Spoiler:

Spoiler:

カエル「これは古代の
太陽石ってやつか？
全く光らなくなってるな。
もう一度光をあて続ければ
戻るかもしれないな。
けど、太陽の力をためるって
どんくらいの時間が必要なんだ。
Frog: This is that Solar Stone
thing from the Ancient Era?
It's stopped shining entirely.
Keeping it exposed to light
again might restore it.
But such time it would need
to store up the sun's power.
Frog: 'Tis the ancient Sun Stone!
But it lacketh luster.
Perhaps the sun can rekindle it!
But how long will that take?
ロボ「これは古代の太陽石？

光を失っているようデス。
しかしもう一度、光をあて続ければ
エネルギーをとりもどせるかも
しれません。
私のすいそくでは６５００万年
以上は必要でしょう。
Robo: This IS the Solar Stone of the
Ancient Era? It SEEMS that it has
lost its light over this long time.
However, if we continue exposing
it to light once again, it may BE
able to regain its energy.
By my conjecture, it WILL likely
require at least 65 million years.
Robo: This is the ancient Sun
Stone?
The years must have robbed it of its
sparkle.
But if we expose it to sunlight for
approximately 65.2167 million years
we just might be able to recharge it.
Putting the Moonstone in the Sun Keep will charge it and turn it into the Sunstone, though this takes many millions of years. With a time machine though, this makes it much easier. Robo has a height of 189cm.
Spoiler:

(Relevant text)
Spoiler:

Official Name: Prometheus
Model Number: F-66Y
Height: 175cm
Weight: 189kg
Spoiler:

179 pixels = 1.75m
1 pixel = 1.75m/179 = 0.00977653631m
0.00977653631m X 100 = 0.977653631m
0.977653631m/2 = 0.488826816m

Surface area of a sphere. As the light only seems to be shining from above, we'll use a demisphere.

A = 4πr^2
= 4 X π X 0.488826816^2
= 3.00275507/2
= 1.50137754m^2

Approximately 1360 watts of sunlight hits the Earths surface every second.

E = 1.50137754 X 1360
= 2041.87345w

Wattage is of course in the amount of joules per second. Given it was left to charge for 65000000 years, it's going to be an extreme amount of watts. The Sun shines in the Sun Keep all of the time, even at night...
ルッカ「ここが夜もひるの場所……。
Lucca: Here, even night is day...
Lucca: It's never dark in here.
ロボ「ココが２４時間
日光が当たっている場所デスカ。
Robo: So HERE is a place where
sunlight hits 24 hours a day.
Robo: The sun shines 24 hrs.
a day here.

はるか昔から、太陽の光が
てり続けているという……。
Magus: The Shrine of Light...
They say the sun's light has kept
shining here since the distant past...
Magus: The Sun Keep...
Where the sun never sets.
...So it's fair to use the entire day.

T = 60 X 60 X 24 X 365 X 65000000
= 2.04984e15s

E = 2041.87345 X 2.04984e15
= 4.18551387e18 joules
= 1.0003618236137667985 gigatons

Final Results
Charging the Sunstone = 1.000 gigatons