You already found the help function, so usage can't be that difficult. It is actually quite simple: enter two values you know and the rest will be calculated. The calculation will be based on the last two items you entered.
This app is completely handcrafted, copyright © 20122024 BertKoor, all rights reserved.
Version 1.1.0 built 22 AUG 2024.
Here are some examples for usage of this app. You can use it to calculate:
NB: Beware of "milli" values specced as mV, mA and mW, such as in the last example. In those cases you'll have to divide the value by 1000 yourself, so 150mA should be entered as 0.150 A. Maybe the next version will support the scaling factor prefixes M (mega), k (kilo), m (milli) and μ (micro).
First click on one of the leftmost blue buttons to select a value known to you. The LCD display next to it will change from  to _ to indicate it is in dataentry mode. Now a displays is selected, you can use the numeric pad.
Use the ⌫ button (backspace) to delete the last digit and the C button to erase the whole value. The R button resets the whole calculator. Press the green ↵ button (enter) when done.
When two values are entered, the other two will be calculated.
When pressing a blue button several times, it will turn red. Then the associated value is fixed, which means it will always be regarded input. This is useful for example when using the mains voltage of 230V, which won't change.
Be careful not to click on a LCD display. They are quite cheap so you might break it.
Your inputted values, which are the basis for the calculation, will have their blue button on the left lit up and the LCD display next to it is green.
The LCD display will show greenlit to indicate a computed value.
All values have a precision of 5 digits. This should be sufficient for most realworld applications. The next major version will feature scaling factor prefixes M (mega), k (kilo), m (milli) and μ (micro) to extend the range.
Values smaller than 0.00005 will be rounded down to zero. When a computed value is larger than 99999.5, it's display will show OVRFLW.
The basis for this calculator are Ohm's law and Joule's first law. In these formulas the used symbols have the following meaning:
Ohm's law can be written in three equivalent formulas:
Joule's first law states P = I^{2} * R. Here R can be substituted by V / I from Ohm's law, which resolves in P = I^{2} * V / I = I * V. This again can be written in three equivalent formulas:
Since P = V * I and I = V / R
⇒ P = V * (V / A)
⇒ P = V^{2} / R
⇒ V^{2} = P * R
⇒ V = √ P * R
And since P = V * I and V = I * R
⇒ P = (I * R) * I
⇒ I^{2} = P / R
⇒ I = √ P / R
That's basically all there is to it.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, INSULT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
Copyright © 2012 BertKoor, all rights reserved.
The following tools were used for creation of this app:
A big thank you goes to the following people:
VAWΩ Calc

for free
Brought to you by BertKoor.nl 
Volt 
V 

Amp 
A 

Watt 
W 

Ohm 
Ω 
7 
8 
9 
⌫ 
4 
5 
6 
C 
1 
2 
3 
R 
0 
. 
? 
↵ 