You're not looking at a solar flare, but rather a massively acute coronal mass ejection.
Solar flares happen all the time and while they can cause temporary disruption to delicate communication technologies all the way up to power grids depending on the size of the solar flare, they very rarely cause 'permanent' damage. Solar flares can cause a massive amount of disruption to the flow of electricity through an electronic circuit, but it's not common for them to damage electrical circuits directly.
We actually have solar flares hit us all the time and we even have an 11 year cycle of solar maxima that regularly hit the earth and have brought down power grids in the past without destroying infrastructure. Solar flares simply aren't powerful enough to do that kind of damage without being a sustained effect over a significant period.
Coronal Mass Ejections on the other hand (see original link) is more or less a massive plasma blast shot from the sun into space, potentially at the Earth. This will do much more damage, but even then you're dealing with a massive one to actually destroy electrical systems. More to the point, if it can do that, there is a small risk to humans, especially at high altitude.
As for the actual impact of a CME; You're dealing with between 2 and 5 days of ejecta, reaching between 20 and 3200 Km/s speeds, and you're dealing with around 1.6×10^12 kg worth of plasma mass. These generally cause issues with Earth's infrastructure, but doesn't wipe it out completely. That is (in part) because a 5 day ejection window means it's pretty hard to hit a moving target like the Earth with the full mass of the CME.
To wipe out all electronics, I'm going to estimate a CME mass of around 10 times the average CME mass, so 1.6 x 10^13 Kg, released in around 5 hrs. That means a CME of around 240 times the normal release load (at least for the 5 hr window we describe). If all that hit the Earth, we'd be in trouble. You'd probably know it was happening because you'd see aurorae as far north as Brisbane Australia (Aurora Australis), and as far south as Los Angeles (Aurora Borealis) just because of the sheer mass of material reacting with the Earth's magnetic field. It's also likely at that magnitude to do at least some damage to life forms even on the surface of the Earth.
It should be pointed out that such a massive CME is highly unlikely because the Sun would have yielded to the causative pressures long before that kind of pressure could build up. Given our current theories on how CMEs form and are released, this kind of scale is likely impossible.
In any event, if you're in the tropics and you're seeing aurorae, turn off as much of your electrical equipment as you can and stock up on sunscreen and anti-plasma burn first aid kits (the first one is a joke by the way; sunscreen wouldn't help you much).