Your solar monitoring app has been showing 20 kWh per day for months. Then the rainy season hits, two days of heavy cloud roll in, and the number drops to 4 kWh. The first instinct is to wonder if something has broken, or whether your installer sized the system wrong.
The honest answer: this is normal, predictable, and already priced into any serious solar sizing calculation. Understanding the actual numbers behind solar panels in cloudy weather removes the worry and helps you make smarter sizing decisions before you install.
TL;DR
- Solar panels under cloudy weather still produce 10-30% of peak capacity, not zero. Output varies with cloud thickness, not just presence.
- Hybrid systems manage consecutive cloudy days well: battery covers night load and short gaps, PLN backs up after 2-3 overcast days. That's the design, not a flaw.
- Indonesia's wet season (Nov-Mar) drops average output 20-25%. Java PSH falls from 4.5-4.7 to 3.8-4.2. Size for the wet-season floor, not the annual average.
- Design for the worst sustained 30-day average, not the single worst day. Designing for the worst day produces an oversized, expensive battery spec.
- Under Permen ESDM 2/2024, excess dry-season production earns no PLN credit. Oversizing panels to offset rainy season isn't financially rational.
- Full off-grid systems need 2-3 days of battery autonomy for Indonesian wet season, depending on your region's cloud persistence.
What output actually drops to on cloudy days
Panels use light, not direct heat, and light penetrates cloud cover even when it's heavily overcast. The output curve follows cloud thickness:
| Sky condition | Output as % of peak |
|---|---|
| Full sun, clear sky | 90-100% |
| Light cloud or haze | 60-80% |
| Partial cloud cover | 40-60% |
| Heavy overcast (no rain) | 20-30% |
| Very heavy cloud and active rain | 10-15% |
For a 5 kWp system with a rated peak around 4.5-5 kW: heavy cloud drops real-time output to 500 watts to 1.25 kW. If your home's midday load (AC off, just lights and devices) is around 500-600 watts, the panels are still covering it. If you're running two AC units and the load is 2-3 kW, panels cover part of it and the battery or PLN fills the rest.
This is instantaneous output, not daily production. An intermittently overcast day (clear morning, cloudy afternoon) still produces 10-15 kWh for a 5 kWp system. A full 12-hour overcast day might yield 3-5 kWh total. Both are far from zero, but the gap from the sunny-day 22 kWh is significant.
The number that matters for sizing decisions is the sustained 30-day average during wet season, not the single worst-day floor. More on this in the sizing section below.
How a hybrid system responds day by day
A hybrid setup (panels, battery, PLN backup) is built specifically to absorb cloudy-weather variability. Here's the actual sequence during an extended overcast period:
Days 1-2 of cloud cover: Panels produce 20-30% of normal. The battery refills partially during the day and covers nighttime load as usual. Most homeowners don't notice any difference. The monitoring app shows lower production numbers, but the house runs normally.
Days 3-4 of sustained cloud: The battery starts each morning at a lower state of charge because partial daytime recharge isn't keeping up. PLN begins carrying a larger share of daytime loads. The system is still functioning correctly: it prioritizes all available solar first, draws from battery second, and pulls from PLN only for the remaining gap.
Day 5 and beyond, unbroken heavy cloud: PLN carries most of the load. The panels still trickle-charge the battery throughout the day, but not enough to cover full household demand. Your bill for that stretch is higher than average. This is precisely the point of keeping PLN in a hybrid system: reliable backup that costs nothing during the 10 months it isn't needed much, and covers you during the 2 months when it is.
LFP batteries (LiFePO4, the chemistry used in HinaESS, Pylontech, and BYD units) handle deep cycling safely to 80% depth of discharge across 6,000-plus charge cycles. Extended cloudy periods cause deeper daily cycling, which is fine as long as the battery is sized for the load. A 10 kWh battery for a 25 kWh/day household will be stressed; a 20 kWh battery for the same household handles a 2-day cloudy gap comfortably.
Sizing your system for Indonesia's wet season
Indonesia's wet season varies considerably by region. The underlying data point is PSH (Peak Sun Hours, equivalent to kWh per square meter per day from Global Solar Atlas):
| Region | Annual average PSH | Wet season PSH (estimate) |
|---|---|---|
| Java (general) | 4.5-4.7 | 3.8-4.2 |
| Bali | 4.7-5.0 | 4.0-4.3 |
| Nusa Tenggara Timur | 5.5 | 4.5-5.0 |
| Sumatra (general) | 4.2-4.5 | 3.5-4.0 |
| Kalimantan | 4.0-4.3 | 3.3-3.8 |
Wet season in Java and Sumatra runs roughly November to March, with peak intensity in December and January. Average output drops 20-25% from dry season.
