Appliance Power Draw Calculation Assistance

[jorgey]

I would like some assistance or confirmation that I am correctly calculating the draw on an appliance (CPAP) that I would like to take camping. I would like to find out if this is correct to determine if it is even feasible to worry about trying to do or not.

I think that I have it figured out and then I read some more and get myself confused.

So, here it is. From the certification sticker on the back of the unit, it shows that it uses 100-115V~ 1.5A (1.7A max). (It is an AC only unit, but suitable for use with an inverter 115v - 200W / 300W surge.)

So if I understand this correctly, using the maximum draw, 115VAC X 1.7Amps AC = 195.5W, then converting it over to DC, take the 195.5W / 12V = 16.29Amps DC. Is this correct? So this device would use 16.3 Ah for each hour of use, correct? Thats a lot of power.

I know that there will be an additional draw that needs to be added for the inefficiency of the inverter as well, but not as concerned about that just yet.

I have done a little testing with a watt meter and found that in real life the unit is drawing less than 50W AC with the heater and humidifier off (which they both always are), but wanted to go with the sticker values for the extreme case.

Thanks in advance for your help and advice.

[SunEagle]

I would like some assistance or confirmation that I am correctly calculating the draw on an appliance (CPAP) that I would like to take camping. I would like to find out if this is correct to determine if it is even feasible to worry about trying to do or not.

I think that I have it figured out and then I read some more and get myself confused.

So, here it is. From the certification sticker on the back of the unit, it shows that it uses 100-115V~ 1.5A (1.7A max). (It is an AC only unit, but suitable for use with an inverter 115v - 200W / 300W surge.)

So if I understand this correctly, using the maximum draw, 115VAC X 1.7Amps AC = 195.5W, then converting it over to DC, take the 195.5W / 12V = 16.29Amps DC. Is this correct? So this device would use 16.3 Ah for each hour of use, correct? Thats a lot of power.

I know that there will be an additional draw that needs to be added for the inefficiency of the inverter as well, but not as concerned about that just yet.

I have done a little testing with a watt meter and found that in real life the unit is drawing less than 50W AC with the heater and humidifier off (which they both always are), but wanted to go with the sticker values for the extreme case.

Thanks in advance for your help and advice.

I have a CPAP machine and have used something called a "kill a watt" meter to measure the electrical usage. That meter gave me pretty good data on the power it uses.

First off it is a Phillips Respironics model. The power supply is rated 100 - 240v at 2.1 amps with an output of 12v DC 5 amps.

So with it just plugged in the electronics draws about 2 watts.

Running it just using the blower is about 10 watts (115v ac, ~ 0.14 amps @ .5 PF) but only ~ 45 watt hours in an 8 hour period

Running it with the humidifier (level #1) and blower is about 30 watts (115v ac, ~ 0.5 amps @ .5 PF) and about 120 watt hours in an 8 hour period.

So even though the power supply has a 2.1 amp rating for the AC side it does not draw anywhere close to that amount on the settings I use.

My back up power for it is a 65Ah 12v battery and 300 watt inverter which has worked but thank god I did not need to use it more than an hour when the power went out one night.

[ewarnerusa]

OP,
I would say your back of the envelope calculations are accurate. I think it's likely that the max AC rating is for when the humidifier and heater are in use, and your actual measurements of how it performs with those features off shows how much electricity it takes to heat things. I read that you said the CPAP is AC only, and I don't know anything about using a CPAP, but I regularly hear them described as operating like SunEagle describes by using an AC to DC converter power supply. So folks just get a 12V power cord to replace the AC power supply and run it straight from a camper's 12V socket rather than through an inverter, saving them from the inefficiencies of inverting DC to AC and then converting back to DC or having to install an inverter if they don't already have one.

My rule of thumb is to divide wattage by 10 for a rough estimate of DC amps required by an inverter. Or take the AC rated amps and multiply by 10 for approximate DC amp draw by the inverter.

[jorgey]

I have a CPAP machine and have used something called a "kill a watt" meter to measure the electrical usage. That meter gave me pretty good data on the power it uses.

First off it is a Phillips Respironics model. The power supply is rated 100 - 240v at 2.1 amps with an output of 12v DC 5 amps.

So with it just plugged in the electronics draws about 2 watts.

