How do you power large resistive loads on a small inverter at reduced power?

Thread Starter

LMF5000

Joined Oct 25, 2017
87
Say you have a 500W pure sine-wave inverter and you want to power a 1500W sandwich toaster or a 3000W tea kettle during a power cut. These resistive devices are only that powerful for a rapid warm-up time - once at temp the toaster cycles on and off with a brief duty cycle, and the kettle only runs for a couple of minutes. The batteries powering the inverter have more than enough energy to make a sandwich or a cup of tea, but not enough power to run the devices at rated power (mostly because it's a 12V system: 3000W @ 12V = 250A. Not feasable!)

I'm looking for a solution for running these devices at 500W so the inverter would be able to run them in a pinch. So basically the toaster takes 3x longer to heat up (and thermostat duty-cycle becomes 3x longer to maintain temperature), and the kettle takes 6x longer to boil water.

I've thought of a thyristor dimmer, but I very much doubt this would play nice with the inverter because of the way it chops up the wave and feeds full power to the load in partial, 50Hz bursts.

A variac would work, but too bulky and expensive.

Buying dedicated low-power "camping" appliances would be a neat solution, but defeats the purposes of this question

I imagine a resistor in series would dissipate too much power to be feasable.

I've had great success using a 10uF 400V capacitor in series to limit the power to a 50W stand-fan (reducing it to 37W), however I don't know whether the idea would scale up to loads with thousands of watts.
 

dl324

Joined Mar 30, 2015
11,485
Say you have a 500W pure sine-wave inverter and you want to power a 1500W sandwich toaster or a 3000W tea kettle during a power cut. [snip] So the batteries powering the inverter have more than enough energy to make a sandwich or a cup of tea, but not enough power to run the devices at rated power.
You can't; an overload should make it shutdown.

That's some electric tea kettle...
 

OBW0549

Joined Mar 2, 2015
3,545
The problem:
Say you have a 500W pure sine-wave inverter and you want to power a 1500W sandwich toaster or a 3000W tea kettle during a power cut.
The solution: throw out the 500W inverter and buy a 3000W inverter.

Your rationalizations amount to what I call "design by wishful thinking." It doesn't work.

Your 500W inverter doesn't give a fat rat's derriere what the overall, long-term average power consumption of your sandwich toaster or tea kettle is; the instant it sees that 1500W or 3000W load it will either promptly shut down (if it's got automatic overload protection) or begin the process of overheating and failing-- which will likely take but a few seconds to a minute at most.
 

LowQCab

Joined Nov 6, 2012
55
Instead of using an Inverter, just put 4 Batteries in Series and run them on DC.
Unfortunately, the cheap mechanical Thermostats won't last very long switching DC.
Better yet, start figuring out how to get rid of your bad eating habits so you won't need these appliances.
 

Papabravo

Joined Feb 24, 2006
14,397
A resistor in series. That would be like saying: "I'm going to throw away half the output of my inverter as wasted heat so that what little is left can be fed to a device that wants 12 times that amount rather than 6 times that amount. IMHO, that would be a sub-optimal choice. Buy the inverter you need, or do without. Those be your choices.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
You can't; an overload should make it shutdown.

That's some electric tea kettle...
My country has a UK style grid. Every outlet in the house is 230V 50Hz 13A single-phase. Kettle uses the full supply.

I'm assuming you're on the US system with a 110V split phase supply if you think 3kW is unusual for an appliance?
 

Ian0

Joined Aug 7, 2020
452
Put the kettle in series with the toaster! Assuming it's only a 2kW kettle, you're down to 850W.
Try a 1.5kW kettle and two toasters, then you're spot on.
You're forgetting one thing - heat loss. The amount depends on excess temperature, so you might end up with tepid tea and warm bread no matter how long you wait.
 

dl324

Joined Mar 30, 2015
11,485
I'm assuming you're on the US system with a 110V split phase supply if you think 3kW is unusual for an appliance?
For a tea kettle. The device my Wife uses to heat water is rated at 1500W. Our coffee pot is about 1000W. I think the circuits for my oven and stove are 20A - so 4800W.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
For a tea kettle. The device my Wife uses to heat water is rated at 1500W. Our coffee pot is about 1000W. I think the circuits for my oven and stove are 20A - so 4800W.
Well yeah, never been to the US but I gather you put high power appliances on a 240V circuit and everything else on 120V? We put everything on 240V through wall plugs. Stoves and large air conditioners get wired directly to the circuit breaker through a cooker switch (no plug). In my house the stove/oven is gas powered. Since we only use gas for cooking (no central boiler systems), we don't have a continuous gas supply with pipelines like in the UK - we use portable 18kg cylinders of LPG. A cylinder lasts about 2 months. When it runs out the gas truck passes by your house and you buy a filled cylinder. If you return the empty cylinder and buy a filled cylinder you pay only for the price of the gas, not the cylinder as well.

