The Measuring Part
Last year I wired up my Dutch household with more sensors than a hospital ICU. A HomeWizard P1 meter on the smart meter, a separate kWh meter on the solar inverter, a Homey Pro hub tracking every smart plug, and an InfluxDB instance running on Kubernetes because apparently I can't do anything casually.
The house: a regular family home in Limburg, Netherlands. 1,400 kWh of solar panels on the roof, a gas boiler for heating and hot water, and roughly €3,100 per year in energy costs. After twelve months of collecting data at 10-second intervals, I sat down to figure out where the money was actually going.
The answer was not what I expected.
The 30% Problem
Here's the uncomfortable truth about my solar panels: only 30% of the electricity they generate is actually used by my household. The other 70% gets exported to the grid.
The reason is timing. My panels produce peak power between 11:00 and 15:00 — up to 1,169 watts on a sunny day. But my household's peak consumption is between 16:00 and 21:00, hitting 1,600-2,500 watts when everyone's home, cooking, and running appliances. By then, the sun has moved on.
During the night, we're pulling 300-540 watts of baseload from the grid with zero solar contribution. In the morning, we're consuming 270-400 watts while the panels are barely waking up.
In the Netherlands, this hasn't mattered much until now. A policy called saldering (net metering) lets you offset exported kWh against imported kWh at the same rate. Export a kilowatt-hour at noon, get credited the full €0.32 when you import one at dinner time. Free battery, essentially.
That ends January 1, 2027. After that, exported power earns roughly €0.09/kWh. Imported power still costs €0.32. Same solar panels, same house, but now that 70% export becomes a real problem.
Everyone Says: Buy a Battery
The obvious fix is a home battery. Store surplus solar during the day, use it in the evening. I looked at the Marstek Venus E 3.0: 5 kWh capacity, lithium iron phosphate, €1,224.
The math:
- Store ~3.8 kWh/day of solar surplus
- 90% round-trip efficiency, so 3.4 kWh usable
- Shift from €0.09 export to €0.22 evening import (on dynamic tariff)
- Add some price arbitrage on cheap night hours
- Total savings: ~€149/year
- Payback: just over 8 years
Fine. Not terrible. But not exactly thrilling either, for a device with a 10-year warranty. I kept digging.
The Counterintuitive Fix
What if instead of storing solar energy, I could just use more of it — right when it's being generated?
This is where air conditioning enters the picture, and where most people's eyebrows go up.
Think about when you want cooling. Hot, sunny days. Now think about when solar panels produce the most. Hot, sunny days. The correlation between cooling demand and solar generation is almost perfect. June through August, my AC would consume roughly 160 kWh — nearly 100% coincident with peak PV output.
But here's the number that actually matters. When AC uses 1 kWh of solar electricity for cooling at a SEER of 6.1, that's 6.1 kWh of cooling delivered. Useful, but cooling doesn't displace another fuel. The real magic happens with heating.
Wait, Heating With AC?
Modern split-unit air conditioners aren't the window-rattling boxes from the '90s. A unit like the TCL Elite F2 is a full heat pump: it heats down to -15°C and carries a heating efficiency (SCOP) of 4.0. That means for every 1 kWh of electricity, it delivers 4 kWh of heat.
Compare that to my gas boiler, which converts 1 m³ of gas (9.77 kWh) into about 8.8 kWh of heat at 90% efficiency. At current prices:
- Gas heating: 1 m³ × €1.54 = €1.54 for 8.8 kWh of heat → €0.175/kWhthermal
- AC heating (dynamic tariff): 1 kWh × €0.22 = €0.22 for 4.0 kWh of heat → €0.055/kWhthermal
- AC heating (on solar): 1 kWh × €0.00 = free for 4.0 kWh of heat → €0.00/kWhthermal
Even buying electricity at dynamic grid rates, the AC heats at a third of the cost of gas. On solar power, it's free heat.
Spring and fall are the sweet spot. March through May and September through October, outdoor temperatures hover between 5°C and 15°C — exactly where heat pump efficiency is highest (COP 4.0-4.5) and where there's still meaningful solar generation. About 70% of spring/fall AC consumption can run directly on PV. In those months, I'm effectively converting surplus solar into free warmth.
Winter is a different calculation. November through February there's barely any solar to work with, and the COP drops to 2.2-3.5 as temperatures dip. But even at COP 2.5 on grid electricity at €0.22/kWh, that's €0.088/kWh of heat — still half the price of gas. The AC won't replace the boiler in a cold snap, but it can handle the mild winter days (above 5°C) and shave 45-55% off annual gas consumption.
The branding is the problem, really. Call it "air conditioning" and people think summer luxury. Call it what it is — a multi-split heat pump that also cools — and suddenly it makes sense year-round.
Now compare the value of a single solar kilowatt-hour through each path:
- Battery route: 1 kWh → battery (90% efficient) → 0.9 kWh evening electricity. Value: €0.20
- AC heating route: 1 kWh → heat pump (SCOP 4.5) → 4.5 kWh heat, replacing 0.46 m³ gas. Value: €0.71
AC extracts 3.5× more value from the same solar kilowatt-hour. Self-consumption jumps from 30% to 47%.
Why a Hybrid Heat Pump Makes It Worse
Here's where it gets properly counterintuitive. If I told you I was considering a €6,000 hybrid heat pump (Daikin Altherma, €3,875 after subsidy), you'd nod approvingly. Responsible. Green. Sensible.
Except for the timing problem.
A hybrid heat pump for central heating consumes most of its electricity between October and March — roughly 1,100 kWh in the cold months. My solar panels produce about 250 kWh in those same months. That's 850 kWh of extra grid imports, piled onto the season when electricity is already most expensive.
It saves gas, absolutely. Around 450 m³/year, worth €390-433 in savings. But it actively deepens the solar mismatch. You're swapping one fuel bill for another while making your solar panels even less relevant.
The multi-split AC, by contrast, spreads its load across the year. Summer cooling consumes power when solar is abundant. Spring and fall heating hits the PV sweet spot. Even winter heating at least benefits from dynamic tariff optimization. The heat pump's consumption pattern is the wrong shape for a solar household.
The Actual Plan
After staring at spreadsheets long enough, the boring-but-correct answer emerged. Not one silver bullet, but three steps in the right order:
- AC this spring (€3,500-4,000) — Start using surplus solar for heating and cooling immediately. Gas bill drops 45%, self-consumption jumps to 47%.
- Dynamic electricity tariff in October 2026 (€0) — When my fixed contract ends, switch to a dynamic rate. In all twelve months of data I analyzed, dynamic was cheaper than my fixed rate. Saves ~€441/year for zero investment.
- Home battery in early 2027 (€1,224) — Now the battery math improves. It captures what AC couldn't use, and does arbitrage on dynamic pricing. Pushes self-consumption to 55%.
Total investment: ~€5,200. Annual savings: €900-1,000. Payback in about 5 years. Annual energy costs drop from €3,100 to roughly €2,100-2,200.
The hybrid heat pump? Maybe in 2028 or 2029, once the easier wins are captured and gas prices give a clearer signal. It's not a bad investment — it's just not the first investment.
What I Actually Learned
The lesson isn't "buy an AC." The lesson is that twelve months of real data told a completely different story than the one I'd assumed walking in. I was ready to order a battery. The data said: wait, there's a better sequence.
Good measurement beats good intuition. The best energy investment isn't always the one that sounds greenest, and the right order matters more than the right equipment.
If you've got solar panels and a timing problem, run the numbers on your own household before listening to anyone — including me. Your mismatch might look nothing like mine. That's the whole point of measuring.
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