# How to calculate the amount of power a circuit needs to function?

#### Devika B S

Joined Mar 8, 2017
144
I am developing a boost converter that boosts 12V input to 48V. My load is 100 ohms. I have done a simulation on MATLAB Simulink and by connecting a voltmeter and ammeter on the load, I observed that it draws 45.73 (output voltage) x 0.457 (output current) = 20.89 Watts power. I understand that the power dissipation across the 100 ohms load resistance is now 20.89 Watts. I connected an ammeter on the input side and noticed that it draws 2.15 Amperes from the 12 Volt source. Does this mean that my circuit needs (2.15 x 12 =25.8) watts to function? Can a 12V 1.2 Ah lead acid rechargeable battery provide this? Plus I need the battery to be charged via a solar panel. Should I buy a 30 Watt panel as its the closest one available to the desired 25.8 Watts?

#### #12

Joined Nov 30, 2010
18,224
Yes, no, and no.
Trying to run a battery with a calculated discharge time of half an hour is asking for trouble. Lead acid batteries don't like to be thoroughly discharged and they don't deliver as advertised when loaded that heavily. You are much better off using a battery that calculates as 10 hours to discharge. Solar panels are labeled as their best result under perfect conditions. The weather on this planet makes that assumption absurd as a way to calculate available power. I would guess you need at least three times as much solar panel.

#### Devika B S

Joined Mar 8, 2017
144
Yes, no, and no.
Trying to run a battery with a calculated discharge time of half an hour is asking for trouble. Lead acid batteries don't like to be thoroughly discharged and they don't deliver as advertised when loaded that heavily. You are much better off using a battery that calculates as 10 hours to discharge. Solar panels are labeled as their best result under perfect conditions. The weather on this planet makes that assumption absurd as a way to calculate available power. I would guess you need at least three times as much solar panel.
Ok then, what battery should I possibly use? I need the battery to have 12V and it should be rechargeable. Plus I am not connecting the solar panel directly to the circuit. I am connecting it via a solar charge controller

#### #12

Joined Nov 30, 2010
18,224
Let me think...what 12 volt battery can provide 2.15 amps for 10 hours...
I got it! A battery that contains at least 21.5 amp hours.

As for the solar controller...That will not make the sun shine better on your solar panels.

#### Devika B S

Joined Mar 8, 2017
144
Let me think...what 12 volt battery can provide 2.15 amps for 10 hours...
I got it! A battery that contains at least 21.5 amp hours.
I don't need it for 10 hours. I need it for a 15 minute demonstration (after which i will charge the battery again). Yeah the point of solar charger appears irrelevant. Anyway I live in a very hot place with plenty of sunlight and this demonstration is expected to take place during the upcoming summer season with absolutely no rain and no clouds as well.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
there's efficiencies to calculate in. Your Boost converter probably does not obey ohms' law. The current waveform is likely different than the voltage.

Solar is rated for usually AM (Air Mass) 1.5 Global for 100 mW/sqcm. You likely won't get that.

Not exactly sure what your demonstrating? The resistor can run at constant power for a certain amount of time?
You can charge a battery? They weigh a certain weight and are easy to carry?