Battery storage is getting cheap fast — but 24/7 solar isn’t here yet

Battery energy storage system (BESS) costs have fallen by a staggering 73 percent between 2018 and 2024.[1] At the same time, Solar PV dropped by 45 percent.[2] This makes solar + BESS increasingly competitive with traditional power plants.

Energy think tank Ember argues that “Solar is no longer just cheap daytime electricity, solar is now anytime dispatchable electricity.”[3] Ember estimates a dispatchable cost of solar + BESS at US$76 per MWh, claiming it is cost-competitive with new gas-fired generation.

At Castalia, we are excited by these declines. We recently wrote about Abu Dhabi’s gigawatt-scale solar + BESS system, which can deliver reliable 24/7 power at costs comparable to gas-fired generation. See our blog post here.

So, is 24/7 solar already here? Well… no, not really. Ember’s claim warrants a more cautious assessment, for the reasons outlined below.

The estimated US$76/MWh cannot be interpreted as the dispatchable cost

Ember’s figure combines daytime solar costs of US$43 per MWh with storage costs of US$65 per MWh applied to half of the solar output,[4] resulting in an energy-weighted average cost of US$76 per MWh. This makes for strong headlines, but is less informative for grid operators.

Power systems operate hour by hour, not on daily averages. At night, stored solar must reflect both generation and storage costs, implying a nighttime cost of US$108/MWh. At this level, the nighttime cost exceeds the typical LCOE of new gas-fired combined-cycle plants of US$69/MWh.[5]

Including the emissions costs of gas-fired generation does not change this conclusion. Using a social cost of carbon of US$64/tCO₂[6] adds roughly US$24/MWh to gas generation costs,[7] shifting the typical cost to about US$93/MWh.

Time-shifting solar is not yet economic in many places—even in island systems with optimal solar resources and high diesel generation costs

Castalia is assisting the Government of Saint Kitts and Nevis on a 50MW solar project with a 30MW/30MWh BESS, in a system with strong solar resources and reliance on imported fuels. The technical due diligence confirmed that a battery design focused on ancillary services delivered greater system value than a battery sized primarily for energy shifting. Scaling storage to provide 24/7 service would have required additional battery capacity, thereby raising costs above those of fuel-based alternatives.

Achieving 24/7 solar is exciting, but we aren’t there yet

Falling BESS prices are unlocking exciting new opportunities and making solar power increasingly viable 24/7. However, headline cost figures based on energy-weighted averages do not reflect the cost of supplying electricity during non-solar hours, which remains the main challenge for solar-based systems. All in all, the outlook is promising: as costs continue to decline, solar + BESS will move closer to consistently outcompeting gas after sunset.

[1]     International Renewable Energy Agency (IRENA). 2025. “Battery energy storage systems: Key to renewable power supply-demand gaps”. Link

[2]     IRENA. 2025. “Renewable power generation costs in 2024” Pg 28. Link

[3]     Jones, D., Kostantsa, R., (Ember) 2025. “How cheap is battery storage?” Pg 3. Link

[4]     Jones, D., Kostantsa, R., (Ember) 2025. “How cheap is battery storage?” Pg 15. Link

[5]     U.S Energy Information Administration. 2025. “Levelized Costs of New Generation Resources in the Annual Energy Outlook 2025.” Pg 5. Link

[6]     Estimates of social cost can vary widely due to different methodologies adopted, assumptions, uncertainties in climate change impact and magnitude, and variations in discount rates. United States Government – Interagency Working Group on Social Cost of Greenhouse Gases, “Technical Support Document: Social Cost of Carbon, Methane, and Nitrous Oxide Interim Estimates under EO 13990.” Page 5. Link

[7]     Assuming that a modern combined-cycle gas turbine emits about 0.40 tCO₂/MWh. Energy for Growth Hub. 2022. “Should lower-income countries build open cycle or combined cycle gas turbines?” Link