How Big An Inverter Do I Need?

[kylakemike]

View attachment 385View attachment 385I currently have this 750 / 1500 watt inverter:

http://www.harborfreight.com/750-watt-continuous-1500-watt-peak-power-inverter-66817.html

I recharge my laptop and my iphone for the time being. I am sure I will add some sort of fan and a cooking device or two at some point. May add a TV. Harbor Freight currently has a 2000 / 4000 watt inverter for $129. Not much of an electrician and am curious if my old inverter is enough or if I should purchase the 2k for $129.

Any advice is greatly appreciated.

 

[sl1966]

First get a total by adding up the watts from each item you plan on using on it. Get an inverter that gives you roughly 25% more than that total. This is quick and dirty though.

If you don't forsee yourself using everything at the same time you can probably scale it back by 1000 watts. Most people say to go with pure sine wave inverters because it won't damage sensitive electronics. This the best but usually a little more expensive route. A modified since wave inveter is much less expensive, works with most electronics, but can potentially damage the more senstive ones. Most of these are anything that uses and AC motor (microwaves, fridges), and/or delicate medical equipment as a few examples.

I'm not very well versed in electrical systems either, but others who are will chime in with more detailed info. This is what I've learned (well, what stuck) over the years in planning out my solar power setup for my rig. I kind of get to dodge this bullet for a short while since I'm going with a Honda eu2000i generator first which has pure sine built-in.

 

[Lucky mike]

dont know if this will help......my motor home (Class A) came from the factory with a 2000 watt inverter.....it runs all you have mentioned without problems.....the odds are your not going to be running all of them at once......I will tell you that inverters also draw power to operate....I would suggest a second deep cycle battery if you plan on running over 1000 watts and isolate it from you chassis battery......or save money and buy a good set of jumper cables you might need them

 

[SternWake]

Using a microwave on an inverter is certainly possible but powering one is a huge stress on the battery(s)

Getting 12 volt devices in the first place, if possible, saves on the inverter losses.

Inverters are great devices for depleting batteries. The alternator is not a magical instant battery recharger as so many people assume.

Charging laptops and Iphones is better done with DC to DC car adapters.

Volts, amps watts are confusing to many.

Lets take a 100 amp hour group 27 battery.

Lets power a 630 watt load on this battery. 630 watts divided by 12.6 volts is 50 amps.

Without taking the Peukert effect into account and assuming a healthy fully charged battery to start with, in one hour, the group 27 battery is below 50% charged and as a rule one should aim to use no more than 50% of the batteries capacity.

So perhaps the question should not be 'how big of an inverter do I need', but I have X amount of battery capacity, what can I hope to power and for how long, and how long is it going to take to replace that energy into my battery.

You can put a 2500 watt inverter on a tiny battery, but it will never support a quarter of that draw even for a minute, so purchasing the 2500 watt inverter without the battery capacity to support such a load is kind of foolish. Bragging rights that cannot be supported.

It is also difficult to actually get a battery truly fully charged where it can provide its full capacity, and without regular full charges, the battery will lose capacity faster than if it did receive them every so often.

If the cabling is adequate, the alternator can charge a battery back up to 80% pretty well, but that last 20% will take hours and hours, no matter the charging source. The alternator is one of the poorest methods for squeezing that last 20% in.

Without the tools to actually measure currents, in or out of the battery, most people just assume everything is fine as long as it works, and then when it does not work anymore it is assumed something is wrong.

One who can measure currents will notice performance loss as the battery ages and not be surprised when it fails, and plan ahead for failure, rather than freak out when the battery is incapable of supporting a fraction of what it once did.

The bigger the inverter, the easier it is to destroy a battery

 

[sl1966]

Wow, thank you for that info.

 

[Blue]

I got that same 2000/4000 watt inverter, I am worried that the draw will be too much for my single deep cycle 29D led/Acid, you should also read the reviews under the inverters, for the 2000 watt a few people are saying to go to a welding suply store and get (I think it was) 2 gage electrical wire for attaching to your house batteries.

also, from what I read the 2000 watt is still NOT enough to run even a low energy AC and will blow the peak wattage at start up... I'm thinking now I should have jumpped up to the 5000 watt and went with a bank of Lithium-Ion batteries, but ya know... hindsight 20-20

wanna read what I did go here...

https://vanlivingforum.com/Thread-Things-learned-from-installing-bat-inverter

 

[kylakemike]

SternWake - Outstanding and thank you so much for the info.

Blue I read that thread as well. I also read every review on this inverter. 1 guage welding cable with good solid connectors seem appropriate.

 

[akrvbob]

I think that cable is too small. I used two-ought (2/0) on mine. That means it is twice as thick as ought (zero gauge). It's about the size of your thumb.
Bob

 

[kylakemike]

I think that cable is too small. I used two-ought (2/0) on mine. That means it is twice as thick as ought (zero gauge). It's about the size of your thumb.
Bob

Appreciate the info. Thanks Bob.

