Know Your Current Draw (Amps): Get a Kill-A-Watt

If you are setting up an inverter or a battery bank and wondering how big it needs to be, the first thing you need to know is how much energy your gear will use.  At only $20, a Kill-A-Watt is a great way to measure it.

If you have labels on your gear, they might tell you the wattage, or the maximum current draw and from that, you can calculate the other.  For example, if you have a 120-volt appliance (in the US, that’s all of them except the oven) and the label says “500 W”, you can calculate the draw on your inverter with Ohm’s law, like this: 500W/120= 4.17 Amps (there is one more step to get the draw from the batteries, but we’ll cover that below).  But what if your label is missing, or you have a number of appliances on the same power strip?  The Kill-A-Watt is an easy way for anyone to make these measurements.  No need to be familiar with Ohm’s law, or to have other measurement equipment.

Using the Kill-A-Watt (by the way, a play on words: Kilowatt) I’m going to measure my favorite minivan camper appliance, my immersion heater.  I use this heater to make coffee, oatmeal, and to heat water for dehydrated foods.  First I’ll plop it in some water and then plug it into the Kill-A-Watt.

Displaying Volts

Pushing the button for the Volts display gives me 112.8 volts.  That’s a little lower than the “120-volt” label on the inverter would have you believe, but it’s normal to see some drop under load, due to a number of factors.

Next, I’ll push a button to display wattage.

Displaying Watts

279 Watts is pretty close to the rating on the immersion heater of 300W.  Actually, that rating was at a full 120V so 279 is very close at the 113 volts actual that the inverter is giving.  Inverters are typically rated by wattage, so this is the number you need to size an inverter.  Just add up the wattage of the appliances you’ll want to run at the same time.  In a minivan RV, you are probably only running one at a time, but some of you might want to make coffee and oatmeal at the SAME time (gasp!).

Onto the number that you need to size your battery bank–the current draw or Amperage.

Displaying Current (Amps)

My immersion heater is “drawing” 2.49 Amps…but wait a minute, because this is at 120 Volts (theoretical from inverter).  If you want to know the current draw at 12v, you have to multiply this number by 10.  So, the current draw from the batteries is 24.9 Amps  Assuming 100% efficiency from the inverter (some are pretty good, but none are 100%)  you could run with that number.  But to be safe, I will add 10% to the number and get 27.39 Amps.  So if you run that heater for an hour, you’ve pulled 27.39 Amp/Hours from your battery.  To give you a ballpark of how much power that it, one hour of this heater would use up almost 75% of the capacity of your average car starting battery (40Ah), pretty much draining it.  Of course, you never really run one of these heaters for more than 10 min.  Also, your battery bank is going to use deep cycle batteries of 80-100 Ah capacity, each.

I carry my Kill-A-Watt when I travel because it’s kinda fun (if you a geek like me) to show everyone how much power their plug-in appliances use.

The Kill A Watt is only $20 on Amazon and I use mine all around the house to find phantom drains that run up the electric bill.  If you use my Amazon link, I’d appreciate it because we will get a portion of the sale to keep this site up (but it won’t cost you a penny more).

If you want an immersion heater like the one I use, here’s another Amazon link.  Buy two, in case you damage one.  They are pretty inexpensive.

Remember that when using an immersion heater, make sure it is IN the water before you plug it in and unplug it before you take it out.  Otherwise, the internal element can overheat and fail almost instantly.  It’s why some people complain that theirs failed, but mine has lasted five years.

Now I know you can make these measurements with a multimeter, but the built-in plug on the Kill-A-Watt makes it easier for the average person to figure out how much inverter and battery they will need.  It should be the starting point for a discussion of either.

Note that I’ve used words instead of the common abbreviations of V (volts), A (Amperes or Amps for short) and W (for Watts), to make this easier for the layperson to understand.  In some places, I’ve interchanged them to further understanding (I hope).  Please let me know if you have any other suggestions at

Disclaimer–Mainly because there are VOLUMES of information that could be added, I expect to hear a lot of “yes, but”.  I welcome that, but may leave it out for brevity.  However, if I goofed a calculation, PLEASE let me know.  Thanks!

–Odyssey Camper

You can find all of my technical and minivan camper conversion DIYs, linked here.

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