Inverter usage maths. I must be wrong somewhere

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TravellingSmurff

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Hi guys,

We have 2x100Ah lithium in parallel. Therefore 200Ah at 12v
No inverter but looking to get one and maybe something like the Renolgy 3000W.
I am trying to calculate usage e.g. how long can I cook on the airfryer, as an example.

Thinking of getting this one: https://ninjakitchen.co.uk/product/...ti-cooker-oven-air-fryer-sfp700uk-zidSFP700UK
It's rated at 1780W
Therefore I am using this math:
Watt-hours to Amp-hours = Wh / V = Ah
= 1780 / 12 = 148Ah
Inverter inafficency = 15%.
= 148Ah + 15% = 170Ah

Am I right? A 1780W appliance, like this air fryer, would cook for an hour and take approximately 170Ah of my 200Ah. This seems like a lot. And the average time off-grid the wife and I do is four days. So, ignoring Solar or any EHU, we can only use the Airfryer for about an hour and a half. Maybe 3 or 4 meals?
I also understand that the air fryer may not use that power the whole time, but measuring the one we have at home does. We used a plug with a meter—don't ask.
Is my maths correct?
Anyway, sorry to ramble. And thanks.
Danny
 
Hey, your math is basically correct. You can change the 1780/12 to 1780/12.8 as most lithiums tend to have a nominal voltage of 12.8. Won’t make that much difference to your equations unfortunately. You need a bigger battery bank and suitable solar for long inverter pulls, with 200ah and low panel arrays your probaly best sticking to smaller ticker appliances and for short burst
 
PeterCarole29 said:
The other thing to consider would be whats the maximum draw will your batteries allow
I dont think your 2 x 100ah lithium can supply constantly a 3kw inverter
Click to expand...
Ah okay, I've never thought about that. Why would it not and is there a certain type or calculation I need to do? Thanks

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funflair said:
Another thing to bear in mind, the air fryer will not be on max all time, and an hour is a long time in an air fryer 😏
Click to expand...
I thought that, but I measured it myself with a TAPO power measuring plug. And it's not consistent but not far off. This was just 4 minutes while I did an egg, closest thing to try :) So it may be better for longer cooking.
Here is the graph
1721942906427.png
 
TravellingSmurff said:
I thought that, but I measured it myself with a TAPO power measuring plug. And it's not consistent but not far off. This was just 4 minutes while I did an egg, closest thing to try :) So it may be better for longer cooking.
Here is the graph View attachment 928112
Click to expand...
:LOL: you replied too quickly before I deleted my post, I have a power monitor on the van inverter and it shows the air fryer cycling but yes they do pull a fair amount of power but we don't run ours for that long, but we have 640ah and 520 watts of solar ;) leave it for more than 4 minutes and you should see it cycle more.
 
As a very rough rule of thumb I divide the wattage by10 to give the current draw allowing for losses. So your 1780 watt air frier will draw approximately 178 amps. Your two 100ah Lithiums in parallel should be able to supply 200 amps but check their spec. It depends on the make. Also realise that you will need to use about 25 sq mm cable crimped properly depending on the length of run fused at 200 amps.
So you may be able to cook some chips for about ten minutes each night but not a lot more to leave yourself with lights etc over the four days.
I have 600ah of lithium with a 50a B2B to replenish them quickly. I use a 1000 watt kettle! A 750 watt sandwich toaster and a 900 watt air frier. I wouldn’t want to use the air frier for more than about 15-20 mins. If I were not moving every day as it would take more than a day to recharge them. Nearly 12 hours constant driving infact if I completely drained them. I rarely drive that long so you will need a bigger battery bank or use the gas.
 
TravellingSmurff said:
Ah okay, I've never thought about that. Why would it not and is there a certain type or calculation I need to do?
Click to expand...
Look up the maximum recommended discharge current. For lead-acid batteries it's usually taken to be a fifth of the Ah capacity, so for 200Ah it would be 200/5 = 40A. That is often written as the C5 value.

Lithiums are better usually, with some of them able to manage C2 or even C. And sometimes the limitation is not the battery cells but the BMS. So you need to look it up for your brand of battery.

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TravellingSmurff one other thing to bear in mind is the wattage draw is not constant with an air fryer.
I have a 1400 watt air fryer.
On start up it draws 1435 watts then after about 1 minute, when it has reached temperature, it drops to 35 watts as the heating element switches off and just the fan runs, then after about another 1 minute the heating element comes back on and it draws 1435 watts for one minute and so on.
So if I do chips for 20 minutes it only draws 1435 watts for 10 minutes and 35 watts for 10 minutes so it uses roughly half the rated wattage.
 
