When I first switched to solar, I placed panels and accumulators randomly. My base ran fine during the day and shut down at night. I doubled the accumulators. Now my base survived the night but the panels filled up accumulators by 10 AM and wasted half the day. The 21:25 ratio fixed it. Here is the math and the layout.
The Ratio Math
The 21:25 ratio comes from two Factorio mechanics: daylight cycle and accumulator charge rate.
A solar panel produces 60 kW during peak daylight (noon). Average output over a full cycle (day + night) is 42 kW per panel because Factorio's day is 83% daylight (summer-style curve) with gradual ramping.
An accumulator stores 5 MJ of energy. To fill an accumulator from empty during daylight, a single solar panel needs roughly 42 kW average x 83% daytime = 35 kW effective charging per panel. One accumulator needs 5000 kJ / 35 kW = ~143 seconds to charge. A Factorio day gives about 166 seconds of net positive solar output.
The community-derived ratio: 21 panels produce 21 x 42 kW = 882 kW average. To store half the daytime energy for night use (25 accumulators x 5 MJ = 125 MJ), the accumulators charge during the 166-second surplus window. 21 panels produce 21 x 60 kW = 1260 kW peak, which charges the accumulators cleanly.
| Stat | Value |
|---|---|
| Solar panel peak output | 60 kW |
| Solar panel average | 42 kW |
| Accumulator capacity | 5 MJ |
| Golden ratio (panels:accumulators) | 21:25 |
| Average power per block (21+25) | ~882 kW |
| Night survival | 100% without backup |
| Tile per panel (including substation) | ~3x3 |
| Accumulators per substation ring | ~24 |
Compact Array Blueprint
The standard compact solar array pattern is a repeating block:
- Place 21 solar panels in a 7x3 grid (7 wide, 3 tall)
- Place 25 accumulators below, in two rows of 12 and 13, staggered
- Leave a 2-tile gap between the panel block and accumulator block for substations
- Each substation covers about 18 panels + accumulators around it
One substation covers roughly 50 tiles. In practice, put one substation per 21-panel + 25-accumulator block, centered between them.
The pattern repeats every 30 tiles horizontally. Stack blocks vertically with 3 tiles of space for roboport coverage and repair access.
When to Switch from Steam
Steam power is fuel-efficient early but scales poorly. Switch to solar when:
- Your base draws more than 10 MW continuous. Below this, steam is fine.
- You have enough space. Solar needs roughly 4x the area of equivalent steam.
- You want UPS-friendly power. Solar has zero moving parts in the simulation. A 1 GW solar array uses less UPS than a single 10 MW steam plant.
- You are tired of refueling boilers. Solar is fire-and-forget once placed.
The mid-game transition: keep steam as backup. Build solar blocks slowly. Set your accumulators to be logic-controlled to enable steam only when accumulator charge drops below 10%. This gives you 24/7 solar with automatic steam failover.
Common Mistakes
Wrong ratio. The most common error is using 20:25 or 25:20 instead of 21:25. 20:25 means accumulators charge slower than daytime surplus -- they never fill. 25:20 means panels produce more than accumulators can store -- you waste daytime potential.
Scaling by hand. Never place solar panels individually. Build a blueprint for one block (21 panels + 25 accumulators + substation), then stamp it in a grid. For 100 MW average, stamp 114 blocks.
Not separating array from base. Build the solar array in its own field, connected by power poles radiating back to your base. If you place panels inside your factory footprint, you will tear them up later for expansion.
Power Output by Array Size
| Array size | Panels | Accumulators | Average power | Tiles |
|---|---|---|---|---|
| Small | 105 | 125 | ~4.4 MW | ~40x40 |
| Medium | 420 | 500 | ~17.6 MW | ~80x80 |
| Large | 1680 | 2000 | ~70 MW | ~160x160 |
| Megabase | 10000+ | 11905+ | ~420 MW+ | Custom |
Bottom Line
Solar power in Factorio is a one-time build investment. Get the ratio right (21:25), build the blueprint, stamp it in a dedicated field, and never think about power again. The UPS savings alone make it worth the space cost for any base above 50 SPM.
Numbers to remember:
- 21 panels + 25 accumulators = 1 block
- 1 block = ~882 kW average
- 114 blocks = 100 MW average
- 1 substation per block
- Solar uses 0 UPS. Steam uses UPS per boiler and pipe.
Previous: Mall Guide Next: Nuclear Power Guide
Community Verification & Resources
- Factorio Wiki -- Solar Panel -- exact power specs and recipe
- Factorio Wiki -- Accumulator -- discharge rate and capacity
- Factorio Prints -- Solar Arrays -- community solar array blueprints
- Reddit -- 21:25 Ratio Explanation -- community derivation
Related: Nuclear Power Guide | Beacon Module Guide | Megabase Transition Guide