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Watts are Watts but not all Amps are the same!

I'll be honest I've been sailing on boats since 2004 and have lived off grid for 8+ years in Hawaii but it wasn't until a few years ago that understanding power really clicked for me. We answer a lot of questions on the phone and via email and hopefully I can explain what we're talking about and get power to 'click' for you also.

Watts are Watts but not all Amps are the same!

You can have 100Watts and it will always be 100Watts, but if you have 10Amps is that at 110Volts (shore power) or at 12Volts (your battery bank)? It's important to know because there is a big difference, for example:

10Amps at 110Volts is 1100Watts

10Amps at 12Volts is only 120Watts

You can see the difference. I think this magic triangle is a good place to start trying to explain it further as it shows the relationship between Watts, Amps and Volts.

Watts = Amps x Volts

Amps = Watts / Volts

Volts = Watts / Amps

On small off grid projects, which is what essentially most boat and RV solar installs are, we tend to get used to talking about Amps and Amp hours because that's what the batteries talk about. For example I have 4x100Amp hour 12V batteries. Most boaters and RVers know what their battery bank capacity is in Amp hours. They also tend to know what they use (consume) each day in Amp hours.

Unfortunately this got me into the bad habit of thinking all Amps are the same. I would look at all the items we were powering at the off grid house, checking out their labels to see how many amps we were using in a day. It came out at a pretty low number and I thought we were doing really well, but I was forgetting that's Amps at shore power voltages (or 110V) with it having gone through our inverter.

So if for example you are looking at the sticker on the back of your TV and you see that it says it's 120V and 145W we can do some quick math (ignore the 60Hz not too important right now!)

So we know that Watts / Volts = Amps so in this example 145W / 120V = 1.2A

Sounds pretty good right, wow my TV is only pulling 1.2A...awesome I can watch that thing all night! sounds too good to be true, well it is. That's 1.2A at 120V. We are more concerned with what that TV is pulling from our battery bank via the inverter:

So again Watts / Volts = Amps so from the battery bank it's 145W / 12V = 12A

Or another way is to times whatever the amps are at 120V by 10 ie 1.2A x 10 = 12A

Either way you are going to get to a much bigger number than you first thought and soon realize you're using much more power than you first thought! If I watch TV for 3 hours every evening that's 3hr x 12A = 36Amp hours

If I have a 400Ah battery bank of lead acid batteries I should really only discharge them to 50% max. So now my 'working' capacity is only 200Ah. So my 3 hours of TV using 36Ah is like using 18% of my working capacity. Add in some fridge, lights, laptop, stereo and a few cell phones and it all starts to add up.

It's a long subject which can head off on some tangents so I will leave it there. Hopefully you can now see how not all amps are the same. Have any questions? Head to the contact us page to see how to get in touch.

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