Photovoltaic Calculator: Solar Output

A photovoltaic calculator turns a proposed solar array into practical estimates: annual generation, monthly output, panel count, roof area, inverter size, self-consumption, export, battery capacity, savings and long-term financial performance. This autonomous calculator does not contact an external solar database, so the quality of the result depends on the assumptions entered by the user.

Use a realistic solar-resource value, enter losses consistently and compare several scenarios. The result is an estimate, not a guarantee.

What the photovoltaic calculator calculates

The calculator supports three production methods and two sizing methods. You can enter the intended installed power in kilowatt-peak, or derive it from the wattage and number of modules. Production can then be estimated from annual specific yield, average peak sun hours or twelve monthly irradiation values.

The result includes:

• installed DC capacity in kWp;
• estimated panel count and minimum module area;
• suggested inverter AC power from the selected DC/AC ratio;
• annual and monthly photovoltaic generation;
• specific yield and capacity factor;
• effective losses from orientation and system components;
• self-consumed energy, exported energy and remaining grid imports;
• first-year savings, simple payback and discounted payback;
• lifetime output, net savings, ROI, NPV and simplified LCOE;
• annual and lifetime avoided grid emissions;
• a basic nominal battery-capacity estimate.

Understand kW, kWp and kWh before calculating

kWp is the rated peak power of the solar array under standard test conditions. A system containing ten 450-watt modules has a nominal capacity of 4.5 kWp. kWh is energy produced or consumed over time. A 4.5 kWp system does not produce 4.5 kWh every hour: actual output changes with sunlight, temperature, orientation, shading and equipment efficiency.

Panel count is calculated as:

Number of panels = required system power × 1,000 ÷ panel wattage.

Round panel count up to a whole module. Module area is panel count multiplied by the area of one module; allow extra roof space for access, edges and obstructions.

Method 1: annual specific yield

Specific yield expresses how many kilowatt-hours a photovoltaic system is expected to produce per installed kilowatt-peak during one year. It is written as kWh/kWp/year and is one of the clearest inputs for a manual estimate.

The calculator uses:

Annual output = installed kWp × reference specific yield × retention factor.

If the reference yield is already a net figure from a completed site assessment, entering additional losses may count them twice. In this calculator, the specific-yield field is treated as a reference value before the selected losses. The user should therefore be consistent about whether the source value is gross or net.

Fraunhofer ISE reports that, after plant losses, average annual photovoltaic yields used in German cost studies can fall roughly within 935 to 1,280 kWh per installed kWp, depending on system type and location. That range illustrates why one national default cannot replace a location-specific assessment.

Method 2: peak sun hours

Peak sun hours convert the day’s solar irradiation into an equivalent number of hours at 1,000 W/m². They are not the same as daylight hours. A cloudy day can have many hours of daylight but relatively few peak sun hours.

The formula is:

Annual output = installed kWp × peak sun hours per day × 365.25 × retention factor.

A 5 kWp array with 3.2 peak sun hours and an 86% retention factor produces an estimated 5 × 3.2 × 365.25 × 0.86 = about 5,026 kWh per year. This method is convenient when a trusted solar-resource source provides average daily irradiation rather than annual specific yield.

Method 3: monthly irradiation

The most detailed autonomous mode accepts twelve irradiation values in kWh/m²/month for the panel plane. Monthly production is estimated as:

Monthly output = installed kWp × monthly irradiation × retention factor.

This approach shows seasonal variation directly. It is useful when the user has a solar survey, meteorological report or another trusted dataset. The values should represent irradiation on the tilted panel plane, not necessarily horizontal irradiation.

If only annual production is known, the calculator’s climate profiles can distribute it across the year. Those profiles are broad seasonal shapes, not weather forecasts. Actual production varies between years; NREL’s PVWatts documentation explicitly treats photovoltaic output as an estimate subject to assumptions, incomplete system information and interannual weather variation.

How the calculator combines solar losses

The simple mode applies one combined percentage. The detailed mode combines losses multiplicatively because each acts on the remaining energy.

For example, 5% shading and 4% inverter loss produce a retention factor of:

0.95 × 0.96 = 0.912, equivalent to an 8.8% combined loss rather than exactly 9%.

The detailed fields cover shading, temperature, inverter conversion, wiring, module mismatch, soiling and system availability. The orientation factor is applied separately. A factor of 100% means no additional orientation penalty; 90% reduces output by 10%.

