Microinverters for single phase grid connections have an inherited issue which is commonly known as double line frequency problem. The problem is about the power pulsations of the AC grid which is double the line frequency. This power is time varying in nature while the power output from the PV module is constant (MPPT is used ). This is shown in Fig.1. This means that there must be some storage device for balance of power. This storage device is often called as decoupling capacitor.

Power composition

The simple multiplication of current and power yields that the AC power oscillates at double the line frequency which may be reflected on the PV side and can badly affect the performance of the MPPT. Therefore, an energy storage element usually a capacitor is required whose value can be calculated as shown in equation below.

C_d= P_in/(ω_oV_dc∆_V)

Where

P_in = Rated power of the PV module
ω_o= Angular frequency of the grid.
V_dc= DC voltage level across the capacitor
∆_V= Allowable peak to peak voltage ripple.

In conventional method a large capacitor usually in mF is used to make sure a ripple free DC  link for a smooth MPPT operation.

Reference:

Haibing Hu; Harb, S.; Kutkut, N.H.; Shen, Z.J.; Batarseh, I., "A Single-Stage Microinverter Without Using Eletrolytic Capacitors," Power Electronics, IEEE Transactions on , vol.28, no.6, pp.2677,2687, June 2013