How we calculate

Every figure on YieldWatt comes from the same model, described here with its data sources and assumptions.

Data last updated: 30 October 2026

Solar data

We use hourly output of a 1 kWp system from PVGIS, the European Commission’s photovoltaic tool, with 14% system losses.

For each city we compute five orientations (east to west) and two mountings: vertical on a railing and tilted at 30°.

Inverter limit and self-consumption

From the hourly series we pre-compute the share of output lost to the inverter limit and the share used at home for a grid of panel-to-inverter ratios and constant base loads (per kWp).

• Panel-to-inverter ratios: 1, 1.25, 1.5, 1.75, 2, 2.5, 3
• Base loads (W per kWp): 0, 25, 50, 75, 100, 150, 200, 300, 400, 600

monthly_kwh_per_kwp[m] = Σ P / 1000 / N
clipping_loss[r]       = Σ max(0, P − 1000/r) / Σ P
self_consumption[r][b] = Σ min(min(P, 1000/r), b) / Σ min(P, 1000/r)

Formulas

Your inputs are interpolated linearly between grid points. The browser and our server use the same formulas.

k      = dc_wp / 1000
r      = clamp(dc_wp / ac_w, 1, 3)
b      = clamp(base_load_w / k, 0, 600)
E_dc   = k × Σ monthly_kwh_per_kwp × (1 − shading)
E_ac   = E_dc × (1 − clip(r))
E_self = E_ac × self_consumption(r, b)
E_exp  = E_ac − E_self
S1     = E_self × tariff + E_exp × export_rate
payback = kit_cost / S1
S_N    = Σ S1 × ((1 − deg) × (1 + g))^t, t = 0 … N−1

Assumptions

• Hourly data understate inverter clipping, because short peaks within an hour are averaged out.
• Shading reduces output before the inverter limit is applied.
• The share used at home is the same in every month.
• The base load at home is constant over the day; the default is 150 W.
• Panels lose 0.5% of output a year by default.

Electricity prices

• EU and EEA countries: household prices for 2,500–4,999 kWh a year, all taxes included, latest half-year, from Eurostat nrg_pc_204
• United States: average residential price per state over the last 12 months available, from EIA
• Great Britain: the energy price cap for the current quarter, entered by our editors with the source.

Rounding

Energy is rounded to 10 kWh above 100 kWh, money to whole units, payback to one decimal and shares to whole percent. We round only what we display.

Licences

• PVGIS © European Communities, 2001-2026. Used with acknowledgement of the source.
• City data: GeoNames, CC BY 4.0
• Tariff data: Eurostat and the U.S. Energy Information Administration, with acknowledgement of the source.