Factorio Solar Calculator Guide: Panels, Accumulators & the 25:21 Ratio
A Factorio solar calculator turns one simple target, such as 60 MW or 1 GW of continuous power, into the solar panels, accumulators and space you need before your factory browns out at night.
Last updated: June 27, 2026. Written for Factorio 2.0 and Space Age-era power planning.
Quick Answer: What Solar Ratio Should You Use?
For Nauvis-style day and night cycles, the standard reliable starting point is 25 solar panels to 21 accumulators. That ratio is close enough for practical builds because panels produce during the day while accumulators cover the night.
- Use 25 solar panels : 21 accumulators as the default continuous-power ratio.
- Each solar panel has a 60 kW peak output, but the useful average is lower across the full day-night cycle.
- A 25:21 block is about one megawatt of continuous power; use roughly 1.04-1.05 MW as a practical planning range.
- Add a safety margin when laser turrets, roboports, beacons, trains or space-platform production create burst demand.
Plan Solar as a Power Block, Not a Single Item
Solar builds work best when panels, accumulators, substations and expansion space are planned together. The calculator gives the counts, while a repeatable block keeps wiring and future growth manageable.
Solar Panel to Accumulator Ratio Formula
The practical formula is to multiply the number of 25:21 blocks until the continuous power target is covered. One block uses 25 panels and 21 accumulators and is roughly 1.04-1.05 MW of continuous power.
| Target continuous power | 25:21 blocks | Solar panels | Accumulators | Planning note |
|---|---|---|---|---|
| 10 MW | 10 blocks | 250 | 210 | Good for a small mid-game factory or a starter solar field. |
| 50 MW | 48 blocks | 1,200 | 1,008 | Useful when replacing steam power and adding early modules. |
| 100 MW | 96 blocks | 2,400 | 2,016 | Add radar, roboport and laser-turret margin before relying on this exactly. |
| 500 MW | 477 blocks | 11,925 | 10,017 | Reserve map space and build with blueprints or construction robots. |
| 1 GW | 953 blocks | 23,825 | 20,013 | Treat this as megabase infrastructure, not a one-off power patch. |
Why Accumulators Matter More Than Peak Solar Output
A solar panel can show strong daytime output, but your factory needs power through dusk, night and dawn. Accumulators store daytime excess and release it when panels cannot cover demand.
Daytime surplus
Panels must generate enough power for the active factory and enough surplus to recharge accumulators before night.
Night coverage
If accumulators drain before sunrise, the factory slows even when the solar panel count looks high at noon.
Burst loads
Laser turrets, roboports, trains, beacons and rapid production changes can temporarily exceed the smooth average.
Planet context
Space Age planets and platform constraints can change what power source is practical, so treat the Nauvis ratio as the baseline.
Recommended Solar Calculator Workflow
Measure the real load
Check the electric network graph after the factory has been running with normal research, modules, robots and defenses.
Choose a continuous MW target
Use the current sustained load plus a margin. A 20-30% buffer is safer when beacons or robot charging are expanding.
Convert the target into 25:21 blocks
Divide the target MW by about 1.05, round up, then multiply by 25 panels and 21 accumulators.
Build in repeatable tiles
Use substations, roboport coverage and walking space so the field can be extended without rewiring every block.
Watch the overnight graph
After construction, confirm accumulators recharge fully during the day and do not bottom out before morning.
Solar vs Steam vs Nuclear: When Solar Is the Right Choice
Solar is quiet, fuel-free and UPS-friendly, but it consumes a lot of space. It is strongest when you can blueprint large fields and want predictable low-maintenance power.
| Power option | Best use | Main tradeoff | Related calculator |
|---|---|---|---|
| Steam | Early game and compact starter bases | Consumes fuel and pollution budget | Use the main production calculator for fuel chains. |
| Solar + accumulators | Mid-game expansion, megabase UPS and low-maintenance power | Large land footprint and high upfront material cost | This solar guide and the main calculator. |
| Nuclear | Dense high-output power blocks | Needs reactor layout, heat and fuel-cell planning | Factorio Nuclear Reactor Calculator |
| Mixed grid | Transition phases and defensive outposts | Requires reading the electric graph instead of trusting one ratio | Power and solar planning pages together. |
Common Solar Planning Mistakes
| Mistake | What happens | Better approach |
|---|---|---|
| Counting only peak panel output | The factory runs fine at noon but loses power overnight. | Calculate continuous power with accumulators included. |
| Building too few accumulators | Accumulator charge reaches zero before sunrise. | Start from 25:21 and add extra storage for burst-heavy grids. |
| Ignoring roboport charging | Construction or logistics spikes cause sudden brownouts. | Measure load while robots are active, not while the base is idle. |
| No expansion margin | Every new module block requires emergency power work. | Reserve land and stamp solar in repeatable blocks. |
| Mixing planet assumptions | A ratio copied from one surface may not fit the next build context. | Use Nauvis 25:21 as baseline and re-check local constraints. |
Need exact solar counts for a bigger factory?
Open the live calculator, set your production plan and power target, then use this guide to translate the result into repeatable solar and accumulator blocks.
Factorio Solar Calculator FAQ
What is the best solar panel to accumulator ratio in Factorio?
The standard reliable ratio is 25 solar panels to 21 accumulators for continuous Nauvis-style power. It is the best default planning ratio before adding extra safety storage for burst loads.
How much continuous power does a 25:21 solar block provide?
A 25 panel and 21 accumulator block is roughly 1.04-1.05 MW of continuous power. Use it as a practical planning unit and round up rather than cutting the final block short.
Do I need extra accumulators beyond the 25:21 ratio?
Sometimes. Extra accumulators help when laser turrets, roboports or factory spikes pull more power than the smooth average. They do not replace enough solar panels, but they improve burst tolerance.
Is solar better than nuclear power?
Solar is simpler, fuel-free and friendly to very large bases, but it needs a lot of land. Nuclear is compact and powerful, but it requires reactor, heat, steam and fuel-cell planning.
Should I use solar before replacing steam power completely?
Yes, but transition gradually. Keep steam or another backup source until the electric graph proves accumulators recharge fully during the day and survive the night.
Sources and Further Reading
These references help verify current solar, accumulator and electric-network behavior.