"Solah" means sixteen and "Kalaa" means potency or power. Combining both terms, "Solah Kalaa" means sixteen potencies or powers. These sixteen potencies are indicated to be as follows: (1) Gnosis, (2) Dhiyan or deliberation, contemplation, concentration etc., (3) pious deeds, (4) perseverance, (5) self-discipline, (6) faith, (7)charity, (8) education or Ilam, (9) meditation or Simran, (10) sublime love, (11) celibacy, (12) spirituality, (13) compassion, (14) pledge or vow, (15) discrimination or Bibek-Budhi, (16) pure intellect.

Now the question one may ask is, "Is God limited only to sixteen potencies or powers?" The Gurbani says no, because, according to the Gurmat, God's potencies are "Anat" (Infinite) or "Biant" (Limitless). As such His powers cannot be limited to sixteen or any other finite number. Therefore, "Solah Kalaa" appears to be just an expression to indicate absolute completeness or perfection. It's like saying "hundred percent", "Sau Fee Sadee", "Solah Aanay" (in old days Indian Rupee was equal to Solah Aanay. People from that era still use the expression "Solah Aanay" to indicate completeness of whatever they are talking about", and so on.

• solh klw sMpUrn PilAw ]Anq klw hoie Twkuru ciVAw ]And ibnod hir nwim suK nwnk AMimRq rsu hir BuMcnw ]16]2]9]: Solah kalaa sanpooran faliaa. Anat kalaa hoi thaakur charriaa. Anad binod har naam sukh Nanak amrit ras har bhunchanaa ||16||2||9||: The sixteen powers, absolute perfection and fruitful rewards are obtained, when the Lord of Infinite Power is revealed. The Lord's Name is Nanak's bliss, he drinks in this Amrit of the Lord. ||16||2||9|| (aggs 1080).
• Agm Agocru ibAMqu suAwmI qw kw AMqu n pweIAY: Agam agochar biant suaamee taa kaa antu na paaeeai: The Lord is Inaccessible, unfathomable and Infinite. His limits cannot be found (sggs 1338).

One (the Gurmukh) realizes these potencies in his ultimate spiritual evolution by becoming one with God. That is, not as an individual (Haume being) separate from God, but by linking or merging with Him. Certain powers like creation, destruction (Pralaya), preservation, revealing, concealing etc., seems as God keeps for Himself.

• Awpy kudriq kry swij: Aape kudarat kare saaj: By His Creative Power, God fashioned the creation (sggs 1170).
• Epiq prlau pRB qy hovY ieh bIcwrI hir jnI: Oupat paralou prabh te hovai ih beechaaree har janee: Creation and destruction come only from God; this is what the Lord's humble servants believe (sggs 1186).
• Anidnu swir smwil hir rwKih gurmuiK shij smwvixAw: Anadin saar samaal har raakhahi Gurmukh sahaj samaavaniaa: Night and day, the Lord takes care of us and protects us. By becoming the Gurmukh, get absorbed in Sahaj: Intuitive Peace and Poise (sggs 112).
• khw gupqu pRgtu pRiB bxq bxweI: Kahaa gupat pragat prabh banat banaaee: Sometimes He is concealed, and sometimes He is revealed; thus God has made the world of His making (sggs 160).
• ijsu vyKwly soeI vyKY nwnk gurmuiK pwie: jis vekhaale soee vekhai Nanak Gurmukh paai: They alone see the Lord, unto whom He reveals Himself. O Nanak, the Gurmukhs realize Him (sggs 36).

The moon also goes through sixteen phases. After attaining all the sixteen phases it attains fullness or completeness. The night it attains completeness is called "full moon night or Punaya". These sixteen phases of the moon are called (1) Amritaa, (2) Maanadaa, (3) Pooshaa, (4) Pushati, (5) Tushati, (6) Rati, (7) Dhariti, (8) Shashani, (9) Chandaricaa, (10) Kaanti, (11) Jayotasana, (12) Sri, (13) Preeti, (14) Angadaa, (15) Poorantaa, and (16) Pooranaamritaa.

There is the expression in the Vaars of Bhai Gurdaas, which goes like "moon enters the house of sun". With the rise of the sun, the moon fades away or the moon becomes veiled or dormant, leading to the moonless night (Amaavas). Similarly, when man's Pure Nature, Soul Consciousness, Intuitive Peace and Poise ("moon") enters the material nature of Maya ("sun"), the Soul Nature ("Joti-Svaroopa" or one's True Nature as Pure Consciousness) fades away and the illusion (Maya), egoism (Haume) or Bikaars (mind's negative propensities) become dominant. Moonless night is metaphor for our darkness of ignorance or falsehood (egoism). When our mind is overtaken by the darkness of egoism, the "moon" of Truth is not to be seen anywhere! But the moment the egoism begins to thin out, the "moon" of Truth begins to rise and gradually grows to it's fullness; bringing Divine Virtues like Peace, Contentment, Knowledge, Compassion, Inner Inquiry (Vichaar), etc.