 

[blars]

2/0 is bigger than 1/0 (or 0) AWG wire, but not twice as big. That would require 4/0. To double the current carrying capacity of AWG wire, you need to reduce the AWG number by 3. wikipedia has a good article: http://en.wikipedia.org/wiki/AWG

 

[sl1966]

I know if you go too small you'll lose current and/or the wire fries. I've read a lot of people going with thicker gauge wire to maximize current/prevent loss. What's the threshold before the wire is too thick/it becomes a waste of money in this case? Is that even a valid assumption?

 

[Donedirtcheap]

Stern made several good points and some of the following may have already been said.
Forget about a/c. Its not going to happen on batteries.
If you want to cook with electricity you can do it but its going to be very expensive and you would be better off using propane.
If you are just running a small tv, phone charger or lap top, a small inverter will be all you need. I use a 400 watt modified to run a 24 inch led tv and charge our phones and laptop. You can use a 2000 watt to do the same thing but I would think that the energy consumption to run it would be greater. Inverters don't just change dc to ac, they also burn energy in the conversion process. Its not an even exchange. If you don't even use the inverter it will drain your battery. I installed a switch so the inverter can be turned off when not in use.

Basically if you are just doing the small items mentioned, you can do it very cheaply, its when you cross the line of powering microwaves, hot plates etc that you will need several batteries and a big inverter and as Stem said you then have to put that energy back in the battery. You can probably cook for weeks on a 1 pound propane canister. Its not worth moving all the weight of those batteries and inverters and solar panels etc to cook and heat when you can just stop at a place that stores all that energy for you. Propane.

There is a pay back issue. Carry a lot of weight and pay a lot for that weight or stop and buy the energy as needed.

He also mentioned battery voltage. I don't mean you, but people often think of a 12 volt battery and think well I can use it until it wont put out then recharge it. Thats what destroys a battery. It's a 12 volt battery but will reach about 13.9 or so volts when charging. As you use it you do not want to go below 12 volts or so imo. Doing so reduces battery life. SO......when you do all this get a small digital voltmeter so you can monitor the battery voltage. There is a good chart in this link that shows voltage vs state of charge. Even if you use a $40.00 400 watt inverter with alligator clips and a group 29 deep cycle battery put a volt meter on it. When not using it, unhook it. Parasitic drains.

http://www.marxrv.com/12volt/12volt.htm

 

[SternWake]

One other factor to consider with the inverter is annoying fan noise. My 800 watt MSW inverter runs the fan full speed even when powering nothing. It also draws 0.9 amps powering nothing.

I have a 13.3 inch RCA TV. AC/DC. I run it directly off battery power.
http://www.amazon.com/RCA-DECK13DR-...8&qid=1392143335&sr=1-3&keywords=RCA+AC/DC+TV

No inverter, no fan, no problem. It draws 0.9 to 1.1 amps depending on brightness and volume.

I have a DC to DC laptop 'car adapter'. Silent, uses 15 to 50% less electricity than the inverter powering the original power brick. Depends on the task being performed by the laptop. Silent, less battery consumption, Win win again.
http://www.amazon.com/PWR-INSPIRON-...1392143953&sr=1-7&keywords=laptop+car+adapter

Don't all phones these days use USB ports as charging ports? I cannot imagine thinking an inverter is needed to plug in a USB charger.

http://www.amazon.com/Blue-Sea-Syst...&qid=1392143799&sr=1-1&keywords=blue+seas+USB

This can charge an Ipad at the full rate. No fan noise, no inverter needed, less electricity consumed.

I bought a 400 watt pure sine wave inverter for my Makita drill/driver batteries, and put it on my electrical wall near my 800 watt inverter.

It is silent until about the 175 watt load range, and then the fan is audible, but still quieter than the 800 watt MSW. It uses less than 0.4amps turned on powering nothing.

I have not used the 800 watt inverter since, and use the 400 watter for everything that cannot just be bought in AC/DC form or have a DC adapter used in place.

My soldering iron, heating pad. I've even run my battery charger off one battery bank to briefly charge the other, as a test.

But honestly I don't use the inverter all that much, it is just one of those things I have to have. But I certainly don't need a 2000 watt one. With only 210 amp hours of capacity, it is foolish to ever believe that I could power anything needing 2000 watts for more than a minute or 2, and I doubt even that.

I'm thinking of freeing up some space on my electrical wall by removing the 800 watt inverter. I do not need its capacity, and 800 watts at 12.6 volts is 63.5 amps. My battery bank could not support such a load for very long anyway. The engine would have to be running and held at 1400 rpm or so to break even, and my alternator circuit is beefed way up beyond most vehicles.

2000 watts? That is 158.73 amps at 12.6 volts. But since the battery voltage will drop well below 12.6 at that much load it is more like a 190 amp load.

My 130 amp alternator could not hope to ever keep up even at 4000 engine rpm. Most people act like an alternator is magical free power that can do anything.

Well it cannot power a 2000 watt inverter and keep the batteries from discharging.

If the 'magical' alternator cannot do it, what chance does a small battery bank have on its own trying to support such a ridiculous load?

Be realistic on inverter choice. First consideration is actual demand needed, then what the battery bank can realistically handle, then what the recharging method will realistically be able to return. And if you only have the alternator, or a puny solar system, then the recharging will be well, well below expectations.

