自发自用，余量上网”是指光伏发出来的电，优先给负载使用，负载用不完，再发到电网。初次接触光伏的朋友，会有很多疑问：光伏发出来的电，怎么能保证优先给负载使用，而不是光伏电送入电网，负载从电网取电？有什么依据能说明负载使用的电是光伏电，而不是电网送过来的电？光伏发电和市电频繁切换，会不会影响逆变器或者设备？

1.同一时刻电流只有一个方向
      从电路原理上，电流都是从电压高的地方流向电压低的地方，在同一个时刻，电流的方向是唯一，就是说，电流在同一个点不能同时既流出又流入。我们以用户侧电表为节点，在同一时刻，电流只有一个方向，要么是光伏电流向电网，要么是电网的电流向负载。所以不存在同一时刻光伏电送入电网，负载从电网取电的情况。
2.光伏发电为什么能优先使用？
      光伏发电是一种电源，它可以输出电能，而且只能输出电能，而电网是一种特殊的电源，它既可以向负载提供电能，也可以作为负载接收电能，根据电流都是从电压高的地方流向电压低的地方这一原理，当光伏发电时，从负载上看，并网逆变器的电压始终比电网的电压要高一点，因此负载优先使用光伏发电，只有当光伏的功率小于负载功率后，并网点的电压才会下降，电网才会向负载供电。分布式光伏，自发自用，余量上网，一般要装两块电表，逆变器出来装一块电表，记录光伏发电，用户侧并网点装一块双向电表，用来记录光伏向电网送电以及用户向电网买电。
      并网馈电系统中，市电主要用于构建电网的电压/频率/相位。并网逆变器不输出电压，而是跟踪电网相位和波形，只向电网输出电流，因为光伏逆变器的电压比电网电压要高，根据电路原理，电流是从电压高的地方流向电压低的地方，所以只要光伏能发电，就一定会先送到负载端。
      从负载角度上，负载消耗电流，是从离自己最近的电流源获取电流。以屋顶系统为例，并网逆变器都是在市电变压器之后，当然是并网逆变器馈电，首先被利用。
3.光伏发电和市电频繁切换，会不会影响逆变器或者设备？
      由于光照不稳定，时大时光，光伏发电的功率也不稳定，因此负载用的电，有时可能是光伏电，有时可能是市电，有时可能是光伏电和市电同时供应，其实这是表面上的。从理论上，用户使用的电都是电网的电，因为逆变器有一个功能，能够把组件发出来的电，变成和电网完全相同的电，具有相同的电压，相同的频率，相同的相位。这个切换过程实际上也是不存的。
4.用户怎么区分正在使用的电，是太阳能光伏电还是电网电？
      从电能质量的角度上看，用户完全分辩不出自己用的电到底来自哪里。其实，也没有必要去区分到底是用哪一种电。如下图所示，并网点安装在变电房，并网点安装双向电表，电流有两个方向，可以计量光伏发出来的电，负载用了多少，上网送出多少。但厂房1到3，电流只有一个方向，就不能单独计量哪一个厂房光伏用了多少电，电网用了多少电。

Spontaneous self-use, surplus Internet "refers to the photovoltaic electricity, priority to load use, load is not used up, and then sent to the grid.   Friends who are in touch with pv for the first time will have a lot of questions: how can we ensure that the electricity generated by PV is preferentially used by the load, rather than the photovoltaic power sent to the grid, and the load receives electricity from the grid?  What is the basis that the load is using photovoltaic power rather than electricity from the grid?  Will frequent switching between photovoltaic power generation and mains affect the inverter or equipment?  

1. The current has only one direction at a time  

In electrical principle, the current flows from a high voltage to a low voltage. At the same time, the direction of the current is unique. That is to say, the current cannot flow out and in at the same point at the same time.   We take the user-side electricity meter as the node, and at the same time, the current only has one direction, either the photovoltaic electricity flows to the grid, or the electricity flows to the load of the grid.  Therefore, there is no photovoltaic power into the grid at the same time, the load from the grid.  

2. Why can PHOTOVOLTAIC power generation be used preferentially?  

Photovoltaic power generation is a kind of power, it can output power, and can only output power, is a special kind of power supply and power grid, it can provide power to the load, also can be used as a load to receive power, according to the current from high voltage low voltage to this principle, when photovoltaic, look from the load,   The voltage of the grid-connected inverter is always higher than that of the grid, so the load preferentially uses photovoltaic power generation. Only when the photovoltaic power is less than the load power, the voltage of the grid-connected node will drop, and the grid will supply power to the load.  Distributed photovoltaic, spontaneous self-use, surplus Internet access, generally to install two meters, the inverter out to install a meter, record photovoltaic power generation, user side and dot installed a two-way meter, used to record photovoltaic power to the grid and users to buy electricity from the grid.  

In the grid-connected feeder system, the mains is mainly used to construct the voltage/frequency/phase of the grid.  The grid-connected inverter does not output voltage, but tracks the grid phase and waveform, only output current to the grid, because the voltage of the photovoltaic inverter is higher than the grid voltage, according to the circuit principle, the current flows from the high voltage to the low voltage, so as long as the photovoltaic power generation, it will be sent to the load end first.  

From the point of view of the load, the load consumes current and obtains current from the current source nearest to it.  Take the roof system as an example, the grid-connected inverter is after the mains transformer, of course, the grid-connected inverter feed, the first to be used.  

3. Will frequent switching between photovoltaic power generation and mains affect the inverter or equipment?  

Because the illumination is not stable, the power of photovoltaic power generation is not stable in large time, so the electricity used for load may be photovoltaic power, sometimes it may be municipal power, and sometimes it may be photovoltaic power and municipal power supply at the same time. In fact, this is superficial.   In theory, all the electricity the user uses is grid electricity, because the inverter has a function to convert the electricity from the components into exactly the same electricity as the grid, with the same voltage, the same frequency, and the same phase.  This switching process is also virtually nonexistent.  

4. How do users distinguish between the electricity being used, solar photovoltaic or grid?  

In terms of power quality, users can't tell where their electricity is coming from.  In fact, there is no need to distinguish which kind of electricity is used.  As shown in the figure below, the connection point is installed in the substation, and the connection point is installed with a bidirectional meter. The current has two directions, which can measure the electricity sent by the photovoltaic, how much the load is used and how much the Internet is sent.  However, from plant 1 to 3, the current is only in one direction, so we cannot measure separately how much electricity is used by the photovoltaic power plant and how much electricity is used by the grid.