EngineerHub.orgWhat this calculator does
Enter average electricity use or a monthly bill, select a representative solar region, and describe the usable roof area. The tool estimates system capacity, panel count and annual production, then distributes output month by month to show seasonal strengths and weaknesses. The foldable economics section estimates savings, undiscounted payback and discounted payback.
Inputs
Results
The regional profile already represents ordinary system losses in a simplified way. Address-level tools such as PVGIS and PVWatts model the location, weather database and system configuration in more detail.
Your estimated solar system at a glance
Monthly solar production and household demand
This subsection uses the feasible roof system calculated above. It separates electricity used in the home from exported electricity because the two may have different values.
This is a simplified cash-flow estimate. Financing costs, taxes, inverter replacement, tariff changes, curtailment, insurance and property-value effects are not modelled.
Background
Optional reading — open any section below.
kWh is energy. It is the unit shown on electricity bills and tells you how much electricity the home used over time. kW is instantaneous power. kWp is the rated peak DC capacity of a solar array under standard test conditions.
A 10 kWp array does not produce 10 kWh every hour. Output rises and falls with sunlight, roof orientation, temperature, shade, snow, dirt and equipment performance. This calculator estimates annual energy by multiplying installed kWp by a regional specific yield in kWh/kWp/year.
Each EngineerHub regional screening preset contains two simplified assumptions: an annual specific yield and a monthly distribution. The annual yield estimates how much electricity one installed kWp may produce over a year. The monthly distribution shows how that annual total is spread across the seasons.
These EngineerHub screening presets are deliberately broad and are not sourced climate normals. Two roofs in the same region can perform differently because of local weather, horizon shading, roof pitch, direction and system losses. The custom option is preferable when you already have an address-specific annual yield from PVGIS, PVWatts or an installer proposal.
The target system size is the selected annual solar-energy target divided by adjusted regional yield. Panel count is rounded up because a fraction of a panel cannot be installed. Roof fit is then checked by dividing usable roof area by panel area plus the selected layout allowance.
Use usable roof area, not the total building footprint. Exclude obviously shaded zones, skylights, chimneys, vents and areas that must remain clear. Fire-access rules, structural loading, wind, snow and exact mounting geometry require a site-specific review.
Self-consumed solar is valued at the retail electricity price you avoid paying. Exported solar is valued at the export compensation you enter. The model then applies annual panel degradation, the entered change in the avoided import price, maintenance cost and discount rate. Export compensation is held constant unless you manually change it.
Undiscounted payback accumulates the modeled annual cash flows without discounting; because those cash flows include degradation and the entered import-price change, it is not the strict cost ÷ year-one-savings definition of simple payback. Discounted payback reduces future cash flows to reflect the time value of money. Both are screening estimates. Financing, taxes, inverter replacement, curtailment, insurance and future tariff reforms can materially change the result.
A home using 900 kWh per month consumes 10,800 kWh per year. If the adjusted regional yield is 1,000 kWh/kWp/year, a 100% annual target requires about 10,800 ÷ 1,000 = 10.8 kWp.
With 440 W panels, the theoretical count is 10,800 W ÷ 440 W = 24.5, so the result is rounded up to 25 panels. Roof-space limits may reduce the feasible system below that target.
This is a first-pass planning tool. It does not calculate structural capacity, wind and snow loading, cable sizes, protection, inverter clipping, module mismatch, detailed horizon shading, hourly self-consumption, battery operation, utility approval or local planning requirements.
The monthly chart compares monthly energy totals. It cannot show whether solar and household demand occur at the same hour. A home may show a monthly surplus while still importing electricity every evening.
- European Commission JRC PVGIS for address-level monthly and annual PV estimates.
- PVWatts for location-based grid-connected PV production estimates.
- U.S. Department of Energy homeowner solar guidance.
- Before buying: obtain at least one site-specific shading and production report, confirm roof condition, and compare the installer assumptions with the values entered here.
Related EngineerHub tools: Battery Storage Sizing Tool and Generator Sizing Tool.
Battery next steps: use the Battery Size & Backup Runtime Calculator for backup kWh/kW, the Home Battery Payback Calculator for self-consumption and tariff economics, or follow the combined solar + battery worked example.
How accurate are the regional presets?
They are EngineerHub representative screening profiles, not address-level forecasts or published regional design values. They show how location and season change output. Before buying a system, compare the result with an address-based PVGIS or PVWatts model and an installer shading survey.
Why does winter production fall so much in northern regions?
Short daylight hours, low sun angles, cloud cover and snow can sharply reduce winter output. A system can meet an annual energy target while still producing far less than household demand in winter months.
Should I size for 100% of annual electricity use?
Not automatically. Roof space, export compensation, self-consumption, future electric-vehicle or heat-pump loads and local utility rules all matter. A smaller system can sometimes have a faster financial payback.
Does a 100% annual offset make the home off-grid?
No. Annual production and annual use can be equal while the timing is completely different. Grid-tied homes commonly export surplus solar in sunny periods and import electricity at night and in winter.
Why are fixed electricity charges excluded from savings?
Solar usually reduces energy purchases, but many utilities continue to charge fixed service fees. Bill savings should therefore be calculated from avoidable energy charges, not the full bill.
What does self-consumption mean?
It is the share of solar electricity used directly in the home rather than exported. Batteries, hot-water control, electric-vehicle charging and shifting appliance use to daytime can increase self-consumption.
How many solar panels do I need for my home?
Divide the required solar-array capacity in watts by the rating of one panel, then round up to the next whole panel. The calculator does this automatically and checks the resulting panel area against the usable roof area you enter.
What is the difference between kW, kWp and kWh?
kW and kWp describe power or rated solar capacity, while kWh describes energy produced or consumed over time. A 10 kWp array does not produce 10 kWh every hour; output changes with sunlight and system conditions.
Can I use one month of electricity use?
You can, but a twelve-month average is much better. Heating, cooling, holidays and seasonal occupancy can make a single month unrepresentative.