Review: relatively low cost for its power storage, and fairly robust. I chose this one because I need recharging on the go, especially backpacking. Last week, I ran it down to one car with 3 charges, mine and other people’s phones.
Since then, it’s been propped up on a south facing window sill in bright sunny May conditions.
Seven days later, it has built up 1 more bar of charge. I accept that these charge indicators are arbitrary, but will it really take three weeks to charge up in full sunshine?

After 1 week in full sun.