Do Rahu and Ketu have shadbala?

Not in the classical scheme, which covers the seven grahas from the Sun to Saturn. The nodes have no physical body and several of the six components are undefined for them.

Is a higher shadbala always better?

It measures capacity to act, not benevolence. A strong malefic placed badly acts strongly in a way the chart may not welcome. Strength says how forcefully a placement expresses, not whether the expression is pleasant.

Shadbala

Shadbala means six strengths. It scores each of the seven classical grahas by adding six independently computed components, producing a total in units called rupas that can be compared against a required minimum for each planet.

The six components

Sthana bala — positional strength, from sign dignity, divisional placement and similar. Dig bala — directional strength, which depends on the house: Jupiter and Mercury are strongest in the first, the Sun and Mars in the tenth. Kala bala — temporal strength, from day or night birth, the lunar month, the year. Chesta bala — motional strength, largest near retrogression. Naisargika bala — a fixed natural ranking, the Sun strongest down to Saturn. Drik bala — the net effect of aspects received.

Reading the total

Each planet has its own required minimum in rupas — the Sun 5, the Moon 6, Mars 5, Mercury 7, Jupiter 6.5, Venus 5.5, Saturn 5. A planet meeting or exceeding its requirement is considered able to deliver what its placement promises. The ratio of actual to required is more informative than the raw total, since the requirements differ.

Where it disagrees with intuition

Shadbala regularly rates a planet strong that looks weak by sign alone, because five of its six components have nothing to do with sign dignity. That is the point of computing it: it is a check on a reading made by eye, and the components are itemised here so it is possible to see which of the six is carrying the total.

Questions

Do Rahu and Ketu have shadbala?: Not in the classical scheme, which covers the seven grahas from the Sun to Saturn. The nodes have no physical body and several of the six components are undefined for them.
Is a higher shadbala always better?: It measures capacity to act, not benevolence. A strong malefic placed badly acts strongly in a way the chart may not welcome. Strength says how forcefully a placement expresses, not whether the expression is pleasant.

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Ashtakavarga — Bhinnashtakavarga and sarvashtakavarga explained — how bindus are assigned, what the 337-point total means, and how the numbers are used for transits.
Navamsha — the D9 chart — The navamsha divides each sign into nine parts of 3°20'. What D9 shows about marriage, inner strength and whether a promising placement actually delivers.
The kundali (birth chart) — A janam kundali is a map of the sky at a birth moment. How the rashi chart is built, why birth time matters to the minute, and how sidereal positions are computed.