离网运行光伏发电系统，一般都要配置蓄电池组作为储能装置。蓄电池的作用是将太电池方阵在有日照时发出的多余电能储存起来，以供晚间或阴雨天时负载使用。蓄电池容量是指其蓄电的能力，通常用该蓄电池放电至终了电压所放出的电量大小来度。铅蓄电池的使用容量是在一定的工作条件下所放出的电量，铅蓄电池使用容量与厂家造质量及电池工作条件有关。确定离网光伏系统蓄电池容量最佳值，必须综合考虑太阳电方阵发电量、负荷容量及直交变换装置（逆变器）的效率等。蓄电池容量的计算方法有种，一般可通过下式求出。

C－蓄电池容量，kW·h（Ah）；D－最长无日期间用电时数，h；F—蓄电池放电效率的修正系数，（通常取1.05）；PO－平均负荷容量，kW；Ｌ为蓄电池的维修保养率，（通常取0.8）；U为蓄电池的放电深度（通常取0.5）；Ｋα为包括逆变器等交流回路的损耗率（通常取0.7～0.8）。上式可简化为：

 
      这就是由平均负荷容量和最长连续无日照时间（用电使用时间）求出蓄电池容量的简计算公式。应当注意到，无日照时间取多长为恰当。若取得过长，计算出的蓄电池容量大，不必要地加大投资。而且，若蓄电池容量增大，必须使太阳电池的容量相应加大，使者容量相匹配才合适；否则相对的蓄电池充电速率减小，蓄电池总是充不满也会影响使用命，因为蓄电池在接近满充电的状态下使用较好。因此，过长预估无日照时间以为安全，实际并不恰当。一般地说，取3倍于太阳电池出力的蓄电池出力认为是比较适当的。由于蓄电池容量一般以安时数表示，故蓄电池容量应该为：

       为蓄电池容量，A·ｈ；V为光伏系统的电压等级（系统电压），通常为12V、24V、48V、110V或220V。
         案例1：按广东星火太阳能电源有限公司提供的晶体电池组件，对广东耐普电池股份有限公司的阀控式密封铅酸蓄电池进行选型。基本要求为：可为400W的负载连续5天阴雨天的情况下供电；蓄电池能放电到其额定容量的75%-80%，性能正常，并保证具有5年使用寿命。

耐普AGM电池放电容量如表4-3所示。

      表1 AGM电池的放电容量/Ah。 

  逆变器的转换效率为0.75，负载为400W,故实际所需功率为400/0.75=533W。功率400W太阳能电池方阵用蓄电池选型容量计算如下：电压为24V，则电流I=533/24=22.2A。  如果连续使用5天，即120h，则放电容量为22.2*120=2664Ah。如果按电池的80%利用率计算，

则对电池的额定容量要求为：容量C=2664/0.8=3330(Ah)

正常使用情况下，按照此设计，正常白天充电，晚上蓄电池放电（以放电12h为例）的情况下（负载工作6-12h），逆变器转换效率按75%进行计算，该蓄电池的放电深度为：U=22.2*12/3330=8.0%
       方案1：用2组1500Ah电池并联使用。上述测试数据可以看出，1500Ah的电池容量比较富余，其10h容量平均可以达到1700Ah，如果采用2组并联，容量可以达到3330Ah以上，可以满足要求。这一方面的优点是容量可以达到要求并有点富余，同时只需要2组电池，维护相应较少，电池占的空间要少。
       方案2：用4组800Ah电池并联使用。800Ah电池10h的放电平均容量为880Ah，如果采用4组并联，其容量可以达到3500Ah，足以达到3330Ah，容量比较富余。这一方案优点是使用4组为800Ah的电池并联，容量更充分、富余；其缺点是要并联使用4组电池，相对于1500Ah的电池，成本要增加，所占用的场地或则体积和空间要增加，维护工作也要大些。

Off-grid operation of photovoltaic power generation system, generally to configure battery pack as energy storage device.  The role of the battery is to store the excess energy generated by the battery array in sunlight for use at night or in rainy days.  Battery capacity refers to its capacity to store electricity, usually with the battery discharge to the terminal voltage of the amount of electricity released.  The service capacity of lead battery is the amount of electricity released under certain working conditions. The service capacity of lead battery is related to the quality of the manufacturer and the working conditions of the battery.   To determine the optimal capacity of battery in off-grid photovoltaic system, the solar array generation, load capacity and efficiency of DC converter (inverter) must be considered comprehensively.   Battery capacity calculation method, generally can be obtained by the following formula.   

