Powering Your Spacecraft

With Realism Overhaul, you'll need to think carefully about the power solution for your spacecraft. Avionics, life support, antennas, and experiments can all draw quite a bit of power, so you'll need to plan for that. On the far right in the VAB toolbar thingy, there's a thing you can mouse over to see how power is generated, how power is used, and how long your batteries will last.

Reducing Power Consumption

To reduce power consumption, consider the tech level of your avionics and communications systems. In the early game, you will need to use science core or, once it's unlocked, deep space avionics for everything that needs to stay powered for a long time, as these will consume very little power when left alone.

Just Use Big Batteries

Even relatively early on, you can get a basic satellite to last a decent while on battery power alone if you just need it to transmit some science. This can also be used for lander probes and a variety of other missions with relatively low power consumption and duration.

Solar Power

Early solar panels are very expensive and will degrade quickly, but they are your only option in the early game. Remember that most solar panels don't track, so if you want to be efficient you should use RCS to get your single panel pointed right at the sun.

How Kerbalism calculates power draw in warp and when the vessel isn't focused

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RTGs

These are relatively early in the tech tree, but they are still kinda expensive and will degrade. Just use where you need them, but they're probably your best bet for Mars or further

Fuel Cells

These are generally the best way to go for crewed missions. They produce a lot of power, and the crew can even drink the water they produce.

Nuclear Reactors

Surprisingly, these are cheaper than RTGs because they don't use plutonium. They may present other challenges though, and are more niche in use case.