Solar System Sizing

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Solar system sizing looks simple at first.

Take your daily electricity use, divide by sunshine, and you get a panel size. That basic idea is correct, but the real design question is more nuanced because sunlight is not uniform, module nameplate power is not what the home actually receives at the socket, and the right design target changes depending on whether the project is grid-tied, hybrid, or off-grid.

That is why good sizing is really a chain of linked decisions.

You start with believable daily load data, convert that into a target energy output, adjust for local peak sun hours, then apply real-world system losses before you trust the final array size.

This guide walks through that logic step by step so you can move from kWh/day to a realistic solar array target in kW.

Solar system sizing workflow showing daily load, peak sun hours, derate factor, array capacity, panel count, and grid versus off grid adjustments

The Core Formula

At its simplest, array sizing works backward from the energy the site needs each day.

Array capacity (kW) =
daily load (kWh) / (peak sun hours x system efficiency)

Where:

Daily load is the site’s energy demand in kWh/day
Peak sun hours, PSH, describe the location’s solar resource
System efficiency is the derate factor that converts ideal panel output into realistic delivered energy

This formula is powerful because it turns system sizing into a transparent chain of assumptions instead of guesswork.

Worked Example

Suppose a household uses 20 kWh/day, the site averages 5 peak sun hours per day, and the design assumes an overall system efficiency of 0.80.

Array = 20 / (5 x 0.80)

Array = 5 kW

That means the project needs roughly a 5 kW array on paper to deliver the required daily energy under those assumptions.

The important phrase there is on paper.

The quality of the result depends entirely on whether the load number, PSH value, and derate factor are realistic for the actual site.

Step 1, Start With Daily Load

Array sizing begins with energy demand, not with roof area and not with panel count.

That demand usually comes from:

An appliance-by-appliance load estimate
Utility bills converted into daily kWh
Short-term meter data averaged over time

If the daily load is wrong, the array size will be wrong no matter how good the rest of the math is.

If you have not done this step yet, read Load Estimation first.

Step 2, Use the Right Peak Sun Hours

Peak sun hours are one of the most important geographic inputs in solar design.

They do not mean the number of daylight hours. They mean the equivalent number of hours per day when solar irradiance averages 1000 W/m2.

That is why two places with similar daylight length can still have very different PSH values.

Typical broad ranges

Region	Typical `PSH`
Germany, Munich	Around `3.0` to `3.5 h/day`
United Kingdom	Around `2.5` to `3.5 h/day`
Vancouver	Around `3.8` to `4.1 h/day`
Adelaide	Around `5.0` to `5.5 h/day`
Houston	Around `5.0 h/day`
U.S. Southwest	Around `6.0` to `7.0 h/day`
Middle East and North Africa	Around `6.5` to `7.5 h/day`

These are useful for orientation, but a real project should use an address-based source such as PVWatts whenever possible.

Why `PSH` Choice Changes by System Type

The correct PSH assumption depends on what the system is trying to achieve.

Grid-tied systems

These often use annual-average solar resource because the grid can absorb shortfalls and surpluses across the year.

Off-grid systems

These should usually be sized against the worst practical solar month, often a winter low-sun condition, because there is no grid fallback. That is why off-grid sizing should usually be cross-checked against Battery Sizing instead of being treated as an array-only calculation.

Hybrid systems

These often sit in the middle. The grid still exists as backup, but battery behavior and backup expectations may justify a more conservative PSH choice than pure grid-tied design.

That is why PSH is not just a weather input. It is a design-goal input too.

Step 3, Apply a Realistic Derate Factor

Panel nameplate power is not what the home receives in real operation.

Solar energy gets reduced by a chain of losses between the module label and the final usable output.

This is why solar sizing uses a derate factor or overall system efficiency.

A practical default often lands in the 0.75 to 0.80 range, though some engineering references show lower overall factors around 0.731 once many real losses are stacked together.

Common loss sources

Loss source	Typical range
Temperature losses	About `5%` to `7%`
Cable and connector losses	About `2%` to `3%`
Inverter losses	About `2%` to `4%`
`MPPT` tracking losses	About `1%` to `2%`
Shading losses	About `0%` to `10%` depending on the site
Soiling	About `1%` to `7%` depending on climate
First-year aging and later degradation	Roughly `0.5%` to `1%` per year after installation

That is why two homes with the same load and the same sunlight can still need different array sizes if one roof is hotter, dirtier, or more shaded than the other.

A Good First-Pass Sizing Workflow

If you want the compact version, use this order.

Estimate believable daily energy use in kWh/day
Find local PSH using a location-aware tool
Choose a realistic derate factor
Solve for array capacity in kW
Convert that result into panel count and roof area
Re-check the answer against inverter size, battery goals, and roof constraints

That sequence keeps system sizing connected to both energy need and physical feasibility.

