# Power over time for RC ish circuit in LT spice

I have a simple circuit in spice that I put a 0-5V step function into. So basically as square-wave going in. Then I can get my current and voltage waveforms out, or do some math to get power. But I really want to know say for 1ms how much power was used. I just want to compare a few approaches and my comparison that other people on my team will understand will be Watts. So I was thinking how many Watts were used in 1ms.

I'm thinking maybe I want to integrate under the power curve? Not sure exactly how to accomplish that in LTspice so I'm asking for some advice. I did read a little about the idt function.

• You can do simple maths in the graph and multiply voltage and current. Commented Dec 14, 2016 at 15:23
• That'll get me a graph of power, but I want to be able to say 1mW was used over 1ms and the response is non-linear. Commented Dec 14, 2016 at 15:26
• @Confused - Do what PlasmaHH said and plot V(t)*I(t). You can export that waveform to Excel/spreadsheet format. It is often easier to do calculations in Excel than trying to figure out how to do it in the waveform plotter. You can sum the discrete instantaneous power values over the time window you're concerned with. (Reimann sum ~= integral for sufficiently small timesteps). Note that the unit is [J] or [W*s] after summing, not [W].
– vofa
Commented Dec 14, 2016 at 15:27
• LTSpice will integrate over a plotted signal. Ctrl + left click (maybe right click - I'm away from my computer and unsure now...). It can be made to integrate over a desired time period. Commented Dec 14, 2016 at 18:19
• @Matthew +1 Thank you I found that by accident, and that is the method that worked great for me. Commented Dec 15, 2016 at 15:36