The key sizing principle: use wet-season PSH as your baseline, not the annual average. If you size based on Java's annual average PSH of 4.7 and actual January PSH is 3.8, your system under-delivers during the most challenging month. If you size for the wet-season floor of 3.8-4.0, your system covers annual needs with a small dry-season surplus.
Practical example: a home in Surabaya uses 20 kWh per day. Sizing for wet-season PSH of 4.0:
- Panels needed: 20 / (4.0 × 0.8) = 6.25 kWp. Round up to 6.5 or 7 kWp.
- Sizing for the annual average PSH of 4.7 instead gives 5.3 kWp, which leaves the system short in wet season.
Calculate the right size for your home →
Off-grid battery sizing for wet season
Full off-grid removes PLN as a fallback, which means cloudy-day gaps must be covered entirely by battery storage. The battery has to hold enough capacity to bridge multi-day stretches of poor solar production.
The general guidance for off-grid in Indonesia:
2 days of autonomy covers most coastal and lowland areas: south Bali, most of East Java, coastal Lombok, West Nusa Tenggara. Extended cloud-only weather (no rain at all) rarely persists more than 2 days in these locations.
3 days of autonomy is appropriate for higher-rainfall highland areas: West Java highlands (Bogor, Puncak, Lembang), Bali's interior highlands (Bedugul, Kintamani, Munduk), and interior Sumatra. These locations see longer persistent cloud cover during wet season, particularly at altitude.
The calculation: battery kWh = daily usage × autonomy days / 0.8 (LFP DoD ceiling).
Example: a home in Bogor using 15 kWh per day targeting 3-day autonomy needs 15 × 3 / 0.8 = 56.25 kWh. Round to 60 kWh (for example, 12 units of HinaESS 5.12 kWh or equivalent BYD or Pylontech modules).
This is a substantial and expensive battery bank. It's one of the main reasons why full off-grid for residential homes with PLN access is rarely the more economical choice compared to hybrid. The cost of additional battery capacity for worst-case coverage typically exceeds the cost of keeping PLN as a backup for the occasions when it's genuinely needed.
The oversizing trap: don't compensate for cloudy months with extra panels
The intuitive reaction to learning about wet-season output drops is often: "I'll just add 25% more panels to compensate." That logic has a structural problem under current Indonesian regulation.
Under Permen ESDM 2/2024, residential solar systems have no net metering. Surplus production during a clear dry-season day doesn't earn PLN credit. It is curtailed and wasted. An 8 kWp system installed to "compensate" for wet-season dips will produce 35-40 kWh on a clear August day, but if your home only uses 20 kWh of it, 15-20 kWh is wasted every sunny day for 6-7 months of dry season.
The smarter approach: size for your actual daily consumption with wet-season PSH as the baseline, accept that PLN will carry a larger share during the 2-3 rainiest months, and don't pay for panel capacity whose dry-season output has nowhere to go.
When this doesn't apply to your situation
You're full off-grid in a high-rainfall area without PLN access. Everything above becomes much more critical. Without a PLN fallback, a 5-day overcast stretch with under-sized battery means no power. Engineering needs to be more conservative, and a site visit during wet season or analysis of local rainfall data is necessary, not optional.
Your roof has significant shading already. A roof with shading from nearby trees or tall buildings under clear sky conditions will see compounding losses when clouds arrive. Combined shading and heavy cloud can push output to 5-10% of peak, well below the ranges in this article. That scenario needs dedicated site analysis.
Your household energy load is unusually high. A home running 4+ AC units around the clock, a pool pump, and a water heater is consuming 40-60 kWh per day. One or two overcast days can cycle the battery deep very quickly. Sizing rules of thumb don't hold well at that consumption level and you need a custom spec.
If your situation matches any of those, the variables are different enough that a generic sizing formula will mislead you. That's the conversation worth having before you sign a contract.
Frequently asked questions
On a heavily overcast day, a solar panel in Indonesia produces roughly 10-25% of its rated peak output. A 5 kWp system that normally generates 4-5 kW at midday might produce only 500 watts to 1.25 kW under thick cloud cover. Partially cloudy skies yield 30-50%. Output doesn't reach zero, but it drops enough that you can't rely on panels alone to cover your full load without battery backup or PLN.