Here's a typical European 3kW tea kettle: https://www.amazon.com/Braun-Multiquick-3000-watt-1-7-Liter-220-volt/dp/B005013GUK?ref_=d6k_applink_bb_marketplace
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
So, just a general comment - so far the only response that seems to understand the intention has been Ian.

Let me try and clarify - I'm not expecting to get 1500W out of a 500W inverter. That would be silly.

I'm trying to limit the power flow to a 1500W toaster to just 500W so I can run the toaster on the inverter with reduced power. Since this is a purely resistive load that merely turns electricity into heat, it should be quite straightforward. At least much easier than trying to do it with say a motor or a microwave.

To be exact, this is a sandwich toaster (aka pannini press, aka George foreman grill). It turns on until it reaches about 220°C, then the thermostat cycles it on and off to maintain about that temperature. The duty cycle is very low, after initial warmup the heating element is on for less than 30% of the time. So 500W should be adequate just for maintaining temperature. Obviously the warm-up period will be at least 3x longer, but the idea is to use existing equipment, not invest hundreds in a multi-kilowatt inverter and a 24V or 48V battery pack to run it .
 
Could try one of those chinsey boost converters off of Amazon. Take 12V in and kick out 24 or 36VDC. Mind you many of those things are so poorly designed they should probably be illegal to sell. Big fire hazards with no overcurrent protection to speak of. Otherwise you can find surplus heating elements for dirt cheap if you know where to look. Doesn't take anything too fancy if you just need to heat water. Hell, you could damn near use a stainless steel pot as your heating element if you could get the voltage closer to 2 or 3VDC.

By the way, what kind of storage battery are we even talking about here? You're not trying to pull half a kilowatt off of a car battery are you?

Bottom line: If you have a legitimate need to cook food during a power outage, put those propane tanks you mentioned to good use. Combustion heating is far cheaper and more eco-friendly than electric heat anyhow. You can toast bread on a pan and boil water in a pot.

Just my 2¢.
 

WBahn

Joined Mar 31, 2012
26,143
Say you have a 500W pure sine-wave inverter and you want to power a 1500W sandwich toaster <snip>

I'm looking for a solution for running these devices at 500W so the inverter would be able to run them in a pinch. So basically the toaster takes 3x longer to heat up
That's wishful thinking. By this reasoning, if we had 15 W available we could heat it up in 100x the time. Of course, what will happen is that it will barely warm up and then stay barely warmed up for the entire time.

If want to power a 1500 W device from an inverted, get an inverter rated at 1500 W.
 

djsfantasi

Joined Apr 11, 2010
6,795
Well yeah, never been to the US but I gather you put high power appliances on a 240V circuit and everything else on 120V?
If by high power appliances, you mean stove and clothes dryer, then yes.

Those two appliances are typically all that run on 240VAC. Everything else runs on 120VAC.

If you have gas, then it’s possible that nothing in the house runs on 240.

However, 240 is run to every house, as two 120VAC circuits with a common neutral. The “outer” connections form a 240VAC circuit for the appliances. And two 120VAC circuits, wired through the remainder of the house for everything else.
 
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crutschow

Joined Mar 14, 2008
25,395
I think a Variac is the only reasonable way to lower the voltage (and power).
I understand the concern about cost (try flea-bay), buy why the concern about it being too "bulky"?

But 500W into a 1500W toaster may never get it hot enough to do more than dry out the bread rather than toast it.
And I've never seen a toaster that "cycles".
 

Thread Starter

LMF5000

Joined Oct 25, 2017
87
I think a Variac is the only reasonable way to lower the voltage (and power).
I understand the concern about cost (try flea-bay), buy why the concern about it being too "bulky"?

But 500W into a 1500W toaster may never get it hot enough to do more than dry out the bread rather than toast it.
And I've never seen a toaster that "cycles".
I clarified that point - it's a sandwich toaster, not the kind with a wire element. I measured the duty cycle today, it's 40 seconds on, 3 minutes off. 22% x 1500W = 330W average. So no problem with power.

Why bulky? The variacs I've seen have typically been the size, weight and cost of the typical inverter. If I'm going down that route I'd just buy another inverter ;)
 
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