From Array Size to Panel Count

Once you know the array target in kW, you can estimate the number of modules required.

Panel count = array size (W) / panel wattage (W)

For example:

5 kW array = 5000 W
5000 / 400 = 12.5 panels

That means you would likely need about 13 panels rated at 400 W, subject to actual string layout, roof dimensions, and available module sizes.

Typical rough counts using `400 W` panels

System size	Typical panel count	Approximate roof area
`3 kW`	`7` to `9` panels	Around `11.9` to `15.3 m2`
`5 kW`	`12` to `15` panels	Around `20.4` to `25.5 m2`
`6.6 kW`	`15` to `18` panels	Around `25.5` to `30.6 m2`
`8 kW`	`19` to `24` panels	Around `32.3` to `40.8 m2`
`10 kW`	`22` to `30` panels	Around `37.4` to `51.0 m2`

These are quick planning numbers, not final layout results.

Roof Space Can Change the Design More Than the Math

A theoretically correct array size still has to fit the site.

That means checking:

Usable roof area
Orientation
Tilt
Obstructions such as chimneys and vents
Shading from trees or nearby buildings

This is why some homes that “need” a larger array mathematically may still end up with a smaller practical installation and a partial offset strategy instead of full annual coverage.

Technicians working across a large rooftop solar installation — Real sizing always meets real roof geometry, access limits, and layout constraints sooner or later. Photo by Trinh Tran on Pexels.

Grid-Tied vs Off-Grid vs Hybrid Sizing Goals

The same house can justify very different array sizes depending on the project objective.

Grid-tied

A grid-tied system often aims to offset around 80% to 100% of annual consumption, depending on export policy, budget, and roof area.

The grid covers shortfalls, so the array does not need to solve every low-sun event by itself.

Off-grid

An off-grid system must survive the worst practical solar conditions and work together with the battery bank.

That usually means:

Using low-season PSH
Adding reserve margin
Designing around battery autonomy days
Sometimes including generator backup

Hybrid

A hybrid system often aims to cover daytime load directly, support some night load with batteries, and let the grid act as the final fallback.

That can make the target array size more strategic than absolute.

Why Off-Grid Sizing Is More Conservative

Off-grid design is usually less forgiving because it cannot rely on the utility as a hidden backup layer.

That is why off-grid sizing often includes:

Worst-month sun assumptions
Larger battery reserve
Higher caution on derate factor
More attention to winter load behavior

It is common for an off-grid array to look oversized compared with a grid-tied system serving the same average daily energy because the design target is reliability, not just annual offset.

After the array target is set, most projects still need Inverter Sizing to confirm that the power electronics can actually support the loads the array was designed around.

Common Solar System Sizing Mistakes

Using daylight hours instead of true peak sun hours
Assuming nameplate panel power translates directly into delivered energy
Reusing a generic derate factor on a shaded or high-temperature site
Ignoring roof-space limits until after the math is done
Using annual-average PSH for an off-grid design that must survive winter
Forgetting that battery-backed systems may need extra array to recharge storage reliably

Most solar sizing errors do not come from a hard formula.

They come from oversimplified assumptions feeding the formula.

A Practical Design Example

Suppose a household uses 15 kWh/day.

Now compare two design paths:

Grid-tied example

Daily load = 15 kWh
PSH = 5 h/day
Derate factor = 0.80

Array = 15 / (5 x 0.80) = 3.75 kW

A first-pass answer would be about 3.8 kW.

More conservative off-grid example

Daily load = 15 kWh
Winter PSH = 3.5 h/day
Derate factor = 0.75

Array = 15 / (3.5 x 0.75) = 5.71 kW

Now the first-pass answer is about 5.7 kW.

Same household, very different system size, because the design goal changed.

Where This Page Connects to the Rest of the Design

Solar system sizing gives you the array target, but it is only one part of the final system.

After this step, most projects still need:

That is why array size should be treated as the center of a design loop, not the end of the process.

Watch or Read More

NREL PVWatts The most practical starting point for address-based solar resource and production estimates.

Alternative Energy Tutorials A simple step-by-step walkthrough of the classic array-sizing formula and planning process.

EcoFlow Step by Step Guide A useful consumer-level explanation that bridges load, panel count, and whole-system thinking.

Solacity Real-World Sizing Helpful for understanding why practical system design often differs from clean spreadsheet math.

Key Takeaways

Solar array sizing starts with daily energy demand, not panel count.
The core formula depends on three things, load, PSH, and realistic system efficiency.
A default derate factor around 0.75 to 0.80 is often useful, but difficult sites may need more caution.
Grid-tied, hybrid, and off-grid systems should not use the same solar-resource assumptions blindly.
A mathematically correct array still has to fit the roof, the inverter, and the battery strategy.

Sources Used for This Page

This page was expanded using the research notes and source list provided for this project, especially the following references.