A general loss assumption is only a starting point; a real project should use roof, equipment and shading data.

Orientation, tilt and shading

A south-facing roof is often productive in the UK, but east-west layouts can generate electricity across a wider part of the day and may improve self-consumption. The best layout is not always the one with the highest annual kWh: household demand timing, roof constraints and export compensation matter.

Shading should not be guessed casually. Trees, chimneys, dormers, neighbouring buildings and parapets can create time-dependent losses and module mismatch. A percentage field is useful for scenario testing, but it cannot reproduce an hour-by-hour shade model or the electrical behaviour of strings, optimisers and microinverters.

Estimating self-consumption and grid export

The calculator applies the chosen self-consumption percentage to annual PV output, then limits self-used energy to annual household consumption. Export is the remaining solar generation. Grid import is annual consumption minus self-used PV energy.

Self-used PV = lower of annual generation × self-consumption rate, or annual consumption.

Export = annual generation − self-used PV.

Grid import = annual consumption − self-used PV.

This annual method is simplified because electricity-use timing strongly affects self-consumption.

UK export payments and tariff assumptions

In Great Britain, the Smart Export Guarantee enables eligible small-scale generators to receive payment for metered electricity exported to the grid. Ofgem makes clear that suppliers set their own export tariffs, so the calculator leaves this value editable. Enter the actual tariff and check metering, certification and contract requirements.

First-year savings

The calculator values self-consumed electricity at the retail purchase price avoided and exported electricity at the entered export tariff:

First-year net savings = self-used kWh × purchase price + exported kWh × export tariff − annual maintenance.

This calculation does not automatically include finance interest, tax, insurance, grants, roof repairs or opportunity cost. Add those to the project cost or annual expenses where relevant.

Simple payback, discounted payback and NPV

Simple payback identifies when undiscounted annual cash flow recovers the initial cost. Discounted payback also reduces future cash flows using the selected discount rate. Net present value subtracts the initial investment and adds discounted annual cash flows.

A project can have positive lifetime cash flow but a weak NPV when the discount rate is high. Future tariffs and behaviour are uncertain, so compare several scenarios.

Degradation, inverter replacement and lifetime production

Photovoltaic output normally declines gradually. The calculator reduces each later year by the entered degradation rate. It can also subtract a one-off inverter replacement cost in a selected year.

Lifetime generation is the sum of each degraded year. The yearly table exposes the assumptions and can be downloaded as CSV.

What the simplified LCOE means

Levelised cost of energy divides discounted project costs by discounted lifetime electricity generation. It is useful for comparing scenarios but should not be confused with a retail electricity tariff. The calculator’s simplified LCOE includes installation cost, maintenance and the entered replacement cost; it does not automatically include tax, financing structure, residual value or major roof work.

Battery capacity estimate

The battery estimate starts with average daily consumption, scales it to the selected backup duration, and adjusts for usable depth of discharge and round-trip efficiency:

Nominal capacity = daily consumption × backup-hours fraction ÷ usable depth of discharge ÷ battery efficiency.

This estimates energy capacity in kWh, not inverter power. It does not model winter demand, essential loads, charge power, reserve settings or solar recharge, so interval data and an electrical design are still required.

CO₂ savings are not a full life-cycle assessment

Avoided operational emissions are estimated from PV generation and the entered grid factor. Manufacturing, transport, installation, replacement equipment and end-of-life processing are outside this calculation.

Common input mistakes

• entering panel watts as system kilowatts;
• using horizontal irradiation as if it were panel-plane irradiation;
• applying losses twice to a net specific-yield value;
• adding detailed losses instead of combining them multiplicatively;
• assuming all annual PV energy can be used in the home;
• using an export tariff without checking eligibility;
• omitting replacement and maintenance costs;
• treating a battery estimate or annual output as a guarantee.

How to produce a defensible estimate

Use a local irradiation or yield value from a recognised dataset or installer survey and record whether it already includes losses. Compare conservative, central and optimistic cases for output, self-consumption, tariffs and cost. Before buying, obtain a site survey, structural assessment, electrical design and written generation estimate.

Frequently asked questions

Sources and further checking

• NREL PVWatts Calculator and modelling information
• NREL PVWatts Version 5 Manual
• Energy Saving Trust: solar panels, costs and savings
• Ofgem: Smart Export Guarantee

Information and source links reviewed: 20 June 2026.