Is it possible to increase the grid by add solar and wind and NOT adding an on-demand backup source (gas, etc.) WITHOUT ADDING RISK?

What I mean is say the grid demands 100. The grid is powered entirely by coal. You give it 120 for 20% redundancy. This is extremely reliable.

The grid demand is now 120. You now need 144 for 20% redundancy. You dont want to use coal. So you add solar and batteries.

Batteries are great because they normalize the volatility of solar generation over time, but they do not make solar truly on demand. So if you add 24 solar to the 120 coal you are increasing the risk on the grid. What often happens is you add 24 solar but you have 24 coal as a backup. Ideally the real-world use will be solar but in case of downtime your grid will not fail.

Your existing demand is not flat. Not flat over a single day, nor a single week nor a single year.

So you already have some spare capacity on the system.

If your demand peak was summer air con, then adding the solar makes the system more stable. This can be seen in a few grids that issue less grid warnings in summer now.

You add 100x1day worth of battery capacity. Which is fairly economical even today (though not economical enough to actually shut down coal). Wouldn't work everywhere (winter in New England needs more than 1 day of backup) but works in some places.

Trivially yes, for any locations with summer energy use peaks. The solar output will be the greatest when the demand for cooling is at its greatest.

Yes, for places with sizeable hydropower. You simply hold the water for longer.

Probably yes, for places where the need for redundancy is rare. Natural gas peaker plants are cheaper to build and simpler to operate with the tradeoff of being less efficient than combined cycle plants.

Take a look at California. Their grid is routinely, daily, generating ~84% of its power from renewables [1] (with ~25GW of existing solar PV capacity, ~6GW of wind, and ~6GW of hydro). They are adding batteries rapidly [2] (with a goal of 52GW by 2045; they are 33% of the way there). They still have ~32GW of fossil gas generation capacity, but it is rarely used constantly at full capacity. They have plans for another ~21GW of solar PV on land that can no longer be farmed due to water shortages [3] [4] (enabling families to keep their land with long term lease payments).

Not everywhere is California, but solar and batteries are the cheapest form of generation in 90%+ of the world [5]. You simply keep building more solar collection, storage, transmission, etc. to orchestrate collecting this "fusion at a distance" and distributing it to loads. The sun rises every day, and will for our lifetimes. We continue to deploy batteries and solar at manufacturing capacity, while continuing to increase manufacturing capacity year over year. You fill any gaps with fossil generation until there are no longer any gaps to fill [6].

Tangentially, Australia is currently testing a battery with a 8 hour discharge capability [7] ("Long-Duration Energy Storage (LDES)"), as they are rapidly preparing for a network/team of battery storage facilities to assume grid health responsibility from their retiring thermal coal generators [8]. Certainly there is much work ahead in understanding and developing longer duration energy storage systems.

[1] https://app.electricitymaps.com/map/zone/US-CAL-CISO/live/fi...

[2] https://www.energy.ca.gov/data-reports/energy-almanac/califo...

[3] 21GW of Solar for California Land That Can No Longer Be Used for Agriculture - https://news.ycombinator.com/item?id=46488648 - January 2026

[4] https://valleycleaninfrastructureplan.com/

[5] Solar electricity every hour of every day is here and it changes everything - https://ember-energy.org/latest-insights/solar-electricity-e... - June 21st, 2025

[6] Renewables reached nearly 50% of global electricity capacity last year - https://news.ycombinator.com/item?id=47615756 - April 2026 (149 comments)

[7] https://www.yahoo.com/news/science/articles/australias-first...

[8] https://www.aemo.com.au/-/media/files/initiatives/engineerin...

(think in systems)