How to obtain Solaris achievement?

You need not only remove your steam engines from your system, but you also need to wait an hour. That is, you will be awarded the achievement once the steam engines disappear from the power tab with the scaling set to one hour.

Specifically; in order to get the Solaris achievement, you need to have 47 Solar Panels connected without any Steam engine's for at least an hour.

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Maths for above if you're interested.

One solar panel provides 60 kW (peak).

KiloWatts can be directly converted to GJ/h: 1kW = 1000 J/s ∴ 1kw = 0.0036 GJ/h.

∴ One solar panel provides: 60 kw = 60*0.0036 = 0.216 GJ/h

Since we need 10 GJ/h -> 10 (GJ/h)/ 0.216 (GJ/h/panel) = 46.3 Panels, since we can't have a partial solar panel, we round up to 47.