Peak sun hours are the single most powerful input in any solar sizing calculation. Get this number right and everything else in your system design falls into place. Get it wrong and your system will either chronically underperform or cost far more than necessary.
The good news is that using peak sun hours to size a solar system is a straightforward, repeatable process once you understand the logic. This guide walks you through every step — from finding your location’s peak sun hours to calculating exactly how many panels, what inverter size, and what battery capacity your system needs.
By the end, you’ll be able to size a solar system for any location in the world with confidence.
Quick Recap: What Are Peak Sun Hours?
One peak sun hour (PSH) = one hour of sunlight at exactly 1,000 W/m² — the intensity threshold at which solar panels produce their rated output.
They are not the same as daylight hours. A location with 12 hours of daylight might only have 5 peak sun hours — because morning and evening sun is weaker than the midday peak. Only the hours at or equivalent to 1,000 W/m² count.
The key insight is this: peak sun hours are numerically identical to solar irradiance in kWh/m²/day. A location with a GHI (Global Horizontal Irradiance) of 5.2 kWh/m²/day has exactly 5.2 peak sun hours per day.
The Core Formula
Every solar sizing calculation using peak sun hours flows from this master formula:
Required System Size (kW) = (Daily Energy Usage ÷ Peak Sun Hours) × System Loss Factor
The System Loss Factor is where the 20% rule and real-world efficiency losses both enter the equation. Breaking this into individual steps makes the logic clear and the calculation accurate at each stage.
Step 1 — Get Your Daily Energy Usage
Your system must be sized to your average daily consumption — not your peak month, not a single day’s usage.
How to calculate it:
- Collect your last 12 months of electricity bills
- Note the kWh consumed figure from each bill
- Add all 12 values together
- Divide by 12 to get average monthly usage
- Divide by 30 to get average daily usage in kWh
Example:
- 12-month total: 9,600 kWh
- Monthly average: 9,600 ÷ 12 = 800 kWh/month
- Daily average: 800 ÷ 30 = 26.7 kWh/day
If you don’t have bills yet (new home or new installation), estimate from your appliances:
| Appliance | Wattage | Hours/Day | Daily kWh |
|---|---|---|---|
| Air conditioner (1.5 ton) | 1,500W | 8 hrs | 12.0 kWh |
| Refrigerator | 150W | 24 hrs | 3.6 kWh |
| LED lights (10 bulbs) | 100W | 6 hrs | 0.6 kWh |
| Ceiling fans (4) | 200W | 12 hrs | 2.4 kWh |
| TV + electronics | 200W | 5 hrs | 1.0 kWh |
| Washing machine | 500W | 1 hr | 0.5 kWh |
| Water pump | 750W | 1 hr | 0.75 kWh |
| Phone/laptop charging | 100W | 3 hrs | 0.3 kWh |
| Total | ~21.15 kWh/day |
Add 10–15% for miscellaneous usage and appliance inefficiency to reach your working daily figure.
Step 2 — Find Your Peak Sun Hours
Your location’s peak sun hours is the number that most directly determines your system size. Two homes using identical energy need completely different panel counts based solely on location.
The three best sources:
Global Solar Atlas (globalsolaratlas.info) — Go to the site, search your address, and read the GHI Per Day value. This is your peak sun hours per day. Free, globally accurate, and backed by World Bank satellite data.
PVGIS (re.jrc.ec.europa.eu/pvg_tools) — Preferred for Europe and Africa. Enter your location and read the monthly irradiance table — this also shows seasonal variation for better system planning.
Footprint Hero Calculator (footprinthero.com/peak-sun-hours-calculator) — Simplest to use. Enter your city or address and it returns annual and monthly peak sun hours directly.
Regional quick reference:
| Region | Annual Average PSH/Day |
|---|---|
| Bangladesh (Dhaka) | 4.8 – 5.5 |
| Bangladesh (Rajshahi) | 5.2 – 6.0 |
| India (Rajasthan) | 6.0 – 7.0 |
| India (South) | 5.5 – 6.5 |
| Southeast Asia | 4.5 – 5.5 |
| Middle East | 6.0 – 7.5 |
| Sub-Saharan Africa | 5.5 – 7.0 |
| Australia (most) | 4.5 – 6.5 |
| Southern Europe | 4.5 – 6.5 |
| USA (Southwest) | 6.0 – 7.5 |
| USA (East Coast) | 4.0 – 5.0 |
| Northern Europe / UK | 2.5 – 3.5 |
Always use your annual average for grid-connected systems. For off-grid systems, use the lowest month’s PSH to ensure the system handles the worst-case period.