funflair said:
:LOL: you replied too quickly before I deleted my post, I have a power monitor on the van inverter and it shows the air fryer cycling but yes they do pull a fair amount of power but we don't run ours for that long, but we have 640ah and 520 watts of solar ;) leave it for more than 4 minutes and you should see it cycle more.
Click to expand...
:) Sorry was too keen. That's what I am slowly thinking, I thought 200Ah of lithium might have been plenty but I am thinking if I want to go no-gas, I might have to upgrade. I do have 400W of solar and it keeps the batteries topped up. But wanted to calculate based on the UK average day :) Thanks
 
OldAgeTravellers said:
As a very rough rule of thumb I divide the wattage by10 to give the current draw allowing for losses. So your 1780 watt air frier will draw approximately 178 amps. Your two 100ah Lithiums in parallel should be able to supply 200 amps but check their spec. It depends on the make. Also realise that you will need to use about 25 sq mm cable crimped properly depending on the length of run fused at 200 amps.
So you may be able to cook some chips for about ten minutes each night but not a lot more to leave yourself with lights etc over the four days.
I have 600ah of lithium with a 50a B2B to replenish them quickly. I use a 1000 watt kettle! A 750 watt sandwich toaster and a 900 watt air frier. I wouldn’t want to use the air frier for more than about 15-20 mins. If I were not moving every day as it would take more than a day to recharge them. Nearly 12 hours constant driving infact if I completely drained them. I rarely drive that long so you will need a bigger battery bank or use the gas.
Click to expand...
This is brilliant, thank you. Once I have this running, I will start a thread on real-life examples so people can see actual data.
If the wife wanted to cook a chicken in the air fryer, that would take approximately 45 minutes in our current one at home. If we got your lower 900W one, my assumption is it would take longer and, therefore, need more time. This makes me feel like we need a higher wattage for a shorter time or a lower wattage for a longer time. Either way, this results in more battery capacity. Thanks again
 
headlight said:
TravellingSmurff one other thing to bear in mind is the wattage draw is not constant with an air fryer.
I have a 1400 watt air fryer.
On start up it draws 1435 watts then after about 1 minute, when it has reached temperature, it drops to 35 watts as the heating element switches off and just the fan runs, then after about another 1 minute the heating element comes back on and it draws 1435 watts for one minute and so on.
So if I do chips for 20 minutes it only draws 1435 watts for 10 minutes and 35 watts for 10 minutes so it uses roughly half the rated wattage.
Click to expand...
Brilliant, thanks for this. My post here above didn't show that; however, I only ran it for 4 minutes. It's the Ninja large, two draw one. Which one do you have please? Thank you
 
No matter how much battery and solar you get, cooking with electric by any method (air fryer, microwave, combi-oven) will always be a huge drain on your reserves. Our combined microwave/grill/convection oven is really efficient but it still draws on average about 100A (bit more on start up, bit less as it cycles). We have 680Ah of battery and 520w of solar but If we are off grid for more than a night or two, I always try to cook with gas if possible, or do something that can be microwaved in less than 10 minutes.

You cannot substitute the amount of energy in a litre of gas without an enormous battery bank. One reason why I would never go without LPG...it ticks every box in my view.

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You said you want some numbers. A 600Ah battery bank is usually thought of as quite large. It contains 600Ah x 12V = 7200Wh of energy. That's 7.2 kWh of energy.

One litre of propane can give you 7.08 kWh of energy, which is about the same as a 600Ah battery bank. So you can see that two 22 litre LPG cylinders is going to give you enormously more energy than a 600Ah battery bank.

As for solar, 1000W of panels would be thought of as quite a large array for a motorhome. It would give about 400Ah per day in summer, which is about 5kWh. For the usual electrics usage (lights, TV, microwave, coffee machine etc) that's plenty. But for serious heating, hot water and most cooking it wouldn't go far.
 
It’s all about sensible power management IMHO. I’m just installing an upgrade to my inverter and I’ve actually gone for ‘only’ a 1200W one. My thinking being, the bigger the inverter, the easier it is to be lazy about not finding lower powered appliances.

Ive got 500A of lithium and combined with roof (600W) and separate rigid panels 920W of solar in total and 80A dc:dc charger.

All we run is: air fryer 900w, the induction hob 1kw max, kettle 800w, microwave 800w power (actually needs 1kw to run), charging two 600wh Ebike batteries and a 4K TV! Everything else runs direct off the 13v batteries. There is quite a lot of power used, with Starlink, electric hot water (heated with excess solar), beer fridge and the usual battery charging (laptops, kindles, phones, vape batteries etc), fans, lighting etc etc etc

With judicious use of power, especially when climatic conditions are not favourable, we don’t need to plug into the mains. We do spend 9 months in Europe, but the angle of the sun is still very low in Jan and Feb so we may use a few non solar generate KWh then. We also don’t cook so much with battery power then either.
 
Spriddler said:
What a fascinating thread.
I had no idea that so much physics and maths expertise was required in order to fry a bit of chicken.
;)
Click to expand...

There is very little maths needed to fry the first piece of chicken. This is all about if you can repeat the chicken frying exercise! 😉🤪
 

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