 

[kylakemike]

Stern and DoneDirtCheap you make excellent points and I appreciate the feedback May I ask what is is that you use to recharge your deep cycle battery?

 

[Donedirtcheap]

Mine are charged with my RV's house charger and they are usually a poor choice in chargers. Ideally you want a three stage charger. I plan on upgrading someday but I set a goal of what I wanted to do right now. If and when I go with solar I will look at the whole system and buy the correct charger etc. Again, as Stern said, your truck alternator is never going to provide the proper charge to a deep cycle battery. So you want to charge with solar?
I'm not into solar yet so Bob or Stern or someone else can help you with that. 3 stage charge controller?
You need to first identify what you want to power, then you need to know how much battery you need to do that and then what do you need to put the energy back int the battery or batteries.

I would read stuff that Bob has said about panels. The way I see it is you don't want to buy too small or some cheesy panel as its one of the components that will be around a long time. Just like internal combustion engines.....there is no replacement for displacement.

http://www.solar-electric.com/solar-charge-controller-basics.html

 

[rvwandering]

We have a 400 watt inverter too, bought specifically because we didn't want to waste power running a larger inverter. Along with all of the usual stuff like powering computers, charging batteries, running fans and the TV we also use it for a drill, soldering irons,a small clothing iron and a little upright vacuum. The inverter fan rarely runs.

For comparison in our previous RV we had a 2000 watt inverter. The only thing we needed that much for was the microwave but using it was so hard on the batteries that we rarely did. The fan ran a lot even when we were not using much electricity.

Our batteries charge from 240 watts of solar and as we drive.

 

[SternWake]

My recharging sources are:

A: 198 watts of solar with a MPPT controller that allows me to set absorption and float voltages
B: 130 amp alternator with upgraded cabling between alternator and batteries.
C: A 2/12/25 amp Schumacher charger.

The Solar, near the summer solstice, will just barely meet the minimum Bulk charge current listed by USbattery, which is the manufacturer of my group31 deep cycle battery(130 amp hours). Since Solar ramps up through solar noon and winds back down afterward, it does not really follow the charging guidelines as Listed by USbattery.

http://pdf.wholesalesolar.com/battery-folder/charging_instruction_2011_2.pdf

I have seen 109 amps from my 130 amp alternator. This was when the alternator was still cold, the batteries depleted and well over 3000 engine rpm. I've seen my single USbattery take 60 amps from the alternator, well over the 10% recommended rate listed by USbattery. The amps taper quickly. I've not been driving much. My vehicles voltage regulator is inside my engine computer. It will drop the voltage to 13.7 prematurely. 45 amps might have been required to hold 14.7v, only 13 amps is required to hold 13.7, slowing down charging significantly.

When hot at idle speed my alternator cannot make more than 32 amps. At night with lights and blower motor on high, 10 amps are flowing from my batteries at hot idle speed.

My Schumacher kind of needs to be babysat as the charging voltages can go too high, especially if the battery still has a load on it, it was not designed to charge a battery that is still under load..

Recently I rewired my system so I can use either battery as a House or engine starting battery.

RV converters like Iota or Progressive Dynamics or Powermax among others are designed to both power all DC loads when 120vac grid power is available, and do three stage battery charging, bulk, acceptance/absorption, and float.

They all have slightly different voltages and durations, and arguments can go on for days which is best for each particular situation.

Bottom line is there is no perfect battery charging regimen. Everything is a compromise, and at the end of the day, batteries are rented.

The first paragraph of the USbattery link above pretty much states it best.

U.S. Battery Manufacturing Company, Inc. recommends the use of ‘opportunity charging’ or charging batteries and
battery packs at every opportunity while in storage or service. Following this recommendation will assure that
batteries are always at the highest possible State of Charge (SOC) to maximize performance and range and to
minimize the battery’s Depth of Discharge (DOD) to optimize performance and life[/quote)

So charge whenever you can using whatever methods at your disposal, and the less you discharge the battery, the longer it will last.

My solar basically does not meet the minumum charging current, even if it is able to return the amp hours used overnight throughout the day, it is not doing so fast enough.

My alternator can Exceed it by a large factor but I never drive long enough for it to take the batteries above 85%, and my Schumacher is a fickle female dog, sometimes, and requires monitoring.

Don't worry about using more than one charging source at the same time. At worst, the one with lower voltage setpoints thinks the battery is fully charged and stops adding current. No need to disconnect one charging source before starting another.

Sometimes if one charging source(solar) has the voltage above 12.8, but the battery is far from fully charged, and then one plugs in, the converter/charger will see this 12.8v, think the battery is fully charged and not contribute anything to the thirsty battery.

When this happens, it might not even be noticeable that it is not contributing. Generally I will turn on my lights and blower motor for a while, long enough to drop battery voltage to below 12.6, then my Schumacher will contribute 12 or 25 amps to the battery(s), on top of whatever my solar is making at the time.

My solar will contribute to the alternator's current too, depending on battery state of charge, and there have been times when my batteries are full charged and the solar is actually powering my fuel injectors, coil and fuel pump.