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Solar PV Inverter Calculator
Size and select the right inverter for your solar PV array. Compare string, hybrid, microinverters and optimisers with DC:AC ratio and cost estimates.
Total DC capacity of panels
One per roof face; 2 strings fit one 2-MPPT inverter
Indicative string inverter base cost (ex-VAT)
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How We Calculate This
This calculator sizes solar PV inverters based on array capacity and installation requirements.
The formula
Inverter AC size = Array kWp × 0.88 (DC:AC ratio ~1.14:1)
- DC:AC ratio: 1.1-1.25:1 is the mainstream UK range; 0.88 is a representative default. Mild array oversizing is normal because nameplate output is rarely reached in the UK.
- G98 connect-and-notify limit: 16 A per phase = 3.68 kW AC single phase, 11.04 kW three phase. Above this a G99 application (DNO pre-approval) is required, often with an export limit.
- A modern string inverter usually has 2 MPPT channels, so two roof-face strings normally feed one unit - extra inverters are only needed when strings exceed the available channels.
- Microinverters: one per panel (modern UK modules ~400-450 W each).
- Maximum DC input voltage is a property of the chosen inverter model (residential string inverters are commonly 480-600 V DC; commercial units up to 1000-1100 V), not of the AC phase - always check the datasheet for string-length design.
Cost figures are indicative supply-plus-install estimates (ex-VAT, UK 2026). 0% VAT applies to qualifying domestic solar installs in Great Britain until 31 March 2027.
Frequently Asked Questions
UK installers usually size the inverter at about 80-90% of the array kWp, giving a DC:AC ratio of roughly 1.1-1.25:1. For a 4 kWp array a 3.5-3.7 kW inverter is typical. Mild undersizing is deliberate and cost-effective because panels rarely reach full nameplate output in UK conditions, so very little energy is clipped. Keep the inverter rating below the array kWp so it switches on reliably in low light.
Under EREC G98 you can fit and notify (connect first, tell your DNO within 28 days) provided the inverter is no more than 16 A per phase - that is 3.68 kW AC on a single-phase supply or 11.04 kW on three phase. Above that threshold you must submit a G99 application for DNO pre-approval before installation, and approval is frequently given only with an export limit set to 3.68 kW. Many 4 kWp+ single-phase systems are therefore export-limited to 3.68 kW or run a G99.
String inverters connect panels in series and convert DC to AC centrally (one unit for the whole array). Microinverters are fitted to each panel individually. Micros perform better with shading as each panel operates independently, but cost more. String inverters are cheaper for unshaded arrays.
A hybrid inverter can manage both solar panels and battery storage. If you plan to add a battery within 2-3 years, installing a hybrid inverter now avoids the cost of replacing the inverter later. They cost 60-80% more than string inverters but save on future retrofit costs.
String and hybrid inverters typically last 10-15 years with a standard 5-10 year warranty (extendable to 15-20 years). Microinverters often have 20-25 year warranties. Most homeowners will replace a string inverter once during the 25+ year life of their solar panels.
Power optimisers are DC-DC converters fitted to each panel that maximise individual panel output before feeding a string inverter. They offer similar shading benefits to microinverters but at lower cost. Systems like SolarEdge use this approach. They add about 40% to inverter system cost.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.