C- Battery capacity, kW·h (Ah);  D- Maximum number of hours of electricity consumption without a day, h;  F -- Correction coefficient of battery discharge efficiency, (usually 1.05);  PO- Average load capacity, kW;  L is the maintenance rate of the battery (usually 0.8);  U is the discharge depth of the battery (usually 0.5).   Kα is the loss rate of ac circuit including inverter (usually 0.7 ~ 0.8).  The above equation can be simplified as:   

This is the average load capacity and the longest continuous sunshine (use time) to calculate the battery capacity simple formula.  It should be noted that the length of time without sunshine is appropriate.  If obtained too long, the calculated battery capacity is large, unnecessarily increase investment.  Moreover, if the capacity of the battery increases, the capacity of the solar cell must be increased accordingly, and the capacity of the messenger is appropriate to match;  Otherwise the relative battery charging rate is reduced, the battery is always dissatisfied will also affect the use of life, because the battery in close to the state of full charge is better used.  Therefore, it is not appropriate to estimate the time without sunshine for too long as it is safe.  Generally speaking, take 3 times the output of the solar battery output is considered to be more appropriate.  As the battery capacity is generally expressed in ampere hours, the battery capacity should be:   

Is battery capacity, A·h;  V is the voltage level of the photovoltaic system (system voltage), usually 12V, 24V, 48V, 110V or 220V.  

Case 1: The valve-controlled sealed lead-acid battery of Guangdong Knapp Battery Co., LTD was selected according to the crystal battery module provided by Guangdong Spark Solar Power Supply Co., LTD.   The basic requirements are: can supply power for the load of 400W for 5 consecutive days under cloudy and rainy conditions;   The battery can discharge up to 75% to 80% of its rated capacity, with normal performance and guaranteed service life of 5 years.   

Table 4-3 lists the discharge capacity of the KNap AGM battery.  

Table 1 Discharge capacity of the AGM battery /Ah.   

The conversion efficiency of the inverter is 0.75, and the load is 400W, so the actual power required is 400/0.75=533W.  The selection capacity of battery for solar cell square array with 400W power is calculated as follows: voltage is 24V, current I=533/24= 22.2a.   If it is used continuously for 5 days, i.e. 120h, the discharge capacity is 22.2*120=2664Ah.  If you use 80% of the battery,       

The rated capacity of the battery is: Capacity C=2664/0.8=3330(Ah)   

In normal use, according to this design, when the battery is normally charged during the day and discharged at night (for example, the battery is discharged for 12h) (the load is 6-12h), the inverter conversion efficiency is calculated as 75%, and the discharge depth of the battery is: U=22.2*12/3330=8.0%  

Solution 1: use two 1500Ah batteries in parallel.  As can be seen from the above test data, the battery capacity of 1500Ah is relatively surplus, and its 10h capacity can reach 1700Ah on average. If two groups are used in parallel, the capacity can reach more than 3330Ah, which can meet the requirements.   The advantage of this aspect is that the capacity can meet the requirements and a little surplus, at the same time, only two batteries are needed, so the maintenance is correspondingly less, the battery takes up less space.  

Scheme 2: Use four 800Ah batteries in parallel.  The average discharging capacity of an 800Ah battery in 10h is 880Ah. If four groups are used in parallel, the capacity can reach 3500Ah, which is enough to reach 3330Ah.  The advantage of this scheme is that four groups of 800Ah batteries are used in parallel, and the capacity is more sufficient and surplus.  Its disadvantage is to use four groups of batteries in parallel. Compared with the 1500Ah battery, the cost should be increased, the site or volume and space occupied should be increased, and the maintenance work should be larger.