Correct inverter size

Hey Everyone :raised_hand_with_fingers_splayed:
I have a 1992 Toyota Hiace LWB poptop called Beastee which was converted in 2005 but I have gutted that and am now rebuilding.
I am currently in the middle of getting the solar sorted.
I have a 200w solar panel and a 120ah lithium battery.
I am just not confident about the size of the inverter needed as I have shore power as well.
The research says not to buy one too big and just to add 25% to the size of the solar panel, so that would be 250w.
Am I on the right track please?

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Depends on what you want to run, 250w is really small. Just make sure it is a pure sine wave. Add up the wattage, it’s on the label of the appliances. Just an example small coffeemaker 600w
Small microwave 600w
Hair Dryer 600-1500w
Small fridge 150w
TV 25-50w
IMO probably a 500 to 1000w , just because it is rated at 500, 1000 or whatever it’s only going to draw what you are using as I said it depends on what you plan to use. We run a 2k but We have a lot of accessories. Correct wire size and length is really important, keep it short as possible.


Thanks John.
I will look at a larger one.
Keeping the wire length short, do you mean between the solar and the inverter or between the inverter and battery or both?
Thanks :grin:

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Between the battery and the inverter. Inverters can draw a lot of current called amperes or amps for short. To figure out amps you divide the watts buy the voltage. Example: If you had a 500w inverter and it was running on a full load, meaning you have 500w of appliances running at one time it would be 500 divided by 12. = 41.7Amps. So you would need wire big enough to carry 42 Amps. Look at it as water the more water( current) going thru the hose the bigger the hose you need. The force or “ pressure” is the volts.
Once you figure out your maximum amps go to Blue Seas Systems website and they will have a chart for what size wire you will need for the amount of current(amps) and length of the run. Any questions give me a shout.
On a special note: Be very careful and wear safety glasses when working with high amp installations. 1/4 Amp can kill you. If you short the wires or battery posts it can blow up the battery.


Check out for inverters, they also make cables and have a great tech line. Just tell them your requirements and they can hook you up. Any questions while shopping just ask.

Your inverter can be whatever size you wish it to be. It’s only going to draw about an amp at idle plus whatever the amperage is of the device connected to it. You have1440 watts of power in your battery when fully charged. Supposed to anyways. Your device will draw it’s rated amperage or a little less at that rate per hour.
Example: if you have a coffee maker that draws 800 watts. That’s 800 watts per hour that it will use at a constant rate of 800 watts for one hour.
If it runs for 30 minutes you will have used 400 watts of your battery power of 1440 watts.

If you are running on plug in power, then bypass your battery system and run it directly off the power cord. Don’t put the wear on your inverter. Also your battery had a limited amount of charges available before it will go bad. The more you charge it, and drain it and charge it, the more you will use your battery. If your charger is providing enough current to run the inverter and item connected, then you can run on your inverter without any wear on your battery. This is commonly done in RV’s with converters, which I have installed in my truck Camper that 100% solar but Can be powered by 120v plug in or generator power.

Your battery has a charge rate that you need to know as to not over charge or over drain the battery. As this can damage a battery. Common for flooded batteries is 30-50% maximum drain and a charge rate of about 10 amps per battery. This varies depending on your battery size and type. Check with the manufacturer if it’s not listed on the battery.

If on 120v plug in power and running off of your battery and charger then you should not draw more than what your charger outputs minus 25%. So if your 12v at 50 amp charger, then you should not go over 450-600 watts ideally as to not drain your battery. That would be used power. Another thing to consider is an inverter had two ratings. Continuous load and peak load. Peak load is the power used when a device is first turned on, typically highest in motor devices which draw large currents to start rotation. Very little in electronic devices without moving parts. Then a good tule of thumb is to not exceed 75% of your inverter continuous rated load.

Example, if your inverter is rated 600 continuous watts, don’t connect anything that draws more than about 450 watts. They will get hot and malfunction if you draw too much current. Current is amps. Power is watts.

