Solar Sizing Calculator
Show the ideal energy-only PV size and an adjusted planning size using efficiency and reserve assumptions.
Use the Solar Sizing Calculator
Show the ideal energy-only PV size and an adjusted planning size using efficiency and reserve assumptions.
Calculations happen locally in your browser. ToolLott does not send these inputs to a server for this tool.
Preliminary engineering calculation only. It does not replace applicable codes, manufacturer data, licensed electrical or mechanical design, protection studies, derating, fault checks, site conditions or professional verification.
How to use it
Replace the worked-example values with the actual project inputs. Review the intermediate quantities and assumptions before using the result elsewhere.
How the Solar Sizing Calculator works
Show the ideal energy-only PV size and an adjusted planning size using efficiency and reserve assumptions. The methodology below exposes the production equation, a QA-verified worked example and the boundary between this screen and detailed design.
Calculation breakdown
ToolLott will explain the current inputs and displayed result here.
Energy-balance PV sizing screen
The production engine evaluates ideal kW = daily kWh / peak-sun-hours; adjusted = ideal / efficiency x (1+reserve). It begins by divide daily energy requirement by entered peak-sun-hours for ideal array kW., then correct for the entered overall system efficiency., and finally apply the selected reserve and clearly separate this energy screen from detailed site modelling. This keeps the calculation auditable instead of hiding design assumptions behind a single number.
ideal kW = daily kWh / peak-sun-hours; adjusted = ideal / efficiency x (1+reserve)ToolLott evaluates the relationship using the entered units and full numeric precision before display rounding.
| Symbol / input | Meaning | Unit |
|---|---|---|
energy | Average daily energy (kWh/day) | user input |
sunHours | Peak sun hours (h/day) | user input |
efficiency | Overall system efficiency (%) | user input |
reserve | Additional design reserve (%) | user input |
Step-by-step method
- Divide daily energy requirement by entered peak-sun-hours for ideal array kW.
- Correct for the entered overall system efficiency.
- Apply the selected reserve and clearly separate this energy screen from detailed site modelling.
Worked example
Grace, a project engineer, has an average 18 kWh/day site load and expects about 5 equivalent peak-sun-hours, so it needs a first-pass PV size before losses and storage are modelled.
Example inputs
- Average daily energy (kWh/day): 18
- Peak sun hours (h/day): 5
- Overall system efficiency (%): 80
- Additional design reserve (%): 10
Calculation / processing
Ideal = 18 / 5 = 3.6 kW.Efficiency-adjusted = 3.6 / 0.80 = 4.5 kW.With 10% reserve: 4.5 x 1.10 = 4.95 kW planning size.
Use the result as a screening or quantity-planning value and compare it with the project criteria, manufacturer data and applicable engineering requirements before making a design or procurement decision.
Assumptions
- The entered values are representative of the condition being screened and use the units shown on the page.
- The equation models only the stated relationship; omitted system effects are not silently estimated.
Limitations
- This page is not engineering certification and does not replace applicable standards, design loads, manufacturer data, site investigation or competent professional review.
- Results can change materially when real geometry, losses, transients, material properties, duty cycles, safety factors or regulatory criteria differ from the simplified inputs.
Common questions
Is this a final engineering design?
No. It is a transparent screening calculation using the entered assumptions and the stated equation.
Why does the page show assumptions and limitations?
Engineering formulas are only valid within their modelling assumptions; exposing them helps users decide what additional design checks are required.
Should I use the rounded displayed value in later design work?
Use suitable calculation precision and the governing project/standard requirements rather than relying on display rounding alone.
Methodology sources
Related ToolLott tools
A realistic way Grace could use this tool
Grace is a project engineer.
Grace has an average 18 kWh/day site load and expects about 5 equivalent peak-sun-hours, so it needs a first-pass PV size before losses and storage are modelled.
The worked data is explicit: energy = 18; sun hours = 5; efficiency = 80; reserve = 10. From that input set the tool returns Ideal PV size: 3.6 kW, so Grace can change one assumption at a time and see what actually drives the output.
The case ends with a measurable/checkable result—Ideal PV size: 3.6 kW. That is the evidence Grace can use to a first-pass PV size before losses and storage are modelled, rather than simply being told the tool was helpful. The page also keeps this limitation explicit: Preliminary screening or quantity calculation only.
Fictional scenario using realistic example data. For Ready tools, the worked result is tied to the tested example shown in the tool. Replace the figures with your own inputs and independently verify important professional, financial, legal, health or safety decisions.
What this calculator is for
Use it to estimate solar PV system size from energy demand, peak sun hours, efficiency and reserve assumptions.
It sits within ToolLott’s Electrical collection, where you can also estimate battery banks, cable sizes, generator sizes, motor current, Ohm’s law values, solar systems, transformers and voltage drop.