Home Appliance Energy Cost Estimator

---
name: home-appliance-energy-cost-estimator
description: Estimates what each home appliance costs to run from its watts, hours per day and your electricity tariff. Reads a CSV, works out kWh and cost per day, month and year, standby (vampire) loads, each appliance's share and off-peak pricing, compares the total with a real bill, and ranks savings ideas such as LED bulbs, 30 C washes and cutting standby by yearly savings. Includes a stdlib Python estimator with --self-test.
---

# Home Appliance Energy Cost Estimator

You help households see where their electricity money goes, appliance by appliance, and which few changes are worth making. You turn watts and habits into kWh and money, show your arithmetic, and keep the advice practical: no shaming, no expensive upgrades that never pay back.

## Files in this skill

- `scripts/energy_cost.py` - reads an appliance CSV and a tariff, prints a cost table, findings and ranked savings ideas (Python 3 standard library only)
- `references/energy-math.md` - formulas, duty cycles, standby, energy labels, tariffs and how to measure real consumption
- `references/typical-wattages.md` - typical running and standby watts and daily hours for common appliances, to fill gaps
- `templates/energy-report.md` - the household report: totals, top consumers, standby, action plan with savings and cost
- `examples/apartment-appliances.csv` - a two-person apartment with 15 appliances (fridge from 2011, halogen spots, tumble dryer, game console standby, seasonal space heater)

## Workflow

### 1. Build the appliance list
Ask for the appliances, their watts (nameplate sticker, manual, or a plug-in energy meter), how long each runs per day and on how many days a week, and the electricity price per kWh (plus off-peak price and standing charge if any) from a recent bill. For fridges and freezers prefer the energy-label kWh/year; for anything with a thermostat set a duty cycle. Fill gaps from `references/typical-wattages.md` and say which values are assumptions. If they have a bill, ask for the monthly kWh to cross-check.

### 2. Run the estimator
```
python3 scripts/energy_cost.py examples/apartment-appliances.csv --rate 0.32 --offpeak-rate 0.21 --standing-charge 0.40 --bill-kwh 330 --currency EUR
python3 scripts/energy_cost.py my-home.csv --rate 0.15 --currency USD --json
python3 scripts/energy_cost.py --self-test
```
Exit code 1 means HIGH findings (standby above 10% of use or the estimate far from the bill), 2 means bad input.

### 3. Review with the user
If BILL_GAP appears, fix the inputs first (missing water heater or electric heating, seasonal use via `months_per_year`, hours that are too generous). Then go through the top consumers, the standby list and the ideas. Savings overlap (a dryer used less also saves less by moving off-peak), so pick a plan rather than adding every idea. Use `references/energy-math.md` to explain any number.

### 4. Deliver
Fill `templates/energy-report.md`: totals, top 5 consumers, standby, and an action plan sorted into free habits, cheap fixes (LED bulbs, smart plugs) and bigger purchases with payback years (price / yearly saving).

## Example result
For `examples/apartment-appliances.csv` at EUR 0.32/kWh (off-peak 0.21, standing charge 0.40/day): 12.3 kWh/day, 373 kWh/month, 4479 kWh/year, energy EUR 118.38/month or EUR 1420.54/year (EUR 1566.54 with the standing charge). Six halogen spots are the biggest consumer (548 kWh, 12%), then the space heater used 5 months (479 kWh). Standby 319 kWh (7%), with MEDIUM findings for the TV (12 W) and game console (15 W). Within 13% of the 330 kWh bill, so no BILL_GAP. Top ideas: LED spots about EUR 149/year, line-drying half the loads EUR 63, 30 C washes EUR 48, off-peak laundry, cutting console standby EUR 40.

## Rules

- Show the formula once and the assumptions for every guessed value.
- Use the user's tariff and currency; never assume a country's price without saying so.
- Estimates are not measurements: suggest a plug-in meter for the top 3 consumers when numbers matter.
- Rank actions by money saved per effort; give payback in years for anything that costs money.
- Safety first: never suggest disabling fridge or freezer cooling, smoke alarms, medical devices or frost protection to save energy.
FILE:references/energy-math.md
# Energy Math

## Core formulas
```
kWh per day (running) = watts x quantity x hours per day x duty% / 1000 x days per week / 7 x months per year / 12
kWh per day (standby) = standby watts x quantity x (24 - average hours running) / 1000
kWh per year = kWh per day x 365        kWh per month = kWh per year / 12
cost = kWh x price per kWh (+ standing charge per day x 365, listed separately)
```
Example: a 2000 W kettle used 15 minutes a day = 2000 x 0.25 / 1000 = 0.5 kWh/day = 183 kWh/year; at 0.32 per kWh = 58.40 a year.

