Firm vs Intermittent — Energy Pulse NZ

Three Different Numbers, One Power Station

When someone says "NZ has 10,000 MW of generation capacity," what does that actually mean? Here's why the answer is more complicated than it sounds:

📋

Nameplate Capacity

100 MW

Maximum rated output under ideal conditions. The number on the box.

🎯
Firm Capacity
~35 MW
What you can rely on at peak demand. The number that matters for reliability.

⚡

Average Output

~35 MW

Typical actual generation over a year. Determines total energy (GWh).

The key insight: Nameplate capacity tells you what a power station could produce. Firm capacity tells you what it will produce when the system needs it most. For intermittent renewables, these are very different numbers.

A Visual Example: 100 MW Wind Farm

Nameplate

100 MW — what it says on the turbines

Firm capacity

~10 MW

What you can count on during a calm winter evening peak

Average output

~35 MW

Typical output across the year (~35% capacity factor)

This doesn't mean wind is "bad" — it means we need to understand what different generation types actually contribute to the system.

Capacity Factors by Generation Type

The capacity factor is the ratio of actual output to maximum possible output. It varies dramatically by technology:

Technology	NZ Nameplate	Capacity Factor	Firm Contribution
🌋 Geothermal	~1,050 MW	~90-95%	Very high — runs 24/7
💧 Hydro (with storage)	~5,400 MW	~50-55%	High — dispatchable on demand
🔥 Gas peakers	~1,500 MW	~10-30%	Very high — runs only when needed
💨 Wind	~1,200 MW	~35-40%	Low — can't guarantee availability
☀️ Solar (utility)	~800 MW	~12-18%	Very low — zero at peak (evening)
🪨 Coal (Huntly)	~500 MW	~20-40%	High — dry year/winter backup

Why gas peakers have low capacity factors but high firm contribution: They're designed to run only during peaks or emergencies — like fire insurance. Low usage, but critical when needed. Their firm capacity approaches nameplate because they can be dispatched on demand.

Capacity factors are NZ-specific estimates based on Electricity Authority and MBIE data, 2023-24. Actual varies by site and year.

Why This Matters for NZ's Energy Transition

44 GW

generation pipeline (nameplate)

82%

of pipeline is intermittent

~15 GW

effective firm capacity equivalent

NZ has a massive pipeline of new generation — 289 projects totalling 44 GW of nameplate capacity. But 82% of that is wind and solar. This creates two important dynamics:

✓ Energy abundance

Wind and solar are excellent at producing cheap energy (GWh). Over a year, a 100 MW wind farm might produce as much energy as a 35 MW baseload plant running 24/7. More renewables = more total energy = lower average prices.

⚠ Capacity challenge

But wind and solar can't guarantee capacity (MW) at peak times. On a calm winter evening at 6pm, solar output is zero and wind might be 10% of nameplate. Something else must fill the gap.

The winter evening problem: NZ's peak demand (~7,000+ MW) occurs on cold, calm winter evenings around 6pm. Solar is zero. Wind is often low. Hydro is being conserved. Gas and coal become critical — but we're retiring gas plants while supply declines.

The Transition Challenge

As NZ adds more intermittent generation and retires thermal plants, we face a question: what provides firm capacity when renewables can't?

Hydro with storage — Our primary flexible resource, but limited by water availability and increasingly needed for firming rather than baseload
Batteries — Growing fast, but current grid-scale batteries provide 1-4 hours of storage, not days or weeks
Gas peakers — Reliable firm capacity, but gas supply is declining and emissions are a concern
Demand response — Industrial users (like Tiwai) reducing load during peaks, but limited scale
Imports — Not possible for NZ (island nation, no interconnectors)

NZ's Capacity vs Energy Reality

Here's how our current generation fleet breaks down — showing nameplate capacity vs actual contribution:

Source	Nameplate MW	% of Capacity	% of Energy (2024)	Peak Contribution
💧 Hydro	~5,400	54%	53%	★★★★★ Primary flexible resource
🌋 Geothermal	~1,050	11%	20%	★★★★★ Always-on baseload
🔥 Gas	~1,500	15%	9%	★★★★★ Peak/emergency backup
🪨 Coal	~500	5%	5%	★★★★☆ Dry year backup
💨 Wind	~1,200	12%	9%	★★☆☆☆ Weather dependent
☀️ Solar	~800	8%	1.4%	★☆☆☆☆ Zero at evening peak

Notice the inversion: Geothermal is 11% of nameplate but 20% of energy (high capacity factor). Wind is 12% of nameplate but only 9% of energy. Solar is 8% of capacity but just 1.4% of energy. This is capacity factors in action.

Data: MBIE Energy in New Zealand 2024-25, Electricity Authority. Solar includes both utility and distributed.

The Firming Gap

As NZ builds more wind and solar while retiring thermal generation, a "firming gap" emerges — the difference between nameplate capacity and what's actually available during peak demand:

~7,500 MW

winter peak demand (and growing)

~6,000 MW

firm capacity available

~700 MW

reserve margin (and shrinking)

What Can Fill the Gap?

Solution	Current Scale	Potential	Challenges
🔋 Grid batteries	~100 MW	~6 GW pipeline	1-4 hour duration; can't cover multi-day events
📉 Demand response	~300-500 MW	~1,000+ MW	Limited industrial base; Tiwai uncertainty
🔥 New gas peakers	Limited new build	Technically feasible	Gas supply declining; carbon liability; investment risk
💧 New hydro	Limited sites	Some potential	Long consenting; environmental constraints
🏔️ Pumped hydro (Onslow)	0	~1,000 MW	Expensive; decade+ timeline; uncertain economics

The honest assessment: NZ doesn't yet have a clear answer to the firming gap. Batteries help with short peaks. Demand response helps at the margins. But for multi-day dry-year events with low wind, we still rely on thermal generation — which is retiring faster than alternatives are arriving.

What Industry Is Debating

Is "100% renewable by 2030" achievable and reliable?
Should NZ build new gas peakers as a transition measure?
Is Lake Onslow pumped hydro worth $15B+ for dry-year security?
Can batteries + demand response + overbuild of renewables close the gap?
Are current price spikes a market failure or an investment signal?

These are genuine questions without easy answers. Understanding firm capacity vs energy is the starting point for any informed position.

Sources: Transpower, Electricity Authority, MBIE EDGS 2024, industry analysis

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