wayoverthere

This sums it up well; i've talked into a GMRS repeater and a 70cm ham repeater on Joaquin Ridge on the west side of the San Joaqin Valley from a hill on the east side of the valley (75ish miles) with 5 watt HTs.  I've also talked into another 70cm repeater on Fremont Peak from a bit further east, around a 115 mile shot over the valley on the 50 watt mobile in my truck.  
In both cases, i had clear line of sight over the valley, as i was sitting around 4500-5000 ft elevation, and all of those repeaters sit around the 3000ft mark on hills/ridges.

No, but that’s a myopic view. The U.S. government has regulations against conducting radio communications on the U.S. side of the border that violate international treaties that they have with other countries. So even though the communications you actually interfere with are in Canada, you can be penalized on the U.S. side. 

MDARC has a good explanation/drawing/info.
https://www.mdarc.org/regulations/line-a-line-c

This is true. I dd not mention it since he was asking about batteries for a 20 watt radio. The current draw of a 20 watt radio won't exceed the battery output of a 10 amp hour battery. 
I would not recommend trying to transmit on a 50 watt radio on high power. You will be at or above the 10 amps of the battery as most 50 watt radios draw 12-14 amps on transmit at high power. Some will draw 8 or 10 amps but why push one's luck. I will recommend a 40 or 50 amp hour battery for a 50 watt radio if you want it to last very long.
The more one transmits the quicker the battery will discharge.
People that do Parks On The Air with QRP radios with 5-10 watts output power will run smaller batteries but those don't last very long even then.

There's really 2 factors to keep in mind when choosing batteries. Overall capacity, and current draw.

@WRYZ926 mentioned the first, as this will play into how long you can run portable.  It's been a bit since I checked the numbers (and don't have the radios handy to check), but i want to say it was around 4a on high power, which would would buy something around 2.5 hours of transmit time on a 10aH battery.  realistically you're likely listening a lot less than transmitting, so the real world use will be longer.
Second consideration, generally, is current draw.  Less of an issue with these smaller radios, but we have to consider how much draw the battery can handle.  the cheaper LiPo batteries i have refer to not drawing more current than their aH rating, which for mine would be a 10a draw on the smaller, and 20a on the larger (mine are Ecoworthy, and the 10aH is roughly the side of a standard red brick).  
Here is a quick example, and their table claims 5.2aH, and can handle a 15.6a draw, and if one isn't transmitting much should be able to last a couple hours at minimum.

It all depends on how long you want to use the radio before having to recharge the battery. How much you transmit will also determine what size battery you will need.
I would start out with a minimum of a 10 amp hour LifPo 4 battery. That will last you from 5-6 hours or longer depending on how much you transmit on 20 watts.

Success! Looks like I may have not been transmitting with the correct offset! I manually input a repeater pair, and got a successful (though faint) signal check response. Excellent.

I've just ordered a programming cable so I can go ahead and program in my local repeaters more easily. But, at last I know the radio is working. I appreciate the help and patience everyone!
 

that's the direction i was thinking as well, just trying to turn the train of thought into something coherent.
i suspect the base programming on those uv5g will be similar what shipped on my Btech gmrs50x1, channels 1-22 were the 7 low power simplex frequencies (1-7), the 0.5 watt FRS frequencies (8-14receive only on mine), and then the 8 high power simplex channels (15-22).  Slots 23-30 were set up for repeater use, with the +5.0mhz transmit offset baked in, though they used the same receive frequencies as 15-22.
if i have a local repeater that the output is 462.575, i'll be able to hear it whether i'm on channel 16 or channel 24, because both receive on 462.575.  However, on transmit, channel 16 is transmitting on 462.575 as well, while 24 will incorporate the offset and have me transmitting on 467.575 (which is where the repeater is listening).

 

Actually, my favorite is a magnetic base with an NMO mount. Then I put a Comet 2x4 or some other quality antenna on it. 

The only other thing I can think of is the transmit frequency. The repeater channels receive on the same frequencies as 15-22, but transmit on 467.xxx MHz channels.