This in mind, you have 1440 watts on off grid use. Deduct anything you have connected to the battery directly. Like a fridge or anything that draws power. Lights whatever is connected. Then what you have left is your free power that you can use with the inverter.
Example, let’s say hi have some led lights that draw 20 watts per hour and you use them about 4 hrs a day. That is 60 watts per day subtracted from your 1440 watts. Then let’s say your battery is rated for no more than 50% discharge. That would be 720 - 60 lights = 680 watts remaining. Now your inverter will likely draw about one amp, that’s 12 watts per hour on 12 v battery. Your devices combined you want to power should not draw more than a total of 680 watts for any 24 hr period. So if you have the inverter on for 8 hrs per day that’s 96 watts per day just to power the inverter. Then factor in your usage and stay below 580 watts total usage. As your inverter will use the first 100 watts of the 680 watts total available.

To factor your usage use this calculation.
Amperage x Voltage = Watts Ohm’s Law

Based on the information you have given I would suggest an inverter size of 600 continuous watts. Make sure to get a true sign wave inverter if you plan to run electronics. With careful monitoring of your use as to not over draw your battery.

If you want one that has more power that you plan to connect 120v plug in power, then get one that doesn’t draw more than your charger for sure and 25% less watts than your charger would be best as to be able to also charge your battery.

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You want all your wires to be as short as possible. But most important anything that draws a high amount of current of charges with a high amount of current should be kept as short as possible. The more wire, the more voltage drop and also the less current it can carry. Current is amps.

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Correction, if you have led lights that draw 20 watts rated, all lights combined usage is 20 watts, and you use them for 4 hrs a day it will use a total of 80 watts per day. Not the 60 watts I stated earlier.

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FYI Battery time is measured in amps, amp hrs in minutes a battery will run on a 25amp load, not watts. I guess you could convert it to watts if you wanted. Watt is a measurement of heat dissipated by resistance. Just like Alternator or battery charger is measured in amps. I would buy a 120 amp alternator and a 20amp charger, not a 1500watt alternator or a whatever watt battery charger. Same misconception is with solar panels I would look at the amps in the panel specs before watts. Some more electo trivia…. A watt isn’t the brightness of a lightbulb it’s the heat dissipated, brightness is measured in lumens and lux. The watt measurement can be very misleading in advertising because it’s the bigger numbers and Americans love bigger. Don’t even start me how it is abused in audio advertising. Lol.

Power and energy consumption is almost always measured in watts or kilowatts. Many batteries give an amperage rating as it is the same, as it is proportional to a watt.

A starter, alternator and battery are typically measured in amps so that it’s easier for the installer to size the wire and instal fuses and breakers. That way they don’t have to convert and do math. Not because you have to measure a batteries power in amps.

A battery does use amperage rating for x amount of amps per hour the battery is capable of outputting. But sometimes it also uses watts or kilowatts. Depending usually on the size of the battery or the manufacturer’s preference, commonly based on what installers need to know to install the item or appliance or panel or battery. Not because you can’t measure it in watts. That is just another way to measure the amount of electricity as the voltage is the same. When you are converting voltage it is easier to use watts as it converts between AC and dc without having to do math and because it measures a total amount of power. Amps does not. Amps only measures total amount of power accurately if the voltage stays the same. As you convert and invert power the voltage changes. So it’s harder to track total power in amps. Unless you want to keep doing the math back and forth. But why? Just use watts, it’s easier.

In an RV or Camper setting you are converting or inverting power to and from AC to DC. To have a relative number that can be used to measure power consumption or capacity, it is easier to use watts. As the voltage and current changes depending on if your refering to the inverted 120v AC appliance or a 12/24/48 DC volt appliance. The total power that is available in a battery is measured in amps and/or watts. If you have a 200 ah battery at 12v, is has a capacity of 2,400 watts of DC power, or is commonly refered to as a 2.4 KWH battery.

Oh and the power company when they charge you for electric, they charge you by kilowatt hour. That would be by every 1,000 watt hours of energy or electricity you used. A kilowatt hour = 1,000 watt hours.

The wattage of a light bulb is the amount of energy it consumes in an hour of use. Not the amount of heat it dissipates. Although they are related, a watt is a measurement of the total amount of power consumed by the light bulb in one hour of use. The lumens are the the measurement of the brightness of the bulb.

Resistance is the amount of force that the current has to overcome to pass through any given point. Different than a watt of power but is still proportional if all other values are the same.