## Watts vs other numbers on the label
- W (watts) is what you need. kW x 1000 = W.
- VA on chargers and UPS units is apparent power; real watts are usually 60-90% of it.
- mAh or Wh on batteries is capacity, not power draw.
- Nameplate watts are the maximum. Heaters, ovens, kettles run near it; TVs, computers and consoles usually draw 40-80% of it. A plug-in meter gives the truth in a day.

## Duty cycle
Anything with a thermostat or compressor switches on and off. The fraction of time it is really on is the duty cycle.
| Appliance | Typical duty cycle |
|---|---|
| Fridge, fridge-freezer | 30-50% (higher in a warm kitchen or with a poor seal) |
| Chest freezer | 35-50% |
| Oil-filled or panel heater on thermostat | 50-80% in cold weather |
| Oven at temperature (after preheat) | 50-60% |
| Washing machine (heats water at the start) | 50-70% of nameplate over a cycle |
| Dishwasher | 50-60% of nameplate over a cycle |
Better: use the energy label's kWh/year (EU label, US EnergyGuide), which already includes cycling.

## Standby
Devices that are "off" but still powered: TV and console instant-on modes (5-15 W), set-top boxes (10-20 W), printers, microwaves with clocks (2-3 W), chargers left plugged in (0.1-0.5 W). Always-on devices such as a Wi-Fi router are running load, not standby, so list them with 24 hours. 10 W of standby around the clock = 88 kWh/year. A switched power strip, a smart plug with a schedule or disabling "quick start" removes most of it.

## Tariffs
- Flat rate: one price per kWh.
- Time of use: cheaper off-peak hours (often night). Set `offpeak_pct` to the share already used off-peak; the script values shifting the rest of the shiftable loads (washer, dishwasher, dryer, EV charger, water heater, pool pump) at (standard - off-peak) per kWh.
- Tiered/block rates: price rises with monthly use. Use the marginal (top tier) price to value savings.
- Standing charge (fixed daily fee): paid even at zero use, so savings never reduce it.
- Taxes and levies: use the all-in price per kWh from the bill (total energy cost / kWh).

## Checking against the bill
Compare the estimated monthly kWh with the bill's kWh (not money). Within 25% is good for a habit-based estimate. Common reasons for an estimate that is too low: electric water heating (often 15-25% of a home's electricity), electric heating or cooling, an EV, a forgotten second fridge, a pool or aquarium. Too high: hours guessed generously, seasonal loads counted all year (use `months_per_year`), nameplate watts used for electronics.

## Measuring
- Plug-in energy meter (cheap): measures kWh over a week; multiply by 52.
- Smart meter app or utility portal: hourly data shows the night-time base load (everything always on); base load x 8760 h / 1000 = kWh/year that never switches off.
- Read the meter before and after a known run (e.g. one oven hour) with everything else steady.

## Payback
payback years = purchase price / yearly saving. LED bulbs: months. Smart plugs for standby: 1-2 years. Heat-pump dryer or new fridge: often 5-10 years, so suggest them when the old unit needs replacing anyway.
FILE:references/typical-wattages.md
# Typical Wattages and Use

Starting values when the user does not know. Always label them as assumptions; real models vary a lot.

| Appliance | Running W | Typical use | Standby W | Notes |
|---|---|---|---|---|
| Fridge-freezer | 100-200 | 24 h at 30-50% duty | - | Label: 150-350 kWh/year (new A-E class lower) |
| Chest freezer | 60-120 | 24 h at 35-50% duty | - | Label: 150-250 kWh/year |
| Washing machine | 1800-2400 | 1-2 h per cycle, 3-5 cycles/week | 0.5-2 | 0.5-1.0 kWh per 40 C cycle |
| Tumble dryer (vented/condenser) | 2000-3000 | 1-1.5 h per load | 0.5-1 | Heat-pump dryer about half |
| Dishwasher | 1200-2000 | 1-2 h per cycle | 0.5-1 | 0.8-1.4 kWh per cycle; eco programs longer but use less |
| Electric oven | 2000-3500 | 0.5-1 h | 2-4 (clock) | Preheat at full power, then cycles |
| Induction or ceramic hob (per zone) | 1200-2500 | 0.5-1 h | 0-2 | |
| Microwave | 800-1200 | 5-15 min | 2-4 | |
| Air fryer | 1200-1800 | 15-30 min | 0 | Cheaper than an oven for small portions |
| Kettle | 2000-3000 | 10-20 min total | 0 | |
| Coffee machine | 1000-1500 | 10-20 min | 1-3 | Keep-warm plates add up |
| Incandescent / halogen bulb | 40-60 | 3-6 h | 0 | LED equivalent 5-9 W |
| LED bulb | 5-12 | 3-6 h | 0-0.5 (smart bulbs) | |
| TV 43-65 inch LED | 60-150 | 3-5 h | 0.5-15 | Quick-start modes raise standby |
| Game console | 100-220 | 1-3 h | 1-15 | Rest/instant-on mode about 10-15 W |
| Desktop PC + monitor | 100-400 | 4-8 h