Why can't I use solar PV system while disconnected from grid?
First a PV panel generates Voltage like a generator by separating positive and negative charges. It will do so in the moment when it is exposed to light. When the circuit between the different potentials is open-circuited, no current can flow, the PV panel will produce voltage but no power. Its just unused. So e.g. your panel produces 100V, then 100V*0A=0W.
Also most inverters are synchronized to net frequency. They do not produce it on their own. So if there is no power from the grid, the frequency information is missing and the inverter will not convert Power from the PV panel. Most power companys here in germany have much less rules and regulations for this type of inverters, because they are inherent safe (or at least more safe). So for customers these are &quot;cheeper&quot; to operate.

If the panels can sit at voltage but no current (like a battery), why does excess current need to be sent somewhere when a small amount of power is utilized (unlike a battery)? What prevents us from using only part of the PV power generated?

&quot;why does excess current need to be sent somewhere &quot; I never heard of that? Where do you get this information from?
My answer: Well nothing. You may use any fraction of energy you want. This is always the case if you do not consume all the power which can be generated and the power grid also does not need the power. But as soon as you or the grid demands the power generated it will be used. This is why home batteries are an idea in some use cases, as they will store energy when nobody needs it. So you can use it by night or to charge your electric car - even in the dark.
Also if you have a hot-water boiler for warm water and/or heating, you can feed energy into the water up until the temperature limit (in most cases around 90°C), while a heat pump only accomplish about 60°C. So from your PV panel you can increase your heat reservoir by 33%