Project

Solar PV Sizing Project

Energy engineering project for sizing a solar photovoltaic system with load profiles, resource assessment, array capacity, inverter sizing, storage, grid connection, losses, economics, risks, and validation.

Branch: Energy Engineering
Content: Project
Updated: Jun 20, 2026
Revision: v1.1.0 · reviewed

This project sizes a solar photovoltaic system for a defined load and site. The goal is to produce a defensible engineering sizing, not a sales estimate. A complete design must account for energy demand, solar resource, orientation, shading, module performance, inverter sizing, wiring losses, storage if needed, grid interaction, economic assumptions, safety constraints, and validation against requirements.

The project can be framed as a residential rooftop system, a small commercial system, a remote off-grid supply, or a hybrid backup system. The workflow is similar, but the design objective changes. A grid-connected system optimized for annual energy is different from an off-grid system optimized for worst-month reliability.

Project Objective

Define the system objective before calculating array size. Example objectives include:

offset 70 percent of annual site electricity;
supply a remote load with 95 percent availability;
reduce daytime grid import below a target threshold;
provide backup power for defined critical loads;
minimize lifecycle cost under a tariff and export rule;
reduce peak demand while preserving facility operation.

The same PV array can be good or poor depending on the objective. The project should state a primary objective and a small set of acceptance criteria.

Example acceptance criteria:

Criterion	Example target
Annual energy offset	at least 70 percent of measured load
Worst-month critical-load support	at least 2 autonomy days with storage
Inverter clipping loss	less than 3 percent of annual PV energy
Export limit compliance	no export above agreed interconnection limit
Battery reserve	at least 20 percent SOC for backup mode
Economic sensitivity	payback or net present value shown for low, base, and high tariff cases

Load Assessment

Start with energy demand. For each load:

E=Pt

where $E$ is energy, $P$ is power, and $t$ is operating time.

Build a daily, monthly, and seasonal load profile. For grid-connected buildings, use interval meter data if available. For off-grid systems, list each device, power rating, usage time, duty cycle, startup current, and criticality.

Important distinctions:

average load versus peak load;
daily energy versus instantaneous power;
critical load versus optional load;
weekday and weekend patterns;
seasonal load variation;
future load growth;
startup or motor surge current;
loads that coincide with solar production versus loads that occur at night.

The project should not size PV from annual kWh alone. A system that looks adequate annually may still fail at night, in winter, during cloudy periods, or during peak demand events.

Solar Resource and Site Survey

PV output depends on solar irradiance, module orientation, tilt, shading, temperature, soiling, snow, wiring losses, inverter behavior, and availability.

A simplified daily estimate is:

E_{PV}=P_{array}H_{PSH}\eta_{system}

where:

$P_{array}$ is DC array rated capacity;
$H_{PSH}$ is peak sun hours per day;
$\eta_{system}$ is total performance factor after losses.

This formula is useful for early sizing but should be checked against monthly resource data. The design month may be the lowest-resource month, the highest-load month, or the month with the worst resource-load mismatch.

The site survey should document:

roof or land area;
azimuth and tilt;
shading from trees, parapets, nearby buildings, poles, or equipment;
structural constraints and roof condition;
cable routing and inverter location;
wind and environmental exposure;
access for maintenance and fire service;
interconnection point and available electrical capacity.

Array Sizing

Rearrange the energy estimate:

\displaystyle P_{array}=\frac{E_{load}}{H_{PSH}\eta_{system}}

Example: a site needs $18\ \text{kWh/day}$ , receives $4.5$ peak sun hours in the design month, and has system efficiency factor $0.78$ .

\displaystyle P_{array}=\frac{18}{4.5(0.78)}=5.13\ \text{kW}

The design might select a 5.4 kW DC array to match module sizes and provide margin.

For monthly energy:

E_{PV,m}=P_{array}H_{PSH,m}N_m\eta_{system,m}

where $N_m$ is number of days in month $m$ . Monthly checks are important because annual averages can hide seasonal deficits.

Loss and Derating Budget

PV systems are affected by losses:

module temperature;
inverter conversion;
wiring resistance;
mismatch;
soiling;
shading;
snow or debris;
module degradation;
transformer or switchgear loss;
curtailment;
availability and downtime.

A performance ratio or total derating factor combines these effects:

\eta_{system}=\eta_{temp}\eta_{inv}\eta_{wiring}\eta_{soiling}\eta_{shade}\eta_{availability}\cdots

The project should state each assumed factor and justify the total. A single unexplained “loss factor” is weak engineering evidence.

