Have home solar. Can I use some to charge a portable backup battery?

Yes, plug in your new battery into any outlet in the house and charge that battery when the Tesla system is generating power. The solar panel generation goes to all your loads in the house before anything is exported.

I agree with Ampster. Just us an electrical outlet to charge your backup battery. Of course that won't work if the grid is already down because most Grid Tied inverters stop working at that point unless you have a hybrid that is designed to charge a battery if the grid is down.

hi, sorry, I didn't finish my thought completely. I would want to use the solar to charge the battery when the grid was down.

That changes things completely since the grid tie inverter would not work. I don't know enough about your grid tie system and how accessible those panels are. It may be a less expensive option to find some panels that would fit the input parameters of your yet to be purchased battery. You would need to match the solar output to the input parameters of whichever product you purchase. That is the concept. The two parameters are voltage and Amperage.

Unfortunately unless you have an hybrid inverter that will work while the grid is down the only other way I know to charge a battery is to use a generator.

I have both a small battery system to use at night and a generator that can be fired up during the day

Forget the grid tied inverter for trying to charge off grid, its not going to happen. If you want to spend thousands if not tens of thousands there is way by installing a hydrid inverter with batteries that support AC coupling like a Sunny Island. When the grid is down, an AC coupled system will create a local microgrid that will trick the Solar City Inverter into producing power and you can do what you want with it. Far better to buy a $500 generator and a gas can.

Save your money and don't buy a portable backup battery, just buy a generator and some earplugs.

I would look into a dual fuel generator that can run on gasoline, propane or natural gas. I have a 12kw one that I fire up every couple of months using a portable propane tank. It should provide just about all of my home loads should the grid go down.

Any recommendations on earplugs?

I did about the same, 11KW gasoline or propane. I figure letting it sit
a long time is OK on propane, gasoline tends to muck up. So the gas
tank has stayed empty till I really need it, meanwhile there is a 250
gallon propane tank hooked to the car shop for possible long runs.
Battery maintainer in place at 13.7VDC, will not drain the battery if
power goes out.

OK so I need to at least test it. My earlier gas gen set was used 4
times in the previous century, never in this. Bruce Roe

I run mine to keep the starting battery charged. So far it starts up real nice because I use the propane. So far I haven't needed to run it long but with hurricane season coming there is no guarantees.

Or remove your hearing aids when the generator is running.

But, seriously, like a lot of other things in life, (neighborhoods, cars, etc.) lower noise levels cost money.
In general, buy cheap and expect more noise.
Don't buy a cheap generator, place it next to the house or on the back porch and expect it to be quiet without some additional noise attenuation measures.
Some thoughts from someone who once designed sound attenuation equipment for a living:
1.) Ironically and for lots of reasons, smaller generators (portable) often make more noise than larger ones.
2.) Some things to consider as an adjunct to simply buying a generator at big box and expecting a quiet experience:
- Put (buy or build) a sound absorbing enclosure around the generator. These are designed and fit for purpose if appropriate for the unit. Google for some ideas but watch out for the u-tube garbage ideas.
- Put the generator on a concrete pad and use isolation mounts on the unit if local codes allow it. (seismic considerations and overall design may need to be considered).
3.) Watch out for advertising that preys on customer ignorance with respect to sound pressure levels and their perception. Adverts may say a product reduces noise levels by 50% and usually lead people to infer that noise is reduced by half. Well, it ain't necessarily so. Sound pressure levels must be reduced an order of magnitude to achieve a 10 Db reduction which is perceived as half as loud. A 50% reduction in sound pressure level is not a 50% reduction in the perception of the sound.
4.) Another game that's played is to not mention the distance from the source when measuring dB levels which, in an open environment, which noise level decreases roughly as the square of the distance from the source.
5.) All that is a result of the math. Db (decibel) is a logarithmic unit (to the base 10) that's used to measure sound pressure levels and is originally used in analysis and design of electrical design and electronic stuff and other things.
The Richter (earthquake) scale is another such logarithmic application. A Richter 6 quake carries 10 times the energy of a Richter 5 quake for example, but probably won't do 10 times a much damage, maybe (and very subjectively) about 2X as much.
Sor sound, that logarithmic (base 10) property means a sound needs to be 10 times as intense to be perceived as twice as loud.

Take what you want of the above. Scrap the rest.

I note some of those backup generators automatically run periodically,
I think I saw one on natural gas doing 5 minutes every week. I consider
that far too often, 5 minutes might be the needed warm up cycle, but
every 3 months would better suit me. Why wear it out and use so much
fuel?

As for the starter battery, running the IC engine is an extremely inefficient
way to keep it charged. Long ago I took that approach, but failure to
keep schedule resulted in a lot of pre mature battery replacements every
year. Using tiny maintainers ended that. My original maintainers wasted
most of the line drain, but the switch mode maintainers I recently upgraded
to are very efficient, also feature no drain back if they get unplugged/lose
power.

Long ago there was a recommendation to bury small engine exhaust
pipes in a hole filled with coarse gravel. Better mufflers really help.
Some of us are not quite ready to admit we need a hearing aid.
That Bell was named after Alexander Graham, then divided by 10
for whole number decibels. Bruce Roe