SpecCalc Hub · Medium
Solar Panel Size Calculator
Estimate PV watts and whole-panel count from daily energy, peak sun hours, system losses and solar coverage.
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Formula
coverage_kwh_day = daily_energy_kwh * coverage system_efficiency = 1 - losses required_pv_kw = coverage_kwh_day / (peak_sun_hours * system_efficiency) panel_count = ceil(required_pv_w / panel_wattage_w)
Assumptions
- Inputs are user-provided.
- Results are preliminary estimates.
- Peak sun hours are provided by the user or trusted data.
Limitations
- Roof angle, azimuth, shading, snow, and interconnection rules are not modeled.
Sources / methodology
Simple PV sizing based on daily energy divided by sun hours and efficiency.
- Formula version
- 1.0.6
- Formula review date
- 2026-05-25
Source links
Quick answer: Use this solar panel size calculator to estimate PV watts and whole-panel count from daily energy use, peak sun hours, system losses and target solar coverage. It is a preliminary planning estimate for roof, off-grid or backup scenarios, not a final solar design or local installation approval.
Input parameters
- Daily energy (kWh/day): 20 kWh/day
- Peak sun hours (h/day): 5 h/day
- System losses (%): 20 %
- Solar coverage (%): 100 %
- Panel wattage (W): 400 W
Output values
- Required PV (kW)
- Required PV (W)
- Panel count (pcs)
- Actual PV with whole panels (kW)
Step-by-step example
- Enter values into the calculator fields.
- Run the calculation and review the result values.
- Compare the result with the assumptions, limitations and sources below.
How to calculate the size of solar panel needed
Start with daily energy use, decide how much of that load should come from solar, then divide by local peak sun hours and system efficiency. The calculator keeps those assumptions explicit so the PV watt estimate can be reviewed instead of hidden behind a black-box result.
Solar panel size calculator vs solar power estimate calculator
Both phrases usually describe the same early planning task: estimate PV watts from load and sun resource. SpecCalc Hub keeps the page focused on daily energy, peak sun hours, system losses and whole-panel rounding, so the estimate is usable for planning without implying a final installation design.
Solar panel sizing calculation formula
The simplified method is: coverage_kWh_per_day = daily_energy_kWh x solar_coverage. Then required_PV_kW = coverage_kWh_per_day / (peak_sun_hours x system_efficiency), where system_efficiency = 1 - losses. Whole panel count is rounded up from required PV watts and selected panel wattage.
What changes solar panel size the most?
Peak sun hours, daily energy use and system losses move the result much more than panel wattage alone. Panel wattage mostly changes how many whole panels you need after the required PV wattage has already been estimated.
| Daily energy | Peak sun hours | Losses | Estimated PV |
|---|---|---|---|
| 6 kWh/day | 5 h/day | 20% | 1.50 kW |
| 6 kWh/day | 4 h/day | 20% | 1.88 kW |
| 6 kWh/day | 3 h/day | 20% | 2.50 kW |
| 6 kWh/day | 5 h/day | 30% | 1.71 kW |
Example: estimating solar panel size for daily Wh usage
If a load uses 6 kWh/day, solar should cover 100%, peak sun hours are 5 h/day and losses are 20%, the planning estimate is 6 / (5 x 0.8) = 1.5 kW of PV. With 400 W panels, that becomes 1500 / 400 = 3.75, so the calculator rounds to 4 panels for a whole-panel estimate.
Roof space and layout still matter
The page estimates PV watts first and only then rounds up to whole panels. A roof or vehicle layout can still reject the rounded panel count because of geometry, shading gaps, orientation, walkway clearance, tilt frames or mounting constraints.
How to estimate roof area from panel dimensions
After the calculator returns a whole-panel count, take the selected module length and width from its manufacturer datasheet and multiply the area of one module by the panel count. Then reserve additional usable area for roof edges, walkways, row spacing, orientation, shading, mounting hardware and obstacles. This planning check does not replace a roof survey or structural review.
UK-style solar sizing note
The calculator can still be used for UK-style planning if you enter realistic local peak sun hours and loss assumptions. It does not check UK standards, roof structure, export rules, permitting, installer practice or local compliance requirements.
Worked examples
Household solar sizing example
A home using 8 kWh/day with 5 peak sun hours and 20% losses needs roughly 2.0 kW of PV for full solar coverage. With 400 W panels, the whole-panel estimate becomes 5 panels.
UK-style planning example
If local planning assumptions are more conservative and peak sun hours are entered as 3 h/day instead of 5 h/day, the same load needs much more PV wattage. That is why a UK-style estimate depends on local irradiance assumptions, not only panel nameplate power.
Mobile or vanlife planning example
A van or off-grid cabin with 1.5 kWh/day of DC and inverter loads can use the page as a first PV estimate, then continue to Battery Runtime and Inverter Size before any real hardware purchase.
Roof and layout caution example
If the estimate says 5 panels but the roof or vehicle can place only 4 with safe spacing and the intended orientation, the energy target must be revised or the layout reworked. The calculator gives the electrical planning layer, not the structural or geometric fit.
Common mistakes
- Use daily energy, not a monthly or annual value, unless you convert it first.
- Use peak sun hours from a credible location-specific source.
- Check roof geometry, shading, mounting clearances and electrical design separately.
What to check next
Check the inputs, limitations, sources and related calculators before using the estimate in a real decision.
FAQ
How do I calculate the size of solar panel needed?
Estimate daily energy first, decide the solar coverage target, then divide that energy by peak sun hours and system efficiency. The result is a preliminary PV watt estimate.
What is the basic solar panel sizing formula?
A simple planning method is required_PV_kW = daily_energy_to_cover / (peak_sun_hours x system_efficiency). Whole panel count is then rounded up from PV watts and panel wattage.
Is this calculator valid for the UK?
It can be used for UK-style planning if you enter realistic local sun-hour and loss assumptions, but it does not check UK installation rules, permitting or compliance.
Why do peak sun hours matter?
Peak sun hours compress local daily solar resource into equivalent full-output hours. Lower sun hours require more PV for the same energy target.
Why does panel wattage change panel count more than required PV?
Required PV is driven mainly by energy target, sun hours and losses. Panel wattage mostly changes how many whole modules are needed to meet or slightly exceed that required PV target.
Why can roof space still block the result?
The calculator can show the electrical PV target, but real roofs and vehicle layouts still have spacing, shading, tilt, walkway and mounting constraints that can reject the rounded panel count.
Is this a final solar design?
No. It is a preliminary estimate only and does not replace roof assessment, shading review, structural checks, electrical design or local code verification.
Related calculators
This calculator provides an estimate for informational purposes only. It is not a certified engineering design, electrical safety approval, or professional installation recommendation. Always verify final decisions with a qualified professional and applicable local codes.