There are standards, many of them, just as there are many different standards for AC power worldwide. Your DC system design will be governed by practical technical considerations in concert with economics. The physics of low-voltage systems are such that they are viable only when current is low; otherwise, a 5V or even 12V system will require prohibitively expensive heavy-gauge wire in order to provide the proper voltage at the point of use. So modern DC systems are typically all 48V or even higher now (people are building solar systems with system voltages of 600VDC!). That does not mean you need to run those higher voltages within your home, but from a practical perspective you probably want either 24V or 48V unless your house and/or current needs are very small. To figure this out, you need to consider the following:
- What devices do you need to supply? Often this is the true governing factor although switching regulators are very efficient and inexpensive up to a few amps.
- How much current do you need to supply to a particular location (this depends on the power consumption of what you're running)?
- What is the distance from the panel to the point of use? The longer the distance, the greater the voltage drop across a wire of a given size (and the higher the cost of larger wire).
As for connectors, there are only a couple of sensible answers. USB is fine for 5V/1A needs, but you're not going to want to run a bunch of 5V wiring separately from the higher-voltage wiring you're going to need anyway. The proper approach here is something like http://www.powerwerx.com/adapter-cables/usbbuddy-powerpole-1..., which will happily work in either a 12V or 24V nominal system. You can of course make other power supplies from all-in-one ICs like http://www.mouser.com/ProductDetail/RECOM-Power/R-78W90-05/?... and a small project box; larger currents and other voltages are available too, of course. These make good replacements for wall warts.
But you don't want to be wiring any of that in your house; instead, you want to use Anderson Powerpoles in a single-voltage (probably 24V or 48V) system. They are properly rated for DC use at these voltages and plenty of current (up to 350A if you need it, which you won't). They can be installed in blocks of 4 2-blade connectors in standard wall boxes, with neat, professional plates. They can be easily crimped onto appropriate-gauge wire by amateurs. They are code-compatible and safe, unlike the dangerous practice of using receptacles designed for 120VAC or some other existing local/regional standard. They provide a reliable connection and reliable disconnection, and if crimped properly they will not fray, crack, or loosen within a very large number of connect/disconnect cycles. The other low-voltage DC "standard" that is popular, the barrel-type "cigarette lighter" connector, provides only 7A at 12V, is bulky, and does not offer a reliable connection. While it is popular in automotive applications, more serious users of DC power -- the Powerpole is very popular among amateur radio enthusiasts -- avoid the barrel type connectors for these reasons.
The only real decision to make is whether to use 24V or 48V, which will depend primarily on the questions I noted above. Higher voltages are certainly possible, but watch out! Switches rated for higher DC voltages (especially at currents much more than 1 or 2A) are hard to find and expensive. Your standard "AC quiet switch" that you can buy for $2 at the local hardware store is rated for 125V AC ONLY. It is not safe to use with any DC system, although in practice it's probably acceptable for 12V systems at a hundred milliamps or so. For more practical applications, you will need to be sure that your equipment is equipped with proper switches, or no switches at all; you will also need to make sure that any hard-wired DC circuits (such as for lighting) are properly switched. Where you have flexibility in the voltage accepted by your equipment, you will need to trade off the higher cost of switches and other passive components at higher voltages against the higher cost (or voltage drop) associated with wiring at lower voltages. You will quickly learn to read spec sheets and rating stamps carefully when you work with DC.
The last thing I'll mention is that you may not really want to do this. At my location, there are often several months with extremely limited power; even the inverter's 25W base consumption dictates that it be on an hour a day or less if at all possible. For that reason, anything I have that needs smallish amounts of power continuously (such as a freezer, reading lamp, phone charger, etc.) gets DC. But in general, it's simply more convenient to use off-the-shelf devices. I can and do make my own power supplies, but I don't really want to go re-power my laser printer or table saw (which by the way has an AC-only induction motor in it). For higher-power devices, off-the-shelf is the way to go; it's much cheaper and the inverter's overhead is amortized over a lot of consumption anyway. If you have a larger system that can easily supply 100W or more on an indefinite basis, you probably don't need to bother much with DC; it'll be easier to just leave the inverter running all the time. A DC-powered well pump or freezer might be worthwhile, but I wouldn't go converting anything else. Most people are putting in enormous (to me) solar systems now; 10kW is common. With a system that large, you'll probably be fine even if it's overcast. For reference, I have 700 watts, and with endless stretches of 6-hour overcast days in winter, DC is the only way to fly. But your needs are likely to be very different indeed, especially if you have (as we're discussing here) adequate storage. The days of custom-wired 12VDC off-grid living are basically over unless your budget is extremely limited.