Here is the difference between the 2730A and 2730B for those interested.

I'll have to listen for those.  Thinking either DMR/Trbo, or P25 as likely possibilities.

Yeah, with the shape of the ground plane of the trunk (dependent on vehicle and frequency), I'd expect some directionality, given most will have more ground plane to the sides than front to back. id also expect the greenhouse to be blocking/shading the signal in some directions too.
It just struck me as a little odd that permanent mounting in that location would mean more loss than a mag mount in the same place, where on the roof it's the other way around.

You know, having seen that graphic many times, I just noticed a couple things I hadn't before. First, there's no rating for a hood lip or fender mount, just a mag mount. Second, the permanent trunk mount is actually rated more loss (-2.8db) compared to a mag mount on the trunk (-2.1db).

I absolutely agree. 

I mean, not really a surprise at that size, but I appreciate that they rate it honestly.

I mean, not really a surprise at that size, but I appreciate that they rate it honestly.

I recommend that you add @Socalgmrs to your “ignore list”.  Life gets better.  
Personally I prefer to have a link to the CFR page on the internet rather than capturing a point in time with PDFs:
https://www.ecfr.gov/current/title-47/chapter-I/subchapter-D/part-95

while there's been lots of valid points raised already, I'll mention something I noticed looking at the specs for that glass mount...wouldn't 1.5dBi effectively be negative gain antenna in the real world? (dBi vs dBd and all)
that aside, I've absolutely run into auto glass on newer cars impeding the signal, which would be an issue with that glass mount inside as well.  aside from the mentioned "hatpin" (uhf 1/4 wave, which are quite inexpensive), Midland's "ghost" (MXTA25) antennas seem to get decent reviews, and that would be fairly unnoticeable hard mounted on the roof without compromising ground clearance (they're less than 4" tall).

For those finding this thread in 2025, Chirp-NEXT now supports editing channels for the KG-805G. No settings for now. It's new support, so still proceed with caution and have a backup using the original software just in case. 

Should probably point here: https://forums.mygmrs.com/guidelines/
The other link was from before a pretty big site/forum upgrade, I suspect things have shifted around a bit. They're also in the menu in the top right, under the "browse" submenu, at least on mobile view.

I guess I've been lucky product wise, but both times I've ordered from Radioddity, I've gotten solid gear (a DMR handheld, and an HF transceiver), but shipping was the issue.
One order got hung up because they shipped DHL to my PO box, the latter was like $50 cheaper for a bundle with some good extras, and said they had us stock, but it shipped from China anyway and took like 3 weeks to get here.
The couple anytone at779uv I've bought (from Amazon, twin to the db20g) have been solid also

I have two Xiegu G90 radios. The first one I bought directly from Radioddity and I won the second G90 in Radioddity's 12th Anniversary raffle back in September. Both radios work great and shipping was very fast both times. I had a technical question and their customer service was very quick to answer my question. The G90 I purchased was one of their bundle deals that I just could not price match anywhere else.
That being said, I will buy from Amazon or other venders that have better customer service and better/easier return policies when possible. And I am will to pay up to $20 more for the peace of mind. Now if the difference is closer to $50 or more is a different story.
I have paid 10-20 dollars more for amateur radios by ordering from DX Engineering, Giga Parts, and R&L Electronics over finding the same radios elsewhere. Those three companies have great customer service and easy return policies.
I avoid eBay, especially for amateur radios since there is a good chance that you will get a Japanese or European version that doesn't work on US amateur radio bands.
 

Sometimes paying a few extra dollars is worth the peace of mind.

@OffRoaderX (aka Notarubicon) tested this myth on his YouTube channel and found that looping the cable didn't affect the SWR at all. That prompted me to test it myself with two different pieces of coax and three different radios from 5 to 35 watts, and I also found no change at all in SWR or transmission range. I'm about to put an antenna on my truck and I plan to leave a few extra feet of coax so I change the position of the radio, plug in an HT, or whatever else may come up. I'll just coil the extra into a loop and secure it with cable ties.