Your house is already a battery.
It just doesn't know it yet.

How much is already stored? Stored heat is just mass × specific heat × temperature band. Water holds 4.186 kJ per litre per °C; a building's mass (plasterboard, timber, concrete, furniture) is rolled into an effective heat capacity in MJ per °C. A real instrumented house was measured at 17.7 MJ/K of effective capacity and 109 W/K of heat loss — a medium-mass, very well-insulated home, a little tighter even than the all-"Good" envelope here. A store only counts toward the "house battery" if it is heated electrically: a gas or solar cylinder, or a wood burner, stores heat but can't shift your electrical load, so the non-electric options drop it out of the total.

Comfortable range. The WHO sets a healthy-home minimum of 18 °C, with 18–21 °C the usual comfort sweet spot and 18–24 °C broadly acceptable for healthy adults — so a 19–22 °C control band sits comfortably inside the guidance while giving 3 °C of headroom to play with. NZ plumbing keeps cylinders at 60 °C+ (Legionella safety), mixed down to ≤45 °C at the tap, which is why the water band runs hot. Because a real cylinder stratifies — hot floats on top and is drawn first — the water you actually get stays hot through a shower even as the stored reserve runs down, so the cylinder makes a far better buffer than its average temperature alone would suggest.

Two peaks, not one. NZ time-of-use tariffs split the weekday into three rates: peak (7–11am and 5–9pm), off-peak (11am–5pm and 9–11pm), and a half-price night rate (11pm–7am). There are two expensive windows to dodge every day. The smart controller's first choice is to do all its heating in the cheap night window and coast through both peaks and the off-peak gap between them on that one charge — which a well-insulated home or a large cylinder can manage outright. Only when a store can't hold heat that long does it add a top-up, and it places that top-up in the cheaper off-peak afternoon rather than letting it fall into a peak. On top of the per-kWh rates sits a fixed daily charge — the lines and connection fee every property pays regardless of use. Smart control and efficiency shrink the energy part of the bill but never that fixed floor, so on a well-insulated, low-use or solar-heavy home the daily charge can quietly become the single largest line on the bill — which the total panel breaks out so you can see it.

How fast does it leak? (1) The building. The house loses heat at its heat-loss coefficient (UA, W/K) times the indoor–outdoor gap. This tool builds that figure up element by element — each surface's area times its U-value (W/m²K, the inverse of the familiar R-value) — for a representative ~120 m² single-storey home, plus an air-leakage term. The specification behind every None / Some / Good button is set out below. Glazing carries outsized weight: MBIE attributes 30–50% of a home's heat loss to windows, and going single → low-E roughly thirds their U-value. BRANZ finds the ceiling is the single biggest leak in uninsulated homes — the usual first retrofit — and that ~830,000 NZ homes still fall short of today's standards. Summed, a fully uninsulated, single-glazed house lands north of 600 W/K (effectively unheatable on one heat pump), a part-retrofitted home around 250–350, and an all-Good envelope near 170.

U-values in W/m²K (whole-assembly, frame included for glazing). For context, "Some" ≈ R1.8–2.5 retrofit batts; "Good" walls ≈ R2.8+, ceiling ≈ R6.0, floor ≈ R3.0. On top of these, an air-leakage term adds ≈95 W/K when everything is None (~1.0 air change/hour for the 288 m³ volume), easing to ≈43 W/K when all Good (~0.45 ACH) — better-insulated homes are usually better sealed. The four buttons plus that leakage are what sum to the "Loses … W/K" shown in the panel.

Time constant. Dividing the building's stored heat by this loss gives τ = C ÷ UA — how many hours it holds its warmth: a leaky lightweight house coasts under 2 hours, a heavy well-sealed one for days. That coast time is the whole game — it sets whether one night charge can carry the house through the next day.

