How much inverter capacity do I have?

Here’s a puzzle for those of you who are electrically minded. Others will want to ignore today’s entry and wait for tomorrows blog.

 

My boat has an extreme amount of inverter capacity: 14kw. There aren’t a lot of boats our size, if any, with this much capacity.

 

It’s a long story why I have so much inverter capacity, and I’ve gone through it before, so I won’t bore you. The one sentence description is that I wanted to be able to run high amperage devices, such as the stove, microwave oven, coffee machine, without having to worry about firing the generator.

 

Recently, I was discussing some electrical work on the boat with an electrician, and had a strange conversation. I mentioned that I had 14kw of capacity, and he corrected me. He said that I only have 7kw. I asked what he was basing this opinion on. “You have four 3.5kw inverters, but they are only 120 volt inverters. Therefore, you have to cut the wattage in half.” I pushed him on the topic, and he held firm. Below is the email I sent:

 

 

Mr. Jones (not his real name)

 

I do think I was right that I have 14kw of inverters, at 240v.

 

I was confused briefly today, when you said that I have four 3.5kw inverters, that are 120v inverters.

 

Four 3.5kw 120v inverters do, in fact (I think) add up to 14kw at 240v.

 

Here’s why…

 

We need to look at amps to understand what is happening.

 

The formula for computing kw is:

 

Watts = volts * amps

 

So…

 

Each inverter is:

 

3500 watts (3.5kw) = 120 volts * ??? amps

 

Solving for amps, we get: 29 amps.

 

In other words, each of my four inverters is capable of producing 29 (120 volt) amps, or a total of 116 amps (at 120 volts).

 

To convert 156 amps at 120v to 240v amps, we divide by 2. In other words, we really only have 58 amps, at 240 volts.

 

Applying the formula again, we find that 58 amps at 240 volts is:

 

Watts = 240 volts * 58 amps

 

Watts = 13,920 – or, roughly 14kw.

 

So .. I have 14kw, at 240 volts.

 

Do you agree, or am I totally off the mark? I’m pretty sure I’m right, in that I know what loads I have transferred to the inverter, and if I only had half what I think I have, things wouldn’t have worked.

 

-Ken Williams

 

 

His response:

 

 

 

Ken,

You have two sets of inverters each set is in series and the two sets are in parallel.

So let’s forget amps for now.

 

You have one set of inverters that are wired 120/240, so the output at 240 is only 3.5KW, the second set is paralleled to the first set, there again its only 3.5KW at 240 so two sets equal 7KW at 240. Now for the 120 side you have two 120 busses, each buss is getting 7KW of power, you can say you get a total of 14KW at 120 volts, however its broken up into two 7KW loads.

 

If the inverters were wired all in parallel for only 120 volts you would indeed have 14KW of inverters. However they are not all in parallel so at 240 its only 7KW which is 29 amps per lag. 7000 divided by 240 not 120.

 

Hope this helps

Later

 

 

 

I stubbornly wrote back:

 

 

 

Mr. Jones:

 

I never give up easily.

 

Let’s simplify this by focusing on just two inverters. Let’s pretend I only had TWO inverters, and look at what I’d have with them in serial configuration, then in parallel.

 

SERIAL

 

Two 3.5kw 120v inverters…

 

My belief is that when you connect two power sources in serial, you add the voltage, but keep the current the same.

 

In other words:

 

We have:

 

120v at 29 amps = 3,500 watts

PLUS 120v at 29 amps = 3,500 watts

 

Giving us: 240v at 29 amps = 7,000 watts ( Remember the formula – volts * amps = power)

 

PARALLEL

 

Two 3.5kw 120v inverters…

 

My belief is that when you connect two power sources in parallel, you keep the voltage the same, but add to the current.

 

In other words:

 

We have:

 

120v at 29 amps = 3,500 watts

Paralleled with 120v at 29 amps = 3,500 watts

 

Giving us: 120v at 58 amps = 7,000 watts ( Remember the formula – volts * amps = power)

 

Do you agree? If you agree with the above .. then I have 14kw at 240v. But, let’s see if you agree with this first.

 

-Ken W

 

 

 

This email elicited a phone call, basically saying I was way off base, but not explaining why.

 

I spoke with another Nordhavn owner, who agreed with Mr. Jones. I then raised this as a topic in my Advanced Electricity course a couple weeks ago. The teacher put it on the chalk board, and his first reaction was to agree with Mr. Jones. Given the number of smart people who believe I am wrong on this topic, I’m willing to say I am probably wrong, but I don’t know when to quit, so I pushed the topic and involved the entire class. After 10 minutes of debate, the instructor and I had lost the class and neither of us was backing down. Finally he said “Let’s take this after class.” I could tell I was shaking his confidence in his (and Mr. Jones’) position, and wasn’t surprised the topic never came back up.

 

Or… perhaps I am wrong. It wouldn’t be the first time…

 

-Ken W

8 Responses

  1. I read the inverter power discussion with some amusement. If you have 14kW, you have 14kW. Period. It makes no sense to say you have X kW at some voltage. You have 14kW, and have half the current available at 240 as you do at 120. Same power though. No difference. Ken, you’re right.

    Scott Adams, Ph.D. EE

  2. In answer to Rick question.

    Two inverter are paralleled to give 120 volts at 7kw, the voltage appears across say line 1 and neutral.

    The second pair are similarly paralled to gives 120 volts at 7kw across say line 2 and neutral. If you connect the two neutrals together the voltage difference between line 1 and line 2 is 240 volts. The 2 inverter pairs appear to be in series.

    If you put a 240 volt 14kw load across these lines, at a given instance, 58.3 amps will flow out of inverter pair 1 and INTO inverter pair 2 thus completing the circuit.

    Remember this is alternating voltage, when inverter pair 1 is at +120 volts inverter pair 2 is -120 volts.

  3. Ken,

    If Mr. Jones is correct and if I’m understanding his answer, what is the point in connecting inverters in series? The second inverter doesn’t add either voltage or amperage. If that’s right, then you could have accomplished the same output with two inverters instead of four. Which doesn’t make sense either.

    I’m eager to learn the answer to this question.

    Rick Heiniger
    N7617 Eliana

  4. I agree with you and JohnA. Two paralleled inverters on a leg give (2 x 3.5kw)/120v = 58.3amps. The other two paralleled inverters form the second leg, assuming the neutrals are tied together.

    With a 240 volt load across both legs, 58.3 amps can flow out of one leg into the other leg. Net effect is 240v x 58.3amp = 14kw.

  5. Ken, I checked the wiring diagrams at the Outback Power website. Looks like your electrician is right on one count, in that you have 2 pairs of series wired inverters. One pair being L1, the other pair being L2. Between L1 and L2 you have 240 VAC output (14KW). Between L1 and neutral or L2 and neutral you have 120 VAC (each 7KW).

    The electricians fatal mistake is to halve the power output as you correctly pointed out.

  6. Ken —

    You are correct. Power is power. Power = Current * Voltage (P=I*V)

    Whether you have inverters in series or parallel, you sum the power.

    In series, you would double the voltage, keeping current the same.
    In parallel, you would double the current, keeping voltage the same.

    I’d look for a new electrician.

    -Bill

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