API & MCP

Everything a person can do on Ledgerly Solar Payback Calculator, a program or AI agent can do with the same inputs and get the identical result.

HTTP API

POST https://ledgerlysolarpayback.usefulweb.app/api/v1/calculate – run Ledgerly Solar Payback Calculator
GET https://ledgerlysolarpayback.usefulweb.app/api/v1/tool – inputs, output schema, examples
OpenAPI 3.1: /openapi.json · /openapi.yaml
Errors: application/problem+json; 422 includes per-field errors.

Ledgerly Solar Payback Calculator

Year-by-year cash flow of a PV system: bill savings from the power you use yourself, feed-in income for the guaranteed years, ageing of the panels, running costs, an inverter replacement and an optional loan. Returns simple and discounted payback, NPV, IRR, the cost per kWh produced and a payback table for other price rises and self-consumption shares. Part of Ledgerly’s Solar & Heat.

Inputs of Ledgerly Solar Payback Calculator
Field	Type	Default	Description
`currency`	text	"EUR"	Currency
`systemCost`	number	14000	System price incl. installation
`subsidy`	number	0	Subsidies and tax refunds
`annualYieldKwh`	number	7600	Yield in the first year (kWh)
`selfConsumptionPercent`	number	30	Used in the house (% of the yield). Without a battery often 25–35 %; the battery simulation gives your value.
`electricityPrice`	number	0.35	Electricity price per kWh
`electricityPriceIncreasePercent`	number	2	Electricity price increase (% per year)
`feedInTariff`	number	0.078	Feed-in payment per kWh
`feedInYears`	integer	20	Years the feed-in payment is guaranteed. After that, exported energy earns nothing in this model.
`degradationPercent`	number	0.5	Yield loss through ageing (% per year)
`maintenancePerYear`	number	150	Insurance, metering and upkeep per year
`costIncreasePercent`	number	2	Increase of the running costs (% per year)
`inverterReplacementCost`	number	1500	Inverter replacement
`inverterReplacementYear`	integer	15	Inverter replaced in year (0 = never)
`loanAmount`	number	0	Financed by a loan
`loanRatePercent`	number	4.5	Loan interest rate (% per year)
`loanYears`	integer	10	Loan term (years)
`discountRatePercent`	number	3	Discount rate for NPV (% per year). What your money would earn elsewhere.
`horizonYears`	integer	25	Years to compare

Example request

8 kWp for €14,000, 30 % self-use

curl -X POST https://ledgerlysolarpayback.usefulweb.app/api/v1/calculate -H 'content-type: application/json' -d '{"currency":"EUR","systemCost":14000,"annualYieldKwh":7600,"selfConsumptionPercent":30,"electricityPrice":0.35,"feedInTariff":0.078}'

Every result contains

inputs (normalised, defaults applied), summary, result (validated against the published schema), units, assumptions, sources, warnings, computedAt and links.

MCP (Model Context Protocol)

Streamable HTTP endpoint: https://ledgerlysolarpayback.usefulweb.app/mcp – protocol revisions 2026-07-28 (stateless) and 2025-11-25 / 2025-06-18 / 2025-03-26 (initialize handshake).
Tools: ledgerly_solar_payback
Local stdio connector: node src/mcp-stdio.ts --tool=ledgerly-solar-payback

{
  "mcpServers": {
    "ledgerly-solar-payback": { "type": "http", "url": "https://ledgerlysolarpayback.usefulweb.app/mcp" }
  }
}