• solh klw sMpUrxo sis Gr sUrj ivrqI hwxI: Solah kalaa sampoorano sasi ghar sooraj viratee haanee: When moon, the master of sixteen phases (Pure Nature) enters into the sun (material nature), it gets faded (Vaar Bhai Gurdaas, 7).
• kUVu Amwvs scu cMdRmw dIsY nwhI kh ciVAw: koorr amaavus such chu(n)dhrumaa dheesai naahee keh charriaa: In this dark night of falsehood, the "moon" of Truth is not be seen anywhere (sggs 145).

Another expression used in the Gurbani and the Vaars of Bhai Gurdaas is "sun enters the house of moon". When the moon is out in its full glory, the sun is not to be seen. Similarly, if man's quality of passion and ignorance ("sun") enter into the quality of Goodness ("moon"), the qualities of passion and ignorance fade away and the quality of Goodness becomes dominant. After transcending egoism and all modes of material nature (Maya), one becomes established in inner Purity, Transcendental Consciousness, Sahaj Avasthaa (Turiya, Inner Peace, Intuitive Poise) or Soul Nature. As indicated in the Gurbani, the sun due to its hot nature is metaphor for the fiery ego or Raajas and Taamas Guna (passion and ignorance, respectively). Whereas the moon because of its cooling and soothing nature is metaphor for the mode of Goodness (Saatav Guna).

• ikau mUlu pCwxY Awqmu jwxY ikau sis Gir sUru smwvY]gurmuiK haumY ivchu KovY qau nwnk shij smwvY: Kio mool pashaanai aatam jaanai kio sasi ghari soor samaavai. Gurmukhi hayumai vichahau khovai tayu Nanak Sahaj Samaavai: "How can the Root, the Source of all be realized? How can the Soul know itself? How can the sun enter the house of the moon?" By becoming a Gurmukh (Spiritual Being) eliminate the egoism from within; then, O Nanak, you naturally enter the Sahaj Avasthaa--sun (Maya's fire) enters into the home of the moon or Soul Nature (sggs 945).
• bwrh solh myl kr ssIAr AMdr sUr smwXw: Baarah solah mel kar saseear andar soor samaayaa: Combining twelve (months) and sixteen (phases of moon) the "sun" merges into the "moon" — Maya's material nature gets absobed into the Pure Being (Vaar Bhai Gurdaas, 7)
• eykwdsI eyk ids DwvY]...bwris bwrh augvY sUr]Aihinis bwjy Anhd qUr]dyiKAw iqhUM lok kw pIau] Acrju BieAw jIv qy sIau: Ekaadasee ek dis dhaavai...Baarasi baahar oogavai soor. Ahinis baajai anahad toor. Dekhiaa tihoon lok kaa peeou. Acharaj bhaiaa jeev te seeou: The person who engages in remembering One Divine ("ek dis dhaavai"), the complete Self-knowledge menifests in his bosom, its like twelve suns have arisen in his inner being! Day and night, he hears the celestial bugles vibrating the unstruck melody within, and he beholds the Father of the three worlds. This is so wonderful! A common man thus becomes God! (sggs 344).
• AWgin myrY soBw cMd]inis bwsur ipRA sMig Anµd: Aangan merai sobhaa chand. Nisi baasur priya sangi anand: Within the courtyard of my Heart, the glory of the moon of Self-realization has shined forth. Night and day, I am in Bliss with my Beloved (sggs 372).
• pUinau pUrw cMd Akws]psrih klw shj prgws]Awid AMiq miD hoie rihAw QIr]suK swgr mih rmih kbIr: Poonio pooraa chand akaas. Pasarahi kalaa sahaj paragaas. Aadi ant madhi hoi rahiaa theer. Sukh saagar mahi ramahi Kabeer: The ever present (beginning, middle and end) Divine is the Ocean of Peace, O Kabeer, immers in that Ocean and meditate on Him. By doing so, the Sahaj Avasthaa will shine forth within, just as on the day of the full moon, the moon shines forth in its full glory (sggs 344).

Also, the term "Sixteen" is found in the SGGS associsted with "Solah Singaar" — sixteen adoration. In addition, it's also found associated with "Solahe". These are the Shabads generally containing sixteen stanzas (for example, see SGGS page 1021, mwrU solhy mhlw 1).

• sKI kwjl hwr qMbol sBY ikCu swijAw]solh kIey sIgwr ik AMjnu pwijAw]jy Gir AwvY kMqu q sBu ikCu pweIAY]hirhW kMqY bwJu sIgwru sBu ibrQw jweIAY: Sakhee kaajal haar tambol sabhai kishh saajiaa. Solah keee singaar ki anjan pajiaa. Je ghar aavai kant ta sabh kish paaeeai. Harihaan kantai baajh seegar sabh birthaa jaaeeai: O my companion, I prepare everything to decorate my body: make-up, garlands and betel-leaves. I embellish my body with the sixteen decorations, and apply the mascara to my eyes. (but, only) If my Divine Beloved comes to my inner Home, then I obtain everything. Without Realization of the Divine Beloved, all these adornments of the body are useless (sggs 1361).

— T. Singh