Running it just using the blower is about 10 watts (115v ac, ~ 0.14 amps @ .5 PF) but only ~ 45 watt hours in an 8 hour period

Running it with the humidifier (level #1) and blower is about 30 watts (115v ac, ~ 0.5 amps @ .5 PF) and about 120 watt hours in an 8 hour period.

So even though the power supply has a 2.1 amp rating for the AC side it does not draw anywhere close to that amount on the settings I use.

My back up power for it is a 65Ah 12v battery and 300 watt inverter which has worked but thank god I did not need to use it more than an hour when the power went out one night.

Thanks SunEagle,

I do have a Kill-a-Watt, that is how I measured the at go that it was only using 50watts in real life. I did not pay attention to or write down the amps so I'll need to put it on again and get some more accurate readings.

[SunEagle]

Thanks SunEagle,

I do have a Kill-a-Watt, that is how I measured the at go that it was only using 50watts in real life. I did not pay attention to or write down the amps so I'll need to put it on again and get some more accurate readings.

Try using the kill a watt over night or at least as long as you would on most nights. Note the time period and then check the watt hours used. You should find that even though you get 50watts for a brief period it might go down depending on how the blower motor reacts to your breathing.

That is why I was surprised when I first measured 30watts but then only got a total of 120 watt hours in an 8 hour period.

[jorgey]

OP,
I would say your back of the envelope calculations are accurate. I think it's likely that the max AC rating is for when the humidifier and heater are in use, and your actual measurements of how it performs with those features off shows how much electricity it takes to heat things. I read that you said the CPAP is AC only, and I don't know anything about using a CPAP, but I regularly hear them described as operating like SunEagle describes by using an AC to DC converter power supply. So folks just get a 12V power cord to replace the AC power supply and run it straight from a camper's 12V socket rather than through an inverter, saving them from the inefficiencies of inverting DC to AC and then converting back to DC or having to install an inverter if they don't already have one.

My rule of thumb is to divide wattage by 10 for a rough estimate of DC amps required by an inverter. Or take the AC rated amps and multiply by 10 for approximate DC amp draw by the inverter.

Yes, almost all of the newer CPAP machines are AC/DC but this one is a little older and is AC only. It is somewhat of a beast and not that convenient to travel with. Been looking at getting a new one, but with all of the "wonderful" changes the last few years with the health care, the $350-$400 machine will cost me close to $2000 as they will not issue a new prescription without a bunch of new tests and sleep studies, etc and they will not cover them like they did initially. Cost a $40 copay for Dr visit, tests, sleep study, and unit last time. Love how much ObamaCare is saving me.

Thanks for confirming that my original calculations are correct. At at least I know that I am on the right track.

[ewarnerusa]

A 300 watt continuous rating inverter should be more than adequate, I'm not sure I would trust one with a smaller rating. You can probably get away with just plugging that small inverter into the camper's 12V socket if it has one, although a 20A draw is probably more than that outlet is fused for. You could either risk increasing that fuse size, or feel confident that your monitoring of actual power draw means that the inverter will only be pulling 5A DC from the socket. And if you're that confident in your actual power needs for how you use the CPAP, you can get away with an even smaller inverter. A 20A draw may also cause enough of a voltage drop when trying to pull that current through the small DC wiring that your inverter faults out on low input voltage. These are all moot points if you wire your inverter directly to the battery; you just need to get that AC power to the CPAP machine.

I prefer pure sine wave (PSW) inverters, but they cost more. I'm not sure I've seen one smaller than 300 watts, either.

[jorgey]

Try using the kill a watt over night or at least as long as you would on most nights. Note the time period and then check the watt hours used. You should find that even though you get 50watts for a brief period it might go down depending on how the blower motor reacts to your breathing.

That is why I was surprised when I first measured 30watts but then only got a total of 120 watt hours in an 8 hour period.

Great suggestion, I will do that.
Thanks!

[Amy@altE]

Samlex makes a nice little 300W pure sine wave inverter, SSW-350-12A . I'm guessing the CPAC would run on modified, but the pure may allow it to run a bit smoother and cooler.

[bcroe]

Thanks SunEagle,

I do have a Kill-a-Watt, that is how I measured the at go that it was only using 50watts in real life. I did not pay attention to or write
down the amps so I'll need to put it on again and get some more accurate readings.

Besides recording actual energy used over time, the Kill-A-Watt figures the power factor, which pure V and A do not. Bruce Roe