Step 3 — Calculate Raw System Size
Divide your daily energy target by your location’s peak sun hours to get the raw DC system size — before any efficiency adjustments.
Formula:
Raw System Size (kW) = Daily Energy Usage (kWh) ÷ Peak Sun Hours
Continuing the example (5.0 PSH):
26.7 kWh ÷ 5.0 = 5.34 kW
This is the theoretical minimum — the panel capacity needed if panels operated at perfect rated efficiency with zero losses. Real systems don’t do this, which is why the next two steps exist.
Step 4 — Apply the 20% Buffer
As established in the previous articles in this series, real-world solar panels don’t deliver their rated output under actual rooftop conditions. Inverter losses, heat, dust, shading, and wiring resistance collectively reduce actual output to 75–85% of rated capacity. The 20% rule compensates for this gap.
Formula:
Adjusted System Size = Raw System Size × 1.2
Continuing the example:
5.34 kW × 1.2 = 6.4 kW
This adjusted figure already incorporates the 20% production buffer — meaning your 6.4 kW array is designed to reliably deliver 26.7 kWh/day after accounting for typical system losses.
Step 5 — Account for System Losses (Derate Factor)
Professional solar installers go one step further and apply a derate factor — a more precise accounting of the specific losses in a system. Where the 20% rule is a general buffer, the derate factor is a precise calculation.
The standard derate factor used by NREL (National Renewable Energy Laboratory) and most professional tools is 0.80 — meaning a real-world system delivers approximately 80% of its theoretical DC panel output as usable AC electricity.
The two approaches compared:
| Approach | Multiplier | When to use |
|---|---|---|
| 20% rule (×1.2) | Divide raw size by 0.833 | Quick estimates, DIY calculations |
| Derate factor (÷0.80) | Divide raw size by 0.80 | More precise professional sizing |
These two approaches give almost identical results — dividing by 0.80 is mathematically equivalent to multiplying by 1.25, only slightly more conservative than the 20% rule’s ×1.2.
Using derate factor for the example:
5.34 kW ÷ 0.80 = 6.68 kW → specify a 7 kW system
The derate factor approach is preferred when you know your specific loss sources — for example, if your roof has 10% shading losses and your inverter runs at 96% efficiency, you can calculate a more precise derate factor than the standard 0.80.
Building a custom derate factor:
| Loss Source | Typical Efficiency |
|---|---|
| Panel temperature losses | 0.92 (8% loss) |
| Inverter efficiency | 0.96 (4% loss) |
| Wiring/connection losses | 0.98 (2% loss) |
| Soiling/dust losses | 0.97 (3% loss) |
| Shading (light) | 0.97 (3% loss) |
| Mismatch losses | 0.98 (2% loss) |
| Combined derate factor | 0.92 × 0.96 × 0.98 × 0.97 × 0.97 × 0.98 ≈ 0.80 |
Multiply all individual efficiency factors together to get your system’s custom derate factor. A shaded roof drops this to 0.72–0.75; an unshaded roof with premium inverter may reach 0.83–0.85.
Step 6 — Calculate Number of Panels
With your adjusted system size confirmed, calculating panel count is straightforward.
Formula:
Number of Panels = System Size (W) ÷ Individual Panel Wattage (W)
Standard panel wattages in 2026:
- Budget: 370–400W
- Mid-range: 400–440W
- Premium: 440–500W+
Continuing the example with 400W panels (7 kW system):
7,000W ÷ 400W = 17.5 → round up to 18 panels
With 440W premium panels:
7,000W ÷ 440W = 15.9 → round up to 16 panels
Verify with daily production check:
Each 400W panel at 5.0 PSH produces:
400W × 5.0 hours = 2,000 Wh = 2.0 kWh/day per panel
18 panels × 2.0 kWh = 36 kWh/day — which after the 20% system losses yields approximately 28.8 kWh of usable energy, comfortably above the 26.7 kWh daily target. ✅
Step 7 — Size Your Inverter
The inverter converts your panels’ DC electricity into usable AC electricity. It must be correctly matched to your panel array — neither too small (clipping production) nor too large (running inefficiently).
