) not up to recent efficiency recommendations - for that sort of money - and you'll probably save a lot more.