by johndavidgordon.com
The total maximum nameplate power your solar panels can produce under ideal test conditions. Note: this rarely happens — lab tests and the real world differ. For example, a new 5.6 kWp array might only produce 5.2 kW even on a good day.
Total installed cost, including any battery storage or extra wiring. Many quotes are ‘after grant’ — check very carefully if your grant is already subtracted!
Your inverter’s AC output limit determines how much you can export, even if your array could produce more power if it was allowed to (very sunny conditions). Oversizing your solar panel array above this boosts yield in shoulder months and winter, but clips output on very sunny days. Some oversizing is normal — especially if you’re DC-coupled. Note: under Ireland’s microgeneration scheme, the export limit is 6 kW AC (sometimes referred to as 6 kVA).
If your system is DC-coupled (battery direct or hybrid inverter), surplus solar can be stored before it hits your inverter’s AC limit, so you’ll see significantly less clipping. Most Irish systems are AC-coupled only. DC/hybrid setups are growing but still under 10% of installs.
A battery stores excess solar for night-time or cloudy spells. Each kWh of usable capacity adds about +5% self-consumption for AC-coupled systems, or +7% for DC-coupled. E.g. a 5 kWh battery ≈ +25% if AC-coupled. Leave this at zero if you don’t have one. If you get the battery cost itemised by your installer, adjust the total cost above to check if it’s worth it — it almost always is. Also check: does “5 kWh” mean usable? Look for Depth of Discharge and round-trip efficiency in the specs.
This is a surprisingly important variable you need to think about. How much of your solar you use directly affects payback massively. The more you run appliances during daylight, the higher this is. Adding a battery typically raises this by about 5% per kWh usable (e.g. a 5 kWh battery ≈ +25%). Around 50–60% self-consumption is typical for Irish homes with a standard 5 kWh battery. Very diligent users, or big battery setups, can approach 90% or above — but this is very unusual.
What you pay your supplier for grid electricity. Most Irish homes pay 34–40 c/kWh (single rate). If you are on a variable rate, use your average.
What your supplier pays you for surplus units exported to the grid. Note: the house always wins — typically Irish electricity sellers charge you double for what you give back. This ≈ 2:1 gap makes self-consumption more financially valuable than exporting.
SEAI grant amount. Be aware: many installers simply adjust their quote to ‘absorb’ this. It’s rarely a true bonus. Don't add this in here if you've already subtracted it from the System Cost, above.
Most domestic solar installs are zero-rated for VAT in Ireland. Use 13.5% for small commercial projects, or 23% if your installer specifically advises. This depends on how the system is bundled with other works or if it’s treated as a general building improvement rather than energy upgrade only. If you’re a homeowner buying a new system from a conventional company, it’s probably 0%.
Annual output loss — standard panels lose ~0.5–0.8% per year. Modern panels degrade slowly, but it’s worth including to stay realistic. This is where Tier 1 panels shine above cheaper brands.
Your total household electricity use per year. You can find this information on your annual bill or smart meter summary. The SEAI frequently trot out the 4,200 kWh/year figure, but most houses are now well above that. Most households invariably increase their electricity consumption after installing solar panels!
Your supplier’s daily charge — you pay this regardless of solar. Typically €0.50–€1.00 per day. Note: The PSO Levy is currently zero or refunded — check your supplier’s latest tariff.
0° = North, 90° = East, 180° = South, 270° = West (clockwise). South-facing arrays produce the most in Ireland. East- or West-facing arrays produce less overall, but can better match your morning or evening usage. In cost-effective setups, even North-facing panels can be worthwhile.
Roof or ground mount tilt — 20–35° is typical for Irish roofs. Ground-mount tilt obviously can be set more freely. Tilt only has a mild effect on annual yield within normal ranges.
100% = no shading. Reduce this if trees, chimneys or nearby buildings block light at any time. Shading can be a huge hidden loss for many Irish arrays.
Check your FIT contract — some suppliers limit total export credits if exports are unusually high (e.g. >50 kWp).
Check my math! — Happy to nerd out on any variable if you like.
This calculator is for general estimates only and does not replace professional design advice!
Please feel free to get in touch — johndavidgordon@gmail.com