Recharging the cylinder: temperature first, then volume. The tank is modelled as two zones — a slug of hot water at the top floating on cold inlet water below. A shower draws hot off the top, so the hot volume shrinks while the water that remains stays hot; that's why the delivered-temperature line barely dips through a shower even as the amber capacity fill drains away. When the element or heat pump comes on it does what a real stratified cylinder does: it restores the temperature of the existing hot zone back to the setpoint first, and only then grows the reserve by heating fresh cold water up from the bottom. Because the hot slug left after a shower is small, nudging it back to full temperature takes very little energy — so the delivered-temperature line snaps back up steeply, and you'd have hot water at the tap again quickly. Rebuilding the reserve is the slow part: every extra litre has to be lifted the full ~50 °C from cold mains (~15 °C) to setpoint, which is far more energy per litre than topping up water that is already warm. That's exactly why, on the charts, the delivered temperature recovers fast while the capacity bar fills in gradually — the two are measuring different things: how hot the water is, versus how much of it there is.

How fast does it leak? (2) The cylinder. A hot-water cylinder leaks far more slowly, through its insulation jacket, and its loss is quoted as standing loss in kWh/day for a tank held at ~60 °C in a ~18 °C space. The slider spans the real range: a well-insulated modern A-grade or extra-wrapped cylinder loses ~1.0–1.5 kWh/day, a standard one ~2–2.5, and an old or thinly-jacketed tank ~3.5–5. Loss scales with surface area, so a 300 L tank loses more in absolute kWh than a 135 L one at the same grade — but since stored heat scales with volume and loss with area, bigger tanks coast proportionally longer. At only ~0.3–1 °C per hour, though, the cylinder's standing loss is small next to a shower: what actually empties it is hot-water use, which is why the hot-water coast time above tracks your shower settings far more than the insulation grade.

Adding solar. Rooftop PV changes the arithmetic, because the cheapest power of all is a kilowatt-hour you generate yourself. Generation follows a half-sine across the winter daylight window (about 7:30am–5pm), scaled by the Weather slider — independent of temperature, so you can model a cold clear day or a mild overcast one. At 0% the panels make nothing; at 100% they hit their full nameplate rating at solar noon, so a 5 kW array yields roughly 4 kWh on a dull day (~20%) and up to ~30 kWh under a clear sky. Any generation first offsets whatever the house is drawing right then — so it only shaves the bill when something is actually running. That's the catch for a night-charging strategy: the cheap night window has no sun. So when the forecast is sunny (weather above ~55%), the smart controller does two extra things. It charges a little less overnight, leaving headroom in the building's thermal mass; then during the midday off-peak window it runs a pure-solar top-up — using only surplus generation, never grid — to pre-heat the house toward the top of its band and finish filling the cylinder. That free midday heat is banked in the mass and carries the house through the evening peak, so the panels displace grid energy that would otherwise have been bought at night. The Net power chart shows grid import above the zero line and solar export below it — when generation outruns what the house is using, the bars dip negative. The separate Where the solar goes chart settles the obvious question — is the sun actually heating my house? — by taking each hour's generation and splitting it into the three places it can end up: home heating, hot water, and export. The three bands stack to the full height of generation, so any ember or blue you see is solar electricity that went straight into a thermal store rather than to the grid. Anything exported is credited at the feed-in rates in the price panel, so the bill is import cost minus export revenue. Because import almost always costs more than export earns, the smart controller prefers to self-consume surplus solar — banking it as heat — rather than export it; you can flip that trade-off by pushing the feed-in rates up toward the import rates. How much solar the stores can soak up is capped by the heating-capacity sliders: a heat pump or element only draws so many kW, and once a store hits the top of its band there's nowhere left to put the energy, so the rest is exported — which is why a bigger array eventually just raises export rather than self-use.

Why it pays. From mid-2026 every major NZ retailer must offer time-of-use pricing, so the peak-to-night spread of 30–40 c/kWh is real — and a typical home can move several kWh off-peak each day with no hardware beyond a smart controller on the appliances it already owns.