Questions about power inverter wires for negative terminal and chassis?

I really like to deal with Watts (P watts = Volts * Amps). When you have multiple voltages (120 VAC and 12 VDC) it gets really confusing (1 amp * 120 VAC = 120 Watts = 10 amps * 12 VDC--Ignoring conversion losses for the moment).

For example, you are using 180 Watts for your desktop computer system--An average amount. However, an average laptop (not the high power gamer type) can use around 30 Watts--Or about 1/6 the amount of power--Very handy if you are going to run with a battery bank (and solar power or what for battery charging). They can use upwards of 90 Watts to recharge the laptop's batteries--But that is only while charging, and is fairly efficient -- Power to battery to unplugged laptop energy usage).

I highly suggest you do a few paper designs--First trying to figure out what your minimum power needs (per day) are... And then what your system should do (recharge from solar panels, genset, driving, plugging into wall outlet after a ouple days on road, etc.).

(and before I forget--Remember a propane fridge needs a chimney/vent to the outside for combustion exhaust--Or bad things could happen to you--such as carbon monoxide poisoning).

I know that it is very exciting to design the power side of the system at the same times you are researching your loads... However--Believe me when I say that (generally) your first step is to reduce your electrical loads as much as possible first--(laptop, not desktop, tablet instead of laptop, LED lighting vs other type of lighting, small vs large AC inverter, small DC fan vs larger AC fan, etc.). Battery powered devices (such as laptops, tablets, LED lighting, RV water pumps, and cell phones) tend already to be much more energy efficient than standard AC appliances.

For most people, they only have a small space for a battery bank, and cannot really carry that much weight. And if you want to use solar panels--There is limited roof space.

There are other solutions out there for off grid power systems used in RV/Vehicles--Such as using LiFePO4 batteries vs Flooded Cell or AGM (much lighter, much smaller, able to "surge" much more power if you have high/short term loads like starting a power tool)--But those tend to be more expensive and a bit more difficult to use (worrying about Batter Management System, very careful to fuse/breaker your wiring to prevent short circuits/fires, if you "kill" a LiFePO4 battery bank--Much more expensive to replace).

But before we go down the power system road--Getting a handle on your Loads first will help a lot. We can do some simple paper design to see if it will meet your requirements (both power, space, and costs), or not.

And some solutions are easier to get lots of power out of (a Honda eu2000i and 1 gallon of fuel can supply you with more energy than 4x golf cart batteries--And weight much less too).

Sometimes the answer is a small battery power system (for lights, cell phone/tablet charging--quiet time/at night)--And the genset when you need to power the "big stuff".

But without better knowledge about your loads and how many hours per day you run them and what the charging source is (solar, not solar, etc.)--I cannot even begin to guess what any solution will be best for you.

We do all of this with simple "paper" design (and modeling with math). We really do want to save you money and have a working system (for you) at the end... We are not in this to sell you hardware (and almost all of us here are volunteers--With no financial incentives at all).

-Bill

I tend to avoid the "calculators"--They hide assumptions and make it difficult to figure out where they are including deratings/safety factors/and assumptions about various solutions (inverter losses, battery losses, etc.).

The equations we use here are relatively simple and we are clear about fudge factors--So you can decide which ones apply to you or not (days of no-sun storage, depth of discharge, and such).

However--We really need to start with your loads and how many hours per day you use them. And your charging sources.

I don' want to say "you cannot do that"--However, I do want you do understand that there are costs associated with your choices. And remember the batteries are not a source of energy--They are simply storage, and you must recharge them every day (or every few days) and those recharging resources are a critical part of the design process.

You have not said what your power usage is planned to be... It could be 3 day weekend (say you run a business that goes to weekend fairs during the summer)--Or you could be living full time off grid. This could be fixed location (national forest areas a few weeks at a time, or you travel daily)--And where they system will be used (forest in deep shade; coastal regions with lots of marine layer; desert south west with lots of sun; up north of Chicago, or what)?

Just to give you some ideas--Off grid power, when you take all costs into account run roughly ~$1-$2 per kWH (more or less, but a good starting point--Remember you are pre-paying 5-10 years of energy usage).

Here is a very quick design based on your request vs what I would do just using "generic" paper parts (i.e., just sizing the system, not a full design).

• (180 Watt computer + 25 Watt satellite receiver + 25 Watt router) * 10 hours per day usage + 10 watt LED lighting*5 hours per night = 2,350 Watt*Hours per day
• 2,350 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery * 2 days storage * 1/0.50 max discharge = 922 AH @ 12 volt battery bank
• using 6 volt @ 225 AH "golf cart batteries"-> 2x 6 volt batteries in series for 12 volts @ 225 AH; 4x parallel strings of 8 total G.C. batteries => 12 volt @ 900 AH battery bank
  
• 922 AH * 14.5 volts charging *1/0.77 panel+controller losses * 0.10 rate of charge = 1,736 Watt solar array nominal (full time off grid) based on battery bank size

And based on Chicago "hours of sun" per day:
http://solarelectricityhandbook.com/solar-irradiance.html

Chicago
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
1.68	2.41	3.43	4.45	5.25	5.93
Jul	Aug	Sep	Oct	Nov	Dec
5.93	5.12	4.29	3.03	1.83	1.48

Toss the bottom three months (use a genset during bad weather):

• 2,350 Watt per day * 1/0.52 off grid system eff * 1/2.41 hours per day = 1,875 Watt array minimum (February break even month)

Now--"my system suggestion" of using low power devices:

• (30 Watt computer + 8 Watt cell phone wifi hot spot) * 10 hours per day usage + 10 watt LED lighting*5 hours per night = 430 Watt*Hours per day
• 430 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery * 2 days storage * 1/0.50 max discharge = 167 AH @ 12 volt battery bank
• using 6 volt @ 225 AH "golf cart batteries"-> 2x 6 volt batteries in series for 12 volts @ 225 AH (2x batteries total)
  
• 220 AH * 14.5 volts charging *1/0.77 panel+controller losses * 0.10 rate of charge = 414 Watt solar array nominal (full time off grid) based on battery bank size
• 430 Watt per day * 1/0.52 off grid system eff * 1/2.41 hours per day = 343 Watt array minimum (February break even month)

So--More or less, using a laptop+cell phone wifi, your solar power system is now 1/4 the size of a full power desktop system+satellite networking at 10 hours per day.

If you only need 1 day of storage, you can cut the battery bank by 1/2 (not the solar array)--But that leads you to more generator use (takes more hours of sun per day to recharge a deeply cycled lead acid battery). You could use a Lithium Ion battery bank (LiFePO4) and even get smaller/lighter than two golf cart batteries (more costly battery technology--But vary nice for RV use).

Anyway--An example of how your choices affect system design. Notice that 120 VAC vs 12 VDC is basically the 85% inverter efficiency--Not a big deal, and many times, a 120 VAC system is much easier to use and wire (at the cost of 1/0.85 or 1.25x larger system). An off grid solar power system runs from 10.5 to 16.5 volts--That wide of voltage range can cause may "12 volt car adapters" to fail--A single (good quality) AC inverter can make things much easier (and you don't have a bunch of 12 volt adapters--which can have their own losses too).

-Bill