The most accurate state of charge reading usually comes from a battery monitor that tracks amp-hours in and out with a properly installed shunt, not from a simple voltage display. Voltage alone can be misleading, especially with lithium batteries, because the battery can sit at a fairly flat voltage for much of its discharge cycle and then drop quickly near empty. That is why many people think their generator is still half full when it is actually much lower, or the other way around after charging.
If you want the most trustworthy answer, look for a monitor that uses a shunt-based coulomb counting method and lets you set the correct battery capacity, charge efficiency, and full-charge parameters. In real-world use, that kind of setup is usually far better than the built-in bars or percentage screen on many solar generators. The catch is that it only stays accurate if it is calibrated correctly. If the monitor is not told the true battery size, or if the battery is never fully charged to the right endpoint, the percentage will drift over time.
For lithium iron phosphate systems, accuracy depends a lot on how the monitor defines “full.” Some batteries do not spend long at the top of charge, so the monitor needs a reliable way to recognize full charge and reset its count. If your system charges from solar, this can be tricky on cloudy days because the battery may never actually reach the reset point. In that case, a good monitor can still be accurate enough, but you may need to fully charge it from time to time so it can resynchronize.
If you are choosing between options, a dedicated shunt-based monitor is usually the safer bet than relying on the generator’s built-in percentage indicator. Built-in displays are often estimates based on voltage, internal algorithms, or a mix of both, and those estimates can be decent but are rarely precise under load. High current draws, inverter losses, and temperature changes all make percentage readings less dependable. A proper monitor will at least show you the real current flow and accumulated usage, which is much more useful than a vague battery icon.
For everyday use, I would trust a well-calibrated shunt monitor the most. It will not be magical, but once it is set up correctly, it gives a much more honest picture of what is left than a voltage-only meter. If you already have a solar generator with a built-in display, compare it against a shunt monitor for a few cycles and see how far off it is under different loads. That comparison usually makes the answer obvious pretty quickly.
