Power inverter: how large is large enough?

Today I had the chance to answer a question on an Italian sailing forum from someone who was asking whether a power inverter he wanted to buy for his boat was “large enough” and how long would his batteries last if he would power this and that appliance. He was pleased with the answer and I thought this would make a nice blog article (yet another technical one!) so here it is.

UPDATE: online tool to size your power inverter available here.

Suppose we want to buy an inverter to power a 1000W AC (Alternate Current) onboard appliance. Its voltage doesn’t matter for what follows, it could be 110V, 220V or 240V. All we need to know is that our appliance is rated at 1000W.

Good appliances, bad appliances

First thing first. How large should our inverter be? Is, say, 1500W good enough to power our 1000W appliance? That would give a 50% safety margin, right? The short answer is, as always, it depends. It depends on the type of inverter, its efficiency to mention one aspect, but mostly on the kind of appliance we want to run through it. Bear with me because this is important.

When it comes to inverters, some appliances are bad while others are good. Bad appliances are most of those that have a motor or compressor like Air Conditioning units (oh yes!), refrigerators, freezers, washers, dryers, dishwashers, vacuum cleaners, fans etc. Good appliances are those that produce light or heat like oven, light bulbs, iron, electric heaters and also almost all power supplies and chargers to modern TV’s, laptops, tablets, phones, etc.

Note for the electrical savvy: Bad appliances are inductive or capacitive loads, good appliances are resistive loads.

Now, the rule of thumb is that if you want to power a bad appliance through your inverter you need to over-dimension it from 3 to 7 times the nominal rating of the appliance. This means that if your A/C unit is rated at 1000W (medium unit), you need something like a monster 5,000W inverter to run it! And this without considering how much battery capacity you need to keep it running until the whole boat reaches the temperature of a chilled beer. More about this below (and I am not referring to the beer!) No surprise that all the boats having an A/C unit also have an on-board genset or run their unit only while connected to shore power.

A good appliance

A good appliance (depending on the season!)

For good appliances on the other hand, you can almost select an inverter with the same nominal rating as your appliance. That is a 1000W inverter to power a 1000W appliance, even though you should always consider some safety margin. Let’s assume we are going for a 1500W inverter so our beloved ones can also plug in their tablets to recharge.

Pure sine wave vs. Modified sine wive

So we are browsing an internet site which lists a lot of different inverters. We know that we need one rated at 1500W continuous power. We find various alternatives, but some are labeled as generating a “Pure sine wave” while others are labeled as producing a “Modified sine wave”. Which one to pick?


Source: arigopower.com

Short answer. When the budget allows, always chose a “Pure sine wave”. This is the closest to what comes out of our house’s outlets, which is a pure sinusoidal voltage wave. A “Modified sine wave” in the other hand resembles more a squarish wave, it is not a pure sinusoid. While all appliances like pure sine waves (they are designed for it), some appliances may not like squarish waves and even refuse to start or fail if powered with such a voltage shape. Sure, they may work fine, but there is no guarantee upfront. Better to play it safe and, when possible, select a pure sine wave inverter.

Warning: batteries under heavy load!

I know. The title gives it away. Inverters are known to put an heavy load on our batteries. How much? A lot! Curious to know the details? Keep reading.

So we have chosen our 1500W inverter with Pure sine wave output and we set out to calculate how much current it will draw from our batteries. Why do we need to know? Two main reasons: 1) we need to dimension correctly the cables connecting the inverter to the batteries so we don’t fry the whole boat and 2) we need to understand for how long we can run our appliance through the inverter before having to recharge the batteries. Makes sense?

The math is really easy.

If we have a 12V battery bank, powering a 1000W appliance through our inverter will draw:

1000W / 12V / 0.90 = 93A

If, on the other hands, we have a 24V battery bank, powering the same appliance will draw:

1000W / 24V / 0.90 = 47A

Now we understand why large boats with a lot of toys (like Kismet!) usually have a 24V house battery bank.

The 0.90 that appears in the calculations above is the efficiency of our inverter. 0.90 stands for 90% efficiency, which means it can turn 90% of your DC power into AC power. Some inverters are rated at lower efficiency (like 80%), some claim to have a 95% efficiency or more. Read the specs to find out. No inverter has 100% efficiency though. If you bought one because you were told so, you should stop reading now and go ask your money back! 🙂

Bottom line is that powering our reasonable 1000W appliance from the inverter is going to draw an astonishing 93A from our 12V battery bank. This is a LOT of current believe me. With this current, connecting the inverter to the batteries is going to require a 35mm² cable (2 AWG), which is a pretty large one if you know what I mean.

