Inverter Output Voltage

This one can be set to 100, 110 or 120 volts…

Dear Mike,

Something I have wondered about are the voltage settings many RV users of inverters have to select our voltage outputs. I can select 120V, 110V and (100V as Japan)

The inverter frequency setting, I realize needs to be 60 Hertz. Is there any advantage to setting an inverter to 110V, as some solar generator’s output, over a setting of 120V? I know neither 110V or 120V will damage any of my electrical equipment, I am seeking longest battery life from my battery bank using both true power devices consuming pure watts and reactive power devices using volt-amps-reactive (VAR). My heating pad might be imperceptibly less warm all night, yet will my coffee made in the microwave oven for two minutes in the morning also be less warm using 110V rather than 120V?

Will I get a few more minutes of battery life if setting my inverter to 110V rather than 120V? Is there a good reason for a solar generator to output 110V rather than 120V? Or maybe just simplifying the stepping up of lithium battery DC voltage to AC voltage with no apparent benefit in how long the solar generator will power anything? - James

Hey James,

Good question. As you alluded to, your resistive heating devices will produce less heat at 110 volts compared to 120 volts. How much less? Let’s take a quick look at Ohm’s Law.

We can see that voltage (E) squared divided by watts (P) equals ohms (R). So for example, a 120-volt 1,200-watt heater has a resistance of 120 x 120 / 1,200 = 12 ohms. To calculated wattage of a 12 ohm resistance load at 110 volts we look on the chart and see that volts (E) squared divided by ohms (R) works out to 110 x 110 = 12,100. And 12,100 divided by 12 ohms = 1,008 watts. So a 1,200-watt resistive appliance rated at 120 volts will only produce 1,008 watts of heat at 110 volts. That’s 84% of the rated heat, which is a heating loss of 16%. That suggests a space heater at 110 volts would run 16% longer on a battery compared to 120 volts, but be producing 16% less heat while doing it.

As you also noted, some appliances may not get as warm, so your electric blanket won’t be a toasty, and your coffee maker may not be as hot. But many of these appliance also include a thermostat that will keep the power on longer until it reaches the proper temperature. So the kWh usage (battery power needed) will be about the same.

What about universal power supplies?

All those modern wall warts and computer power supplies are rated for incoming AC power from 90 to 250 volts (or so). And that means the PWM (Pulse Width Modulated) Power Supply will vary its duty cycle to make the correct output voltage. So they will draw more amperage at 100 or 110 volts compared to 120 or 220 volts. So kWh use from the battery should be about the same.

Let’s not make readers crazy with Reactive Power or Power Factor!

I’m afraid that discussion is beyond the scope of this newsletter, but suffice to say that any differences won’t be of any consequence at 110 volts compared to 120 volts.

So what should you set your inverter AC voltage to?

I would set it to 120 volts since that would actually minimize AC current draw from PWM power supplies, and current draw is what causes voltage drop which causes heating of the conductors. Now, this is going to be a really small difference, probably too little to worry about. But my gut feel is to always set inverters to 120 volts when possible since most appliances are designed for that voltage and will be happier (as it were).

Let’s play safe out there… Mike