Inverter Sizing

The inverter must handle AC power demand, operate safely with the DC array, and satisfy grid or standalone requirements.

For AC apparent power:

\displaystyle S=\frac{P}{PF}

If reactive power support is required:

S=\sqrt{P^2+Q^2}

For grid-tied systems, the DC-to-AC ratio is a design choice:

\displaystyle r_{DC/AC}=\frac{P_{DC,array}}{P_{AC,inverter}}

Some DC oversizing can improve annual energy yield because the inverter operates closer to useful output during low irradiance. Too much oversizing causes clipping:

\displaystyle E_{clip}=\int \max(P_{DC}(t)-P_{AC,max},0)\,dt

Check:

maximum DC input voltage at low temperature;
MPPT voltage range;
maximum input current;
AC output rating;
surge capability for motors or islanded loads;
reactive power and power-factor requirements;
protection, isolation, and anti-islanding requirements;
efficiency curve at expected operating points.

Storage Sizing

If storage is required, estimate usable battery capacity:

\displaystyle E_{bat}=\frac{E_{critical}N_{autonomy}}{DOD\eta_{bat}}

where:

$E_{critical}$ is daily critical load;
$N_{autonomy}$ is days of autonomy;
$DOD$ is allowable depth of discharge;
$\eta_{bat}$ is battery efficiency.

Storage should also check power rating:

P_{bat,rated}\geq P_{critical,peak}

Battery design must account for state of charge, charge rate, discharge rate, temperature limits, cycle life, fire safety, ventilation, protection, maintenance, and end-of-life capacity fade. A battery that has enough kWh may still fail if it cannot deliver peak kW or charge quickly enough before the next event.

Grid Connection and Protection

For grid-connected systems, interconnection is part of the design. The project should address:

point of interconnection;
export limit or net-metering rule;
inverter protection functions;
breaker and disconnect ratings;
grounding and bonding concept;
short-circuit and overcurrent protection;
power factor and reactive power settings;
monitoring and metering;
utility approval and commissioning tests.

If export is limited:

P_{export}(t)=\max(P_{PV}(t)-P_{load}(t)-P_{charge}(t),0)

The control strategy must keep this value below the allowed export limit. A design that relies on average load to absorb PV output may violate export limits during low-load, high-sun conditions.

Economics and Sensitivity

Economic analysis may include capital cost, replacement cost, electricity tariff, export value, maintenance, incentives, degradation, discount rate, project life, and financing.

A simple payback estimate is:

\displaystyle Payback=\frac{C_{initial}}{S_{annual}}

where $C_{initial}$ is initial cost and $S_{annual}$ is annual savings. This is only a screening metric. A stronger project also checks net present value or lifecycle cost under tariff, export, degradation, and maintenance uncertainty.

Do not hide assumptions. A payback estimate is only as credible as the tariff, export, degradation, and maintenance assumptions behind it.

Risk Review

A useful project report should identify the risks that could make the sizing wrong:

load growth after installation;
shading growth from vegetation or new construction;
roof replacement during project life;
export rule or tariff change;
inverter replacement cost;
battery degradation faster than assumed;
equipment downtime;
soiling or snow not maintained;
overheating or ventilation limits;
interconnection approval delay.

The report should state which risks are handled by design margin, monitoring, maintenance, contractual assumptions, or future review.

Validation and Deliverables

Final deliverables should include:

project objective and acceptance criteria;
load table and load profile;
solar resource basis and design month;
site survey notes and shading assumptions;
DC array sizing calculation;
inverter sizing and DC/AC ratio;
storage sizing if applicable;
loss and derating budget;
monthly energy estimate;
one-line electrical concept;
interconnection and protection assumptions;
economic sensitivity;
risk register;
commissioning and monitoring plan;
items requiring licensed professional or authority review.

Commissioning should compare measured system behavior with the design assumptions. Useful checks include inverter startup, power output under known irradiance, meter direction, export control, battery charge and discharge behavior, communication monitoring, and emergency shutdown or isolation where required.

Engineering Lesson

Solar PV sizing is an energy balance problem constrained by electrical ratings, resource variability, economics, interconnection rules, and operation. A strong project does not simply divide annual kWh by panel wattage. It explains when energy is needed, when sun is available, how losses reduce output, what equipment limits apply, and what happens during low-resource periods, storage depletion, or grid events.

REF

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