Standard inverter sizing ratio:
Inverter AC Rating = Panel Array DC Capacity ÷ 1.1 to 1.25
A DC-to-AC ratio of 1.1–1.25 is standard practice. Panels rarely produce their full rated output simultaneously in the real world, so a slightly smaller inverter captures almost all real production while reducing cost and running more efficiently.
For the 7 kW example:
- Conservative: 7,000 ÷ 1.1 = 6.36 kW → specify 6.5 kW inverter
- Standard: 7,000 ÷ 1.2 = 5.83 kW → specify 6 kW inverter
Also verify the 120% electrical safety rule:
Your solar system’s combined output must not exceed 120% of your home’s main electrical panel rating. A 200-amp main panel allows a maximum solar breaker of 40 amps (200 × 0.20 = 40). Confirm your panel’s busbar rating before finalizing system size.
Step 8 — Size Your Battery (If Needed)
A battery stores surplus daytime generation for evening and nighttime use. For grid-connected systems it’s optional — but for off-grid systems and backup power requirements, it’s essential.
Formula:
Battery Capacity (kWh) = Nighttime Usage (kWh) ÷ Battery DoD × Days of Autonomy
Step 8a — Estimate nighttime consumption:
Most homes use 40–60% of daily consumption between 6 PM and 6 AM.
26.7 kWh × 0.50 = 13.35 kWh nighttime usage
Step 8b — Apply battery Depth of Discharge (DoD):
- LFP lithium: 85–95% DoD → divide by 0.85
- Lead-acid / AGM: 50% DoD → divide by 0.50
For LFP (1 night storage):
13.35 ÷ 0.85 = 15.7 kWh → specify 15–16 kWh LFP battery
For off-grid (3 nights autonomy with LFP):
Full Worked Examples
Example 1: Average Home in Dhaka, Bangladesh (5.0 PSH)
- Daily usage: 26.7 kWh (800 kWh/month)
- Peak sun hours: 5.0
- Raw system size: 26.7 ÷ 5.0 = 5.34 kW
- With 20% buffer (×1.2): = 6.4 kW → specify 6.5 kW
- Panels (400W): 6,500 ÷ 400 = 16–17 panels
- Inverter: 5.5–6 kW
- Battery (LFP, 1 night): 15–16 kWh
Example 2: Large Home in Rajshahi, Bangladesh (5.8 PSH)
- Daily usage: 46.7 kWh (1,400 kWh/month)
- Peak sun hours: 5.8
- Raw system size: 46.7 ÷ 5.8 = 8.05 kW
- With 20% buffer (×1.2): = 9.66 kW → specify 10 kW
- Panels (440W): 10,000 ÷ 440 = 23 panels
- Inverter: 8–9 kW
- Battery (LFP, 1 night): 27–28 kWh
Example 3: Off-Grid Cabin (4.5 PSH, 3 Days Autonomy)
- Daily usage: 10 kWh
- Peak sun hours: 4.5 (using lowest month — worst case)
- Raw system size: 10 ÷ 4.5 = 2.22 kW
- With 30% off-grid buffer (×1.3): = 2.89 kW → specify 3 kW
- Panels (400W): 3,000 ÷ 400 = 8 panels
- Inverter: 2.5–3 kW
- Battery (LFP, 3 days): (10 × 3) ÷ 0.85 = 35.3 kWh battery bank
Example 4: Small Apartment in London, UK (2.8 PSH)
- Daily usage: 10 kWh (300 kWh/month)
- Peak sun hours: 2.8
- Raw system size: 10 ÷ 2.8 = 3.57 kW
- With 20% buffer (×1.2): = 4.28 kW → specify 4.5 kW
- Panels (400W): 4,500 ÷ 400 = 12 panels
- Inverter: 3.5–4 kW
- Battery (LFP, 1 night): 5–6 kWh
This example illustrates why northern European solar needs significantly more panels than tropical or subtropical locations for the same energy output — 12 panels for 300 kWh/month vs. just 5 panels for the same usage in a 5.5 PSH location.
Common Mistakes to Avoid
Using daylight hours instead of peak sun hours. A 12-hour day is not 12 peak sun hours. Using daylight hours underestimates required system size by 30–60% in temperate climates. Always use irradiance data from the Global Solar Atlas or PVGIS.