Even a copper wire used to transfer electricity has a resistance. Even though a volt meter which measure resistance registers zero. The wire does have small amounts of resistance that is not measured by typical volt meters. If you look at the National Electrical Code book, it gives resistance of different wire types.

A resister limits power passing through it. This is commonly used to control the current or flow of the power. In cases where the full amperage of dc current is not needed. Unlike AC voltage DC voltage current is direct and pushed to a device rather than the device using only what it needs as with the case of alternating current. Resisters are used to control the amount of current flowing to a device. This is how I was taught that power travels and how it works. If you have an NEC approved literature that states otherwise, I would love to read it. Please send me the link.

When you have a 100 watt light bulb, it consumes 100 watts per hour of energy or power. That’s where Is gets it’s name. Your right is does not put off 100 watts of light it puts of several thousand lumens of light.

Appliances for AC voltage are also measured in watts used. If you look at an energy star rating, it will give you the amount of energy estimated that the appliance will consume or use in a period of one year, in dollars typically. “This appliance costs $50 a year.” Or something like that. That cost is based off its watts or kilowatts used per year and the average cost of electricity. However, when you connect the unit to your home, it will have an amperage rating as all residential homes are 120/240 volts. So you will have a 15/20/30 or 50 amp plug and wire. It gives you watts for total power and amperage for connecting it because that is the information that you need to connect it to your house electrical panel as the breakers and wires are rated by amperage carrying capability. The wire and breakers do not care how many watts is passing through it, it only cares about how much current is passing through it. So you need to know current when connecting it. You need to know wattage when factoring the total amount of power it will consume in a year.

When you by wire, you buy it for the amount of current that it can carry. For example, a number 12 copper wire can carry 20 amps of AC current (for you trolls out there, that amperage rating is for a residential copper wire used to power 20 amp circuits) That is not measured in watts because the voltage can be adjusted. It doesn’t matter how many watts you can put through it. Amps is the ideal way to measure the amount of current a wire can carry as voltages change. Your house has 120/240v, a commercial Bulding has that but it starts at 277/480v. A transformer is used to lower the voltage to 120/240 where needed. You can carry more wattage or current at higher voltages. As they are proportional to each other. 120v x 20 amps = 2,400 watts. 277v x 20 amps = 5,540 watts. Same wire can carry more power at a higher voltage. They still have the same current going through them, 20 amps. This is why many compagines do not measure power in amps, but measure it in watts instead.

Resistance is not wattage. They are different but also proportionally to each other. You can look at an ohms law guide book and it will tell you all the differences.

The best way I have heard it explained is like this: electricity is like water. Current is the flow rate of water flowing through a pipe, pressure of the water is how much voltage. Wattage would be the total amount of water that passes through the pipe regardless of the pressure or flow rate, gallons of water per minute. Which would be watts per hour in electricity.

There is no misconception in solar panels. If you have a 100 watt solar panel and a properly working charge controller. At noon and faced directly at the sun it produces 100 watts of power. It’s too difficult to measure a solar panel in current and voltage as the voltage and amperage are constantly changing. It doesn’t stay the same. So they use watts instead. Plus you may combine your panels in series and raise the voltage even more. You still need to know the total amount of power you will be able to produce in ideal conditions, that is watts. Total amount of power. It doesn’t matter what rate the power passes through the wire unless you are sizing the wire. If you have 4 100 watt solar panels, you have a total of 400 watts. You may connect those at 12v or you may connect them at 24 or 48 volts. It doesn’t matter because you still have 400 watts of power. No matter what voltage you change the panels to run on. If you run them in parallel, and they are 12v panels, you have 12v, 33.33 amps and still 400 watts.

A watt is a watt is a watt. Regardless of the voltage or amperage changes, a watt always stays the same.

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Sorry I seemed to have offended you. I am well aware of all you mentioned. The next time someone on this site is looking for a battery for their van make sure they ask for one in KW. Stick to siding.

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I get your numbers, when you can build and operate a bench like this, give me a call. Just trying to help with some BASICS.

Thanks for all the information guys, very helpful.
Sorry I hadn’t replied and no one offended me at all.
I appreciate the help.
I’ve not been well and had to overcome a few hurdles to get back here but definitely back on track, so thanks again.

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