Tools
Solar calculators
Every calculator here shows its working: the formula it uses, the assumptions baked into it, how to read the result and a worked example with real numbers. A figure you cannot audit is not worth acting on.
Payback and ROI
Year-by-year cash flow across 25 years, separating self-consumed from exported energy, with NPV, IRR and levelised cost of energy.
Inputs: Installed cost, yield, self-consumption, tariffs, degradation
Battery sizing
Finds the smaller of the two ceilings that decide storage capacity: evening load and available solar surplus, corrected for depth of discharge.
Inputs: Daily load, surplus, depth of discharge, cycle life
Inverter sizing and DC/AC ratio
Cold-morning open-circuit voltage, hot-roof MPPT voltage, string current and an estimate of clipping loss.
Inputs: Module datasheet values, string length, inverter ratings
Cable sizing and voltage drop
Volt drop, percentage of nominal, heat dissipated and the cross-section needed to reach a 1% or 3% target.
Inputs: Current, route length, cross-section, conductor temperature
System sizing
Works backwards from annual consumption to the capacity, module count and roof area a system needs.
Inputs: Consumption, specific yield, module power, roof packing
Self-consumption
Splits generation between energy used on site and energy exported, and shows what a battery adds in kWh and money.
Inputs: Generation, consumption, daytime load share, battery size, tariffs
Carbon savings
Lifetime avoided emissions net of embodied carbon, with carbon payback time and the system's own gCO₂e/kWh.
Inputs: Capacity, yield, grid intensity, embodied carbon, life
Row spacing and shading pitch
Minimum pitch between tilted rows for a chosen design hour, plus ground coverage ratio and the capacity you fit per hectare.
Inputs: Latitude, tilt, module length, design solar altitude
Tilt and orientation
How much annual yield a given tilt and azimuth keeps compared with the optimum, plus summer and winter bias.
Inputs: Latitude, tilt, azimuth, reference yield
Cell temperature and thermal losses
Cell temperature from ambient and irradiance, the instantaneous power loss it causes, and the annual energy penalty.
Inputs: Ambient temperature, irradiance, mounting type, temperature coefficients
Annual energy yield
Annual and monthly production from irradiation and an itemised loss chain, with the resulting performance ratio.
Inputs: Capacity, irradiation, temperature, soiling, wiring, inverter and mismatch losses
Degradation and lifetime yield
Lifetime energy after first-year and annual degradation, with the warranty floor and the year output crosses it.
Inputs: Year-one yield, first-year LID, annual degradation, warranty terms, system life
Levelised cost of energy
Discounted lifetime cost divided by discounted lifetime energy — the only figure that compares solar with a tariff honestly.
Inputs: Capex, opex, capacity, yield, degradation, discount rate, lifetime
NPV and IRR
Net present value, internal rate of return and discounted payback for a solar investment.
Inputs: Capex, annual savings, escalation, degradation, opex, discount rate, life
Loan versus cash
Monthly repayment, total interest and whether the system pays for itself from day one under finance.
Inputs: System cost, deposit, interest rate, term, annual savings
Battery payback
Whether stored energy is worth more than the storage costs, per cycle and over the warranty.
Inputs: Battery cost and capacity, daily surplus, tariff spread, efficiency, warranty cycles
Backup runtime and autonomy
How many hours of essential load a battery covers in an outage, with and without daytime solar.
Inputs: Usable capacity, reserve, essential loads, inverter output limit, solar recharge
Peak shaving and demand charges
Battery power and energy needed to shave a demand peak, and the annual charge saving it produces.
Inputs: Site peak, target peak, peak duration, demand charge, battery cost
EV charging from solar
Kilometres per year a solar array can put into an EV, and what that saves against grid and public charging.
Inputs: Annual mileage, vehicle efficiency, array surplus, charging window, tariffs
Heat pump and solar
Heat pump electricity demand against solar supply, and the seasonal mismatch nobody mentions.
Inputs: Heat demand, SCOP, array size, specific yield, winter share
Fuse and breaker sizing
String fuse rating, combiner protection and AC breaker size from module and inverter data.
Inputs: Isc, number of parallel strings, module fuse rating, inverter AC current
Cable ampacity and derating
Derated cable capacity for temperature, grouping and installation method, checked against the design current.
Inputs: Base ampacity, ambient temperature, grouping, insulation type, design current
Roof load from a PV array
Added dead load per square metre, point loads at fixings and ballast mass for flat-roof systems.
Inputs: Module mass and area, mounting mass, tilt, wind zone, ballast density
Soiling and cleaning economics
The energy soiling costs, the value of a clean, and the interval at which cleaning stops paying.
Inputs: Soiling rate, rainfall reset, array size, yield, cleaning cost and tariff
Export limit and curtailment loss
How much energy an export limit actually costs, and what a battery would recover from it.
Inputs: Array size, export limit, self-consumption, yield profile, battery capacity
How to use these together
Start with inverter sizing to fix the array and string layout, then run cable sizing for the routes that layout implies. Take the resulting system into the payback calculator to see whether the economics hold at your tariffs, and only then test whether storage improves them using the battery sizing tool. Working in the other direction — picking a battery first — is how systems end up with capacity nothing charges.