I've tried:
energy_production_night_multiplier = 0.01 & 0.1 & 0.5

I've also tried:
#Base energy production of the helmet.
energy_base_value = 150 (with the 3 'multiplier' settings above)

...and I tried:
energy_production_multiplier = 1.0 to a larger number but it defaulted back to "1.0" after restarting minecraft.

Thank you for the report, I'll investigate it asap.

Would you please provide more information about your environment (Modloader, Versions, other mods)?

Too many mods to mention so I'll backup the savegame & remove all the mods - BRB...

(Forge 47.2.0 / mc 1.20.1 / JustABattery-47.1.0.jar / SolarHelmet-47.0.1.jar .. I had another issue whereby the helmet would not charge when I placed it on my head 'manually', but when I put the 'solar helmet' in the toolbar & 'right-clicked', it then started working/charging)

It might be the mod: 'inventorio-1.20-forge-1.9.2.jar' &/or 'curios-forge-5.4.2+1.20.1.jar' combo?

...BRB

[EDIT: Oops! The game didn't load; I tried 'safe mode' & 'optimise world' (which made my savegame disappear ... but it reappeared when I made a new game & cancelled it & went back into the main menu & back into 'single player'... WTF?!!?]

I can confirm this issue. I actually haven't reintroduced the nighttime and rain multipliers, after changing the energy calculation.
It might take some time to add these to the current calculation.

Thanks. I'll close this issue since I have set the other settings to make it OP (&& justabattery too)... so it doesn't matter to me.