Guide · Reviewed against RECI guidance and installer-published documentation. Last verified: 2026-10-22.

Generator sizing guide for Irish homes (kW, kVA and the 80% rule)

Sizing is where generator projects are won or lost. Too small and the set stalls on start-up surges or trips under load; too large and you pay thousands extra for the installation, then run the engine below its efficient range forever. This guide gives you the methodology Irish installers actually use - loads, surge, the 80% rule, and the kW/kVA distinction - so you can walk into a quote conversation knowing your number.

First, the vocabulary: kW, kVA and power factor

Generator ratings are given in kVA (apparent power) and kW (real power). They are connected by the power factor; for residential single-phase work, assume:

When an installer says "10 kVA", they mean the apparent-power rating. When your load list adds up fridge, pump and heating watts, those are real power (kW). Mixing the two without the 0.8 factor is how undersized units get sold. If a quote states only kVA, ask for the kW figure too - a competent installer answers without hesitation.

The 80% rule

A generator should not be run continuously above about 80% of its rated output. Two reasons:

1. Headroom. Motors - well pumps, septic pumps, heat-pump compressors, oil-burner fans - draw a start-up surge several times their running draw. That surge rides on top of your continuous load, and the set must absorb it without stalling or tripping.
2. Engine health. Runs at the extreme end of rating run hotter and age faster; the 80% ceiling is the industry's standing rule of thumb.

So the sizing target is: expected continuous load x 1.25, then round up to the nearest available kVA rating. That multiplication is simply the 80% rule rearranged.

Step-by-step: building your load list

Step 1 - List the circuits you want powered

Whole-home backup covers everything and is the most expensive option. Essential-circuits backup - well pump, heating controls, fridge-freezer, broadband, some sockets and lights - is the typical Irish answer and often cuts the kVA requirement (and the price) dramatically. Decide which you want before you size anything.

Step 2 - Find the running watts for each load

Typical running draws for Irish homes (indicative - your appliances' nameplates and the installer's survey are authoritative):

Step 3 - Add surge, not just running, load

The rule of thumb: take your largest motor's start surge as roughly three times its running draw, add it to the running total of everything else, and confirm that combined figure sits inside the 80% ceiling. Rural homes with deep-well pumps and heat pumps are exactly the profiles where skipping this step produces a set that dark-starts beautifully and then trips the first morning someone showers while the heat pump kicks in.

Step 4 - Apply the 80% rule and round up

Continuous load x 1.25, rounded up to the nearest standard rating. Standard single-phase ratings cluster around 7, 10, 13, 17, 20 and 26 kVA depending on brand.

Worked examples

Example 1 - Rural bungalow, oil heating, private well (typical Connacht or Donegal profile)

Well pump 1,100 W running, oil boiler 300 W, fridge 250 W, septic pump 600 W, lights and sockets 400 W, broadband 50 W. Continuous total: about 2,700 W. Add the well pump's start surge (3 x 1,100 W = 3,300 W in place of its running draw momentarily): design peak roughly 5,600 W. Apply the 80% rule: 5,600 x 1.25 = 7,000 W, i.e. about 8.75 kVA at 0.8 power factor. Recommended bracket: a 10 kVA set - round up, keep headroom.

Example 2 - Suburban estate home, gas heating, heat pump added (Kildare or Dublin commuter belt)

Heat pump 4,000 W running, gas boiler controls 150 W, fridge 200 W, IT and lighting 400 W. Continuous total: about 4,750 W. Heat-pump start surge adds several thousand watts momentarily: design peak roughly 8,000-9,000 W. With the 80% rule: about 11,000-11,500 W, i.e. roughly 14 kVA. Recommended bracket: 13-17 kVA depending on the heat pump's data sheet - this is why you tell installers about the heat pump before they quote.

Example 3 - Essential circuits only, same suburban home

Drop the heat pump from backup (it stays off in an outage; immersion and a couple of oil-filled radiators cover essentials). Continuous load drops to about 750 W, design peak to about 2,500 W with fridges and pumps surging. A 6-7 kVA set covers it with room to grow - thousands of euro cheaper than Example 2, which is the essential-circuits argument in one line.

Use the calculator, then verify with a survey

Our sizing calculator automates exactly this method - home size, heating type, well pump, special loads - and returns a recommended kVA bracket plus an indicative installed budget band (EUR, ASSUMPTION). Treat its output as a planning number: the installer's site survey, your actual appliance nameplates and the well pump's real start current are what the final quote should rest on.

Signs a quote is mis-sized

• A kVA figure appears with no load list or questions about your well pump or heat pump.
• Whole-home backup is quoted when you asked about essential circuits - or never asked at all.
• Nobody mentions the 80% rule or start-up surges during the site visit.
• The quote jumps a bracket after you mention the heat pump, with no explanation of the surge maths.

Storm Éowyn turned sizing from a nice-to-have into the difference between riding out January 2025 comfortably or not at all. Ten minutes of load-list maths is the cheapest part of any installation - do it first, then bring the number to the professionals.

Draw figures and brackets on this page are indicative planning values (ASSUMPTION). Actual loads vary by appliance and installation - confirm with nameplates and an installer survey.