Using summer PSH instead of annual average. Summer peak sun hours look great but your system needs to perform year-round. Using the summer peak produces a system that undersizes for winter months — sometimes severely, particularly in locations with strong seasonal variation.
Skipping the derate factor entirely. Forgetting that panels don’t produce their rated wattage in real conditions is the most common cause of disappointed solar buyers. A 400W panel does not deliver 400W continuously — it delivers closer to 320–340W on an average basis after all system losses. The 20% buffer and derate factor exist specifically to close this gap.
Applying PSH from the wrong data source. Using a generic regional figure from a table instead of your specific location’s data introduces unnecessary error. Two cities 100 km apart can differ by 0.5–1.0 PSH based on local cloud patterns and elevation. Always use the Global Solar Atlas with your precise address.
Not adjusting for roof orientation. PSH values from irradiance maps assume optimal tilt facing true south (northern hemisphere). An east or west-facing roof delivers 15–25% less energy than the map value suggests. If your roof isn’t ideally oriented, multiply your PSH figure by 0.78–0.85 before using it in the formula.
Using peak month PSH for off-grid sizing. Off-grid systems must survive the worst-case period, not the best. Size your off-grid battery and panel array based on the lowest PSH month — typically December or January in the northern hemisphere.
Frequently Asked Questions
What is the formula for using peak sun hours to size a solar system?
The core formula is: Required System Size (kW) = (Daily kWh Usage ÷ Peak Sun Hours) × 1.2. Divide your average daily energy consumption by your location’s peak sun hours to get the raw system size. Multiply by 1.2 to apply the 20% production buffer that compensates for real-world system losses. The result is your total DC panel capacity.
How does changing peak sun hours affect system size?
It has a direct, proportional effect. Doubling your peak sun hours halves your required system size for the same energy output. A home needing 10 kWh/day at 3.0 PSH (London) needs a 4.0 kW system. The same home at 6.0 PSH (Dubai) needs only a 2.0 kW system. Location is the most powerful variable in solar system sizing.
Should I use monthly or annual peak sun hours in the calculation?
For grid-connected systems, use the annual average — the grid handles seasonal shortfalls, and annual average gives the most accurate year-round sizing baseline. For off-grid systems, use the lowest month’s PSH to ensure the system handles the least-sunny period independently without running out of power. For a battery-backed grid-connected system, annual average works but check that your battery capacity covers the worst winter months.
How many solar panels do I need per kWh of daily usage?
A useful quick estimate: at 5.0 PSH with 400W panels and a 20% buffer, you need approximately 0.6 panels per kWh of daily usage. For 10 kWh/day: 6 panels. For 30 kWh/day: 18 panels. At lower PSH locations like northern Europe (3.0 PSH), this rises to approximately 1.0 panel per kWh of daily usage — nearly twice as many panels for the same output.
What is a derate factor and how does it differ from the 20% rule?
Both account for real-world system losses — they are two ways of expressing the same concept. The 20% rule multiplies the raw system size by 1.2 (or equivalently, divides by 0.833). The derate factor approach divides by 0.80 — meaning it assumes 80% of rated DC panel capacity reaches your appliances as usable AC electricity. The standard NREL derate factor of 0.80 is slightly more conservative than the 20% rule’s 0.833, but the practical difference in system sizing is minimal. For most residential calculations, the 20% rule and a 0.80 derate factor produce results within 5% of each other.
Can I use peak sun hours to estimate my monthly solar production?
Yes — it’s straightforward. Monthly Production (kWh) = Panel Wattage (kW) × Daily PSH × 30 × Derate Factor. For a 6 kW system at 5.0 PSH with 0.80 derate: 6 × 5.0 × 30 × 0.80 = 720 kWh/month. This is the expected monthly generation — compare it to your monthly consumption to assess what percentage of your electricity the system covers.
Does panel efficiency affect the peak sun hours calculation?
Panel efficiency determines how much physical roof space a given wattage requires — it doesn’t change the peak sun hours calculation itself. A 20% efficient 400W panel and a 22% efficient 440W panel both use PSH identically in the sizing formula — you divide your daily consumption by PSH regardless of panel efficiency. Efficiency matters when roof space is limited — higher efficiency panels produce more watts per square meter, letting you fit more capacity in a constrained area.
