What size solar system do I need?
solar_pv_system_sizeWork out the solar PV system a UK home needs: the capacity in kWp, the number of panels, the battery to go with it, and what it would do to the electricity bill.
This runs the MCS arithmetic backwards. The other solar calculators here each answer one question about a system somebody has already specified. This one starts from what a household uses and arrives at the system that covers it, which is the question a householder actually has.
The chain is short and every link is published. Annual consumption sets the generation the array has to make. The MCS irradiance dataset behind MIS 3002 says what one kWp makes on that roof, which gives a capacity. Capacity divided by panel size gives a panel count, rounded up, because panels come whole. The MGD 003 tables then say how much of that generation the household will use and how much it will export.
Only the postcode and the roof direction are required. Direction is a compass point, not an angle: 'S', 'SE', 'SW', 'E', 'W', 'NE', 'NW' or 'N'. Give the annual consumption from a bill if it is known, since the 3,500 kWh default is only the British average. Shading can be described in words instead of measured, and a roof area caps the panel count where the roof is the constraint.
No cost is assumed. Installed prices vary far too much between houses to guess at, so a payback figure appears only when a cost from a real quotation is supplied.
Use this to size a system or to sanity-check a quotation. Use 'solar_pv_output' instead when the capacity is already known and the question is only what it will generate.
Method from MIS 3002. These calculators follow the published MCS methods, for understanding a system and checking a quotation. Anything that gets installed still needs a design and a site survey from a certified installer.
Input Schema
| Name | Required | Description | Default |
|---|---|---|---|
| shading | No | How much shading the roof has, in words: 'none', 'light', 'moderate' or 'heavy'. Defaults to 'none'. Use 'shadeFactor' instead if a proper MGD 005 assessment has been done. | |
| postcode | Yes | UK postcode of the property, for example 'BS1 5TR'. Only the outward code matters. Required. | |
| direction | Yes | Compass point the roof faces: 'S', 'SE', 'SW', 'E', 'W', 'NE', 'NW' or 'N'. Required. | |
| occupancy | No | Weekday occupancy: 'home_all_day', 'in_half_the_day' or 'out_all_day'. Decides how much of the generation is used at home instead of exported. Defaults to 'in_half_the_day'. | |
| panelWatts | No | Nameplate rating of a single panel in watts. Defaults to 440, which is typical of a current domestic module. | |
| roofAreaM2 | No | Shade-free roof area available for panels, in square metres. Optional; when given it caps the number of panels. | |
| inclination | No | Roof pitch in degrees from horizontal. Defaults to 35, which is typical of a British pitched roof. A flat roof is 0. | |
| shadeFactor | No | A measured shade factor between 0.5 and 1 from an MGD 005 assessment. Overrides 'shading' when given. | |
| installedCost | No | The installed cost of the system in pounds, from a quotation. Optional; supplying it gives a payback period. No cost is assumed if it is left out, because prices vary too much to guess at. | |
| includeBattery | No | Whether to size a battery alongside the panels. Defaults to false. | |
| batteryModuleKwh | No | Nominal capacity of one battery module in kWh. Batteries are sold in whole modules, so the answer rounds up to one. Defaults to 5. | |
| exportPricePerKwh | No | What the household is paid for a unit exported, in pounds. Optional. | |
| annualConsumptionKwh | No | The household's electricity use in kWh a year, from a bill. Defaults to 3500, which is the British average and the figure MGD 003 falls back on, but a real bill sharpens the answer considerably. | |
| electricityPricePerKwh | No | What the household pays for a unit, in pounds. Optional; supplying it puts a value on the answer. |
Output Schema
| Name | Required | Description | Default |
|---|---|---|---|
| zone | Yes | ||
| notes | Yes | ||
| method | Yes | ||
| panels | Yes | ||
| battery | No | ||
| shading | No | The shading description used, when one was chosen in place of a measured factor. | |
| direction | Yes | ||
| panelWatts | Yes | ||
| references | Yes | ||
| capacityKwp | Yes | The installed capacity: the panel count multiplied by the panel rating. | |
| exportedKwh | Yes | ||
| importedKwh | Yes | Units still bought from the grid over a year. | |
| inclination | Yes | ||
| roofLimited | Yes | True when the roof area given, not consumption, set the size. | |
| shadeFactor | Yes | ||
| paybackYears | No | Installed cost divided by the annual benefit. Only present when a cost was supplied. | |
| postcodeUsed | Yes | ||
| annualBenefit | No | Bill saving plus export income. | |
| installedCost | No | ||
| newAnnualCost | No | ||
| occupancyLabel | Yes | ||
| selfConsumedKwh | Yes | ||
| annualBillSaving | No | ||
| idealCapacityKwp | Yes | The capacity that would exactly cover a year of consumption, before rounding up to whole panels. | |
| roofAreaNeededM2 | Yes | ||
| currentAnnualCost | No | ||
| annualExportIncome | No | ||
| consumptionAssumed | Yes | True when the 3500 kWh default stood in for a figure the caller did not give. | |
| annualGenerationKwh | Yes | ||
| annualConsumptionKwh | Yes | ||
| selfConsumptionPercent | Yes | ||
| specificYieldKwhPerKwp | Yes | Kk: the kWh a kWp produces in a year at this zone, pitch and orientation. | |
| gridIndependencePercent | Yes | The share of the household's own use met by the system. | |
| generationAsPercentOfUse | Yes | Annual generation as a percentage of annual consumption. Above 100 means the system makes more over a year than the house uses, though not at the same times. |