BMSBattery (China) sells a 48v 30AH Lipo battery pack [1] for $530 + $120 shipping, with delivery in 2-3 weeks. That's 1440 Wh! And the battery includes a "BMS", circuitry that prevents under/over-voltages, over-temperature, cell balancing, and allows for simultaneous charging and discharging. This last feature is really useful if you are using the battery with a solar setup and want to be able to source loads bigger than your panels can supply from time to time.
If you need batteries ASAP in the bay area, you can get similar products from AA Portable Power Corp / Batteryspace.com [2] located in northeast bay (caution - these do not include BMS so no simultaneous charging+discharging).
So, if you are spending $500 for the battery, you can get much higher capacity (Wh) Li-ion batteries around the same price point from suppliers serving the diy ebike market.
Caveat is that these batteries are typically in the 36-48v, so depending on your application, you may need to add a buck converter (to regulate voltage down to 12v or 5v) or a boost/ac inverter for 120v. Sometimes having a higher voltage can lead to some efficiencies vs lower voltages - can use thinner wires, less power loss over same wire lengths, voltage converters / inverters may be more efficient.
"DROK"-brand voltage converters are around $30 on amazon[3].
If you want to charge your 36-48v Li-ion battery, try the $38 MPT-7210A "Docooler® MPPT Solar Panel Battery Regulator Charge Controller 10A"[4]. If you have two ~20v panels, wire them in series so the panel voltage going to the charge controller is ~40v; it won't have to work as hard to boost the voltage for battery charging and will be more efficient.
[1]: https://bmsbattery.com/ebike-battery/715-high-power-48v30ah-... [2]: http://www.batteryspace.com/lithium-26650-battery-pack-50-4v... [3]: https://www.amazon.com/s/ref=sr_nr_scat_10967761_9119938011_... [4]: https://www.amazon.com/gp/product/B01HCL7LEW