Note for the electrical savvy: the cable dimension above was calculated for a maximum 3% voltage drop on a total cable run of 3 meters from the batteries to the inverter (1.5 meters each way). For my American readers out there, sorry for using the metric system. I am from the old continent, enough said!

We should also consider that with all that current the fuse/breaker that we need to install to protect the system is also going to be a large one. Everything goes hand in hand when large currents are involved.

How long is it going to run?

Ok, let’s assume we have our inverter installed and connected with proper size cables to the batteries. Fuse and everything else is in place as well.

Now, we are at the anchor in a beautiful bay, a well deserved cocktail in our hands. We power on our inverter, plug-in our appliance and start enjoying it (whatever it is, I am not going to judge…) How long is it going to run if our batteries are not being charged at all in the mean time?

Here is the math. Let’s assume our battery bank has a total capacity of 200Ah, is fully charged and efficient (shiny new!), and no other load is on it beside the inverter with our appliance connected to it.

200Ah / 93A = 2.15 hours = 2 hours 9 minutes

Yes, that’s it. Only 2 hours and few minutes of enjoyment and then either we turn on the engine to recharge or go to bed in full dark…

But wait, it is even worse than that. Deep-cycle batteries, which is what a house bank should consist of, do not like to be discharged at more than 80% of their nominal capacity. This is called maximum Depth of Discharge or DoD. If we exceed the 80% DoD the batteries will last much less than expected and die prematurely on us. This means we should actually calculate the run time of our appliance like this:

200Ah * 0.80 / 93A = 1.72 hours = 1 hour 43 minutes

where 0.80 is the 80% DoD.

That’s not too bad though!

Well, you could say, 1 hour and 43 minutes may be more than enough to enjoy my toy… I have bad news and good news for you. Bad news first. There is another factor we did not consider yet. Bear with me once again.

There is something else that deep-cycle batteries do not like beside being discharged beyond 80% of their capacity. If we want them to stay healthy for a long time we should not draw from them in a continuous manner a current which is higher than 1/8 of their nominal capacity. This maximum current rate is known as C/8, where C is the nominal capacity of the battery in Ah and 8 is expressed in hours.

So our 200Ah battery bank should not be loaded continuously at more than

200Ah / 8h = 25A

What does it mean for our inverter and appliance? Well that means that we need a much larger battery bank if we want to power our 1000W appliance in a continuative manner. How large should the battery bank be if we want to run our appliance continuously? Here is the line of thinking.

We need a battery bank that can provide 93A in a continuative manner respecting the rule of C/8. Looks like we need:

93A * 8h = 744Ah

Including some safety margin, we could build a bank made of 4 x 200Ah deep-cycle batteries wired in parallel. The parallel wiring maintains the voltage at 12V and raises the capacity to 800Ah total. This is a really large battery bank, which takes a considerable amount of space to install and may weight around 250kg (500 pounds). Beside that, it doesn’t come cheap either!


Super-large battery bank

The good news now. Our astonishing 800Ah of total battery capacity, discharged at maximum 80%, can now yield a total of:

800Ah * 0.80 = 640Ah

With our appliance still absorbing 93A, it can run for

640Ah / 93A =  6.88 hours = 6 hours 52 minutes

before we need to switch everything off and top up our batteries. This is, of course, 4 times the run time with a 200Ah battery bank, with the difference that now we are respecting the C/8 rule. All that money we threw into our massive battery bank is worth something at least!

What if our boat uses a 24V system? Our batteries need to deliver half of the current (see above), so we need only half of the capacity

47A * 8h = 376Ah

Cool, we need only half of the batteries than at 12V! Then let’s use a 48V system since we are at it. Wait, where is the trick? Well, we still need to “make” the 24V first. As a matter of fact there is no difference. We will need 2 sets of 200Ah 12V batteries each wired in series to raise the voltage to 24V while the capacity is still 200Ah and then wire each set in parallel to bring the capacity to 400Ah total. Same amount of batteries, same real estate, same weight, same $$$. The only thing that changes is that we now need smaller wires because of the lower current. That’s about it.


We have gone a long way in understanding how to size our inverter and what effect its use has on our battery bank, including how large it should be.

To recap, a 1000W good appliance powered through a 1500W pure sine wave inverter draws 93A from a 12V battery bank. In order to respect the limitations on maximum continuous current (C/8) and maximum Depth of Discharge (80%) of the batteries, we had to size our battery bank accordingly. At 12V we needed a bank of around 750Ah total, which we decided to build using 4 x 200Ah batteries wired in parallel for 800Ah total. This bank is able to power our appliance respecting the 80% DoD limit for 6 hours and 52 minutes before needing to be recharged.

If your situation is different than the example described above, you can go through all calculations swapping in your own numbers to find out how large your inverter and battery bank should be.

For example, if you run your appliance for a shorter time, say a 1000W hair-dryer for 10 minutes, the continuous current is still the same, 93A, but you will only draw a total of 93A * 10/60 = 15.5Ah from your battery bank. The battery bank will still need to be large enough to respect the C/8 rule, but will only be discharged by around 2% (15.5Ah/800Ah). Or they could keep your hair-dryer running for 6 hours and 52 minutes, which should be a fair enough time for your significant other to dry their many hair. 🙂

I hope this was useful to you. As usual, feel free to rate it with the stars above or share your comments/remarks/critiques in the section below. Till next time!

Fair winds,

Online Calculation Tool

Doing calculations by hand could be quite boring. Check out our online Power Inverter Sizing Tool. Just plug-in your numbers and get all details automatically.

Some words of caution

  • Working with electrical equipment, especially AC, could be really dangerous if you don’t know what you are doing. You may be seriously injured and even die. Better to ask a professional to do the job or assist you.
  • All cables and safety devices should be sized according to needs, including a safety margin. If you don’t do that there are high chances you are going to fry the boat, the crew and your pets.
  • The marine environment is particularly hostile toward electrical devices. There are vibrations, moisture and salty water, excessive heath (engine room for example) and potentially explosive gases (like petrol vapours). Make sure you select and install your equipment keeping this in mind.
  • Different countries have different regulations when it comes to what you can or cannot do on your boat and how you should install certain pieces of equipment. Make sure you respect all those that apply to your case.
  • Last but not least, I am not a lawyer, but if you break it you own the pieces!


6 thoughts on “Power inverter: how large is large enough?

  1. Nice assembly of info and links to nice tools. It really shows the value of using 24 V configs too. Well done!

    • Thanks Keith! Indeed having a 24V system helps to save on cable size especially with high current loads (windlass, winches, power inverter to mention few). Fair winds!

  2. Very interesting calculation. You are right. To deliver same amount of power, 24V system pushes out less current. Voltage times current equals to power. Then, the smaller wire is required. This is very smart.

  3. This is a very interesting article. Though I use inverters for both purposes, residential and industrial inverters, some of the factors to consider in using them which are mentioned in this article were unknown to me. The difference in inverter usage for good and bad appliances was definitely a new knowledge for me and will consider it in future.

  4. Thanks for the excellent write-up. I have learned much.

    How do I identify optimize charging current? Looks like I need to upgrade to a larger on-grid inverter solution to handle faster charging and longer outages. My house hold load is about 1,250 VA.

    Currently running 1.5kVA inverter, 220 Ah/24V (2X12V std batteries). Maximum charge current is 5A.
    Considering a 3.5kVA/48V inverter, 400 Ah/48V 98X12V deep cycle tubular Batteries). Maximum Charge current is 16A. But am not sure if this is enough charging capacity.

    Should I consider shopping for inverter and charger separately?

    Thanks in advance.

    • Hi Dan,
      Before attempting any recommendations, I would need to understand a bit better your setup.

      I understand you have a battery bank + inverter, but what is charging it? Is this a backup system to keep going during power outages?

      If so, you should have a dedicated battery charger connected to the main power supply (grid), unless you have a combined inverter + charger unit. Usually the most modern battery chargers have some kind of switches/selectors to setup the capacity and the type of the battery bank they are serving (e.g. 200Ah AGM). This setup is used to optimize the charging curve in terms of current vs. sensed battery voltage.

      It may be that you have a very “basic” charger that simply uses a constant current (5A). That is a pretty low current compared to what your bank could absorb, especially when it starts from less than 80% residual charge. If this is the case, it will take a long time to recharge your batteries (e.g. 80% residual on 200Ah = 160Ah, meaning 40Ah to full richarge. At 5A it will take nominally 8 hours to richarge that amount. If you could richarge at 20A it would take only 2 hours nominal).

      I would definitely look into upgrading your charger to a smarter and more powerful one.

      Hope this helps!


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