These two terms are quite commonly used interchangeably. Although the term “electric keyboard” can also be used to describe both, there are some definite differences between digital pianos and portable keyboards. The differences normally come down to features, size and cost.
Body and Keyboard Size
Portable keyboards, just as the same states, are portable. They normally weigh less than 20 pounds and have smaller ranges; 61 and 49 key models are among the most common. There are also some optional keyboard stands that may come with the instruments for an additional cost. They are easily storable and can be played table-top.
Digital pianos are however not storable, but they supposed to sit atop a custom fitted stand. Digital pianos can weigh anywhere in the range from 25 to 60 pounds, and normally come with 88, 61, or 78 keys.
Keys and Sensitivity
Keys on portable keyboards are very plastic and light when compared to digital or acoustic pianos. Touch-sensitivity, which enables volume change depending on how hard the keys are pressed—may not be a feature; and polyphony, which gives the ability to play many notes simultaneously is at times limited.
Keys on digital pianos are normally of full-length and feel like an acoustic. Certain models even replicate the piano feeling incredibly well, with heavier keys to touch in bass octaves, a feature commonly referred to as the “graded hammer-action.” Polyphony varies and may range anywhere from 32 to 64 notes, and at times even more; and touch sensitivity is standard and can even be adjusted.
Voices and Song Banks
Portable keyboards have many rhythms, voices and built-in songs, and most times will include a variety of organs, woodwinds, percussion and pianos; as well as other lovely random sound effects. You should keep in mind that voices can sound electric and phony, and the volume of the speakers may be limited.
Digital pianos on the other hand tend to offer few voices, and may not contain any rhythms or songs at all. The voices that are offered normally sound genuine (depending on quality and brand), and they tend to be the staples: harpsichord, grand piano, strings and pipe or jazz organ.
Price Comparison
Normally a portable playing instrument fetches a lower price tag, which hovers around $100 to $500 depending on the size of the keyboard, features and the brand. For instance, a Casio WPK with a 76key option will cost about $200.
Digital keys can be found around the $300 mark, but may also fetch in the range of $500 to $1500. An expensive digital piano may cost you as much as a whopping $20,000, and that will be grand piano/digital hybrid.
Bottom Line
A digital piano is a widely preferred cost-effective alternative to an acoustic piano. The best suited practice instruments for practice, beginners for just for fun are portable keyboards.