What size generator do I need? Learn to size yours right

What Size Generator Do I Need? Complete Diesel Generator Sizing Guide & Calculators

When a storm is approaching, the first question many homeowners type into Google is what size generator do i need. Picking the right size keeps your home safe, protects appliances, and avoids wasted fuel.

In this guide, you get a simple step-by-step method, clear examples, easy sizing calculators, and a quick diesel generator chart you can use today. We explain power factor and generators explained in plain English, and show you how to use a runtime calculator generator to plan fuel and refills.

We also include real Australian scenarios from homes, farms, and worksites, plus notes on AS/NZS 3010 (standby generating systems) and ISO 8528 (generator ratings) to help you stay compliant and safe.

Why Right-Sizing a Generator Matters – generator sizing for house

Getting the size right matters for every home and site in Australia. Here is why:

• Protects equipment: Undersized units can trip, sag voltage, and damage motors, fridges, freezers, and electronics.

• Improves safety: Correct sizing helps keep voltage within limits, reducing nuisance trips and overheating. This is critical with transfer switches under AS/NZS 3010.

• Lower total cost of ownership: An efficient match reduces fuel burn, cuts wet-stacking on diesels, and extends engine life.

• Compliance with electrical codes: Correct capacity with an approved changeover switch (AS/NZS 3010) supports safe isolation from the grid.

• Handles surge loads: Start current vs run current generator demands are different. Right sizing ensures big loads (A/C, pumps, welders) start cleanly.

Reference: (source)

Core Electrical Concepts You Must Know – power factor and generators explained

kVA vs kW Explained – kva to kw calculator

Generators are rated in kVA and kW. It is simple:

• kVA (apparent power): The “total” power the generator supplies.

• kW (real power): The usable power that does the work.

They are linked by power factor (PF):

kW = kVA × PF

Most diesel generators are rated at PF = 0.8. That means an 8 kVA unit can deliver about 6.4 kW of usable power (8 × 0.8).

If you use a kva to kw calculator, always enter your PF. For most sites, use 0.8 unless your generator data plate says otherwise (see ISO 8528 ratings on the nameplate).

Reference: (source)

Power Factor in Plain English – power factor and generators explained

Power factor (PF) shows how well your devices turn electrical energy into work.

• Lagging PF: Common with motors. Current lags voltage. Needs more current for the same useful power.

• Leading PF: Less common at home. Can occur with some capacitor banks.

Quick example: 10 kVA at PF 0.8 gives 8 kW usable. If PF slips to 0.7, usable kW drops to 7 kW for the same kVA.

In short, poorer PF means more current and more heating. Good sizing and correct PF assumptions protect your generator and wiring.

Reference: (source)

Start Current vs Run Current – start current vs run current generator

Motors and compressors draw a high inrush current at start-up. This is often 6–8× the full-load current. The generator must handle this surge for at least 5–10 seconds.

• Run current: Normal steady draw after start-up.

• Start current: Very high, short surge. Drives the “motor starting kVA allowance” you must include.

Plan for the largest single start and ensure your generator’s voltage dip and frequency response stay within ISO 8528 limits when that start hits.

Reference: (source)

Step-by-Step Generator Sizing Method

Create a Load Inventory – fridge/freezer wattage generator, generator for air conditioner size, generator for welder size, pump start generator size

Start with a list of what you will power at the same time. Note running watts and starting watts.

Tip: Check each device’s nameplate or manual. If watts are not listed, use volts × amps to estimate. For pumps, use 3–7× the run watts to estimate start surge. For welders, use the highest duty cycle and peak current and convert to watts.

References: (source) (source)

Convert Watts to kVA & kW – kva to kw calculator

Once you total the running and starting watts, convert to kVA and kW so you can match a generator.

• kVA = Watts ÷ (1000 × PF). Use PF = 0.8 unless your generator says otherwise.

• kW = kVA × PF.

Example using the table above (assume fridge + A/C + pump running):

• Total running watts = 600 + 3,500 + 1,000 = 5,100 W (5.1 kW).

• Largest start surge at one time (worst case) ≈ A/C 10,500 W.

• Convert running to kVA at PF 0.8: kVA_run = 5,100 ÷ (1000 × 0.8) = 6.375 kVA.

• Convert surge to kVA at PF 0.8: kVA_start = 10,500 ÷ (1000 × 0.8) = 13.125 kVA.

Use an online tool to sanity check your numbers and explore options: FG Wilson Simple-Sizer lets you model loads and check motor starting impact.

Reference: (source)

Add Motor Starting kVA Allowance – motor starting kVA allowance, start current vs run current generator

Rule of thumb: Your generator’s motor starting kVA rating should be equal to or greater than the largest single motor start. For example, a 5 HP air compressor (approx. 3.7 kW run) can draw 6× running amps briefly:

• Run power ≈ 3.7 kW → at PF 0.8 → 3.7 ÷ 0.8 = 4.6 kVA.

• Start kVA ≈ 6 × 4.6 = 27.6 kVA (short duration).

Pick a generator with a motor starting kVA allowance that can supply that 27.6 kVA without excessive voltage dip (per ISO 8528 transient response limits).

Reference: (source)

Apply 25% Future-Proof Margin – runtime calculator generator

After you cover running load and the worst surge, add a 25% margin. This covers unlisted loads, future growth, and hot-day starts. It also reduces the chance of nuisance trips by your RCDs and breakers.

Later, use a runtime calculator generator with your chosen size to estimate hours of operation between refuels based on tank size and load.

Application-Specific Cheat Sheets

Home Backup Power – generator sizing for house with 8kva diesel generator and 10kva diesel generator

Example: A typical Aussie 3–4 bed home with fridge, lights, modem/router, TV, a few power points, and a small A/C may sit around 6 kW running with a 12 kVA start spike if the A/C or pump kicks in. At PF 0.8, 12 kVA surge means 9.6 kW surge.

• Recommendation: Start with an 8kva diesel generator for lighter homes or partial circuits.

• Step up to a 10kva diesel generator if you want to include a larger A/C or more circuits.

• Ensure a compliant changeover switch and earthing under AS/NZS 3010. Use a licensed electrician.

Air Conditioners – generator for air conditioner size with 14kva diesel generator or 16kva diesel generator

To estimate start kVA for a ducted split or multi-split, use the nameplate Locked Rotor Amps (LRA):

• Start kVA ≈ (LRA × Voltage) ÷ 1000.

For multi-split systems or homes with two compressors, you may need a 14kva diesel generator or a 16kva diesel generator to keep starts smooth and voltage steady. Ask your sparky to stagger starts with soft starters if possible.

Welders – generator for welder size with 13kva diesel generator or 22kva diesel generator

Welders have high surge and duty cycle swings. A typical MIG/Stick on a contractor trailer can demand a short surge above 10 kW while striking an arc.

• For light site work, a 13kva diesel generator is often the floor.

• For heavier wire feed or stick at high amps alongside a compressor or lights, consider a 22kva diesel generator.

Always confirm the welder’s input current at your planned duty cycle and voltage (single- or three-phase).

Pumps & Irrigation – pump start generator size with 27.5kva diesel generator or 34kva diesel generator

Pumps often need 3–7× starting current. On farms across QLD and NSW, we see 25–40 HP pumps for bores and pivots. These often call for a 27.5kva diesel generator or a 34kva diesel generator, especially when line lengths are long and voltage drop matters. Add a soft starter or VSD where suitable and confirm with your pump supplier and electrician.

Quick-Reference Diesel Generator Size Chart – 5.5 kVA to 400 kVA

Worked Case Studies – kva to kw calculator in action

Case 1: Tiny cabin (QLD hinterland)

• Loads: LED lights, fridge/freezer, phone chargers, router.

• Run total ≈ 2 kW. Start surge ≈ 3.5 kVA (fridge).

• PF 0.8 → Run kVA ≈ 2 ÷ 0.8 = 2.5 kVA. Start ≈ 3.5 kVA.

• Add 25% → ≈ 3.1–3.2 kVA.

Recommendation: 5.5 kVA diesel generator for headroom and smooth restarts. Quick check with a kva to kw calculator confirms plenty of room for a small pump or toaster.

Case 2: 4-bed home + central A/C (Western Sydney)

• Run total ≈ 7 kW. Start ≈ 14 kVA (A/C start), PF 0.8.

• Run kVA ≈ 7 ÷ 0.8 = 8.75 kVA.

• Start kW equivalent ≈ 14 × 0.8 = 11.2 kW.

• Add 25% margin.

Recommendation: 10kva diesel generator for whole-home essentials, with a soft starter on the A/C if needed. This supports generator sizing for house without pushing the unit too hard.

Case 3: Contractor trailer with welder & compressor (Brisbane)

• Run total ≈ 15 kW (welder at duty cycle + air compressor + lights).

• Start surge ≈ 25 kVA (compressor start + welder arc strike).

• PF 0.8 check: 25 kVA ≈ 20 kW start capacity.

Recommendation: 22kva diesel generator to cover the peak without tripping, and allow use of tools while welding.

Case 4: Pivot irrigation pump 30 HP (Darling Downs)

• 30 HP ≈ 22 kW running power.

• Start ≈ 3× to 6× → assume ≈ 66 kVA surge worst case.

• PF 0.8 → 66 kVA ≈ 52.8 kW start.

Recommendation: 65kva diesel generator with a soft starter where possible and proper protection devices. Verify cable sizes and earthing per AS/NZS 3010 and your electrician’s advice.

Case 5: Regional clinic with theatre lights & UPS (Far North QLD)

• Run ≈ 150 kW (critical circuits, HVAC, lighting, UPS charging).

• Start ≈ 220 kVA across HVAC and medical compressors.

• N+1 preferred for resilience.

Recommendation: 250kva diesel generator minimum, or a 350kva diesel generator for redundancy and smoother transients. Confirm rating methodology per ISO 8528 (PRP vs ESP) and select the correct alternator.

Note: After Cyclone Yasi, we helped a Townsville clinic re-commission a mid-size diesel set. Using a simple kva to kw calculator and a proper load audit, we upsized to prevent voltage dips on critical HVAC starts. It paid off the next wet season with zero outages.

Fuel Consumption & Runtime – runtime calculator generator

Diesel fuel burn depends on load and size. A handy rule at 75% load is about 0.24 L per kW-hour. This varies by engine model and altitude/temperature.

How to estimate runtime:

• Find your average load in kW.

• Check your tank size (e.g., 120 L base tank).

• Use a runtime calculator generator to estimate hours between refuels based on your load and tank.

Avoid running a big unit lightly loaded for long periods. Oversizing can cause wet-stacking (unburnt fuel in the exhaust) and higher fuel costs. Pick a size that keeps your average load around 50–80% for best efficiency and engine health.

Reference: (source)

FAQ

What size generator do I need for a refrigerator and freezer?

Most fridges and freezers run around 600 W but can surge to about 1,800 W at start. A small set like a 5.5 kVA unit will cover a fridge/freezer wattage generator need plus lights and a router. Add 25% for future loads and check nameplate watts.

Can I start my 3-ton air conditioner on an 8 kVA unit?

Often no. A 3-ton A/C can need about 10,500 W at start (~13 kVA at PF 0.8). You may need 10–14 kVA depending on soft starters and other loads. Use the generator for air conditioner size method: LRA × voltage ÷ 1000 to estimate start kVA.

Does power factor change over time?

Yes. Motor wear, capacitor ageing, and load mix can shift PF. That is why we explain power factor and generators explained with a default PF of 0.8 for sizing. Confirm PF on the generator nameplate and equipment specs.

What is the difference between start current vs run current generator?

Run current is the steady draw after a device is up to speed. Start current (inrush) is a short, high surge, often 6–8× higher for motors. Your generator must meet the highest start current without tripping or large voltage dips.

How big should my pump start generator size be for a 2 HP well pump?

A 2 HP pump runs around 1.5 kW. Start can be 3–7×. Plan for about 10–12 kVA start capacity to be safe, or add a soft starter. Check with your pump supplier for exact LRA and your sparky for wiring length and voltage drop.

Is my home allowed to connect a generator to the switchboard in Australia?

Yes, but only through a compliant transfer/changeover switch and correct earthing per AS/NZS 3010. A licensed electrician must assess the installation and make sure backfeed to the grid is impossible.

Conclusion & Next Steps

You now have a simple process to answer what size generator do i need:

• List your loads and note running and starting watts.

• Convert with a kva to kw calculator and include the biggest start surge.

• Add a 25% buffer for future growth and hot-day starts.

• Use a runtime calculator generator to plan fuel and tank size.

Match your numbers to the diesel chart above, then check AS/NZS 3010 compliance. For generator sizing for house and farms, always confirm with a licensed electrician before purchase and installation.

References used in this guide: (source) (source) (source) (source)

6-Point FAQ

• What size generator do I need for a refrigerator and freezer? Most fridges/freezers run around 600 W and surge to 1,800 W. A 5.5 kVA unit is usually enough with lights and a router. Check the fridge/freezer wattage generator info on your nameplate and add 25% margin.

• Can I start my 3-ton air conditioner on an 8 kVA unit? Usually not without a soft starter. A 3-ton can need ~13 kVA surge. Follow the generator for air conditioner size method using LRA.

• Do I need to consider power factor? Yes. Power factor and generators explained: kW = kVA × PF. Most gensets are PF 0.8; always include this in your sizing.

• What is start current vs run current generator? Start current is the high inrush (often 6–8×) when motors start; run current is the steady draw after start-up.

• What pump start generator size is right for a 2 HP pump? Allow roughly 10–12 kVA start (3–7× run) or use a soft starter/VSD. Confirm with your pump supplier and electrician.

• What standards apply in Australia? Use AS/NZS 3010 for generator installations and ISO 8528 for generator ratings. Always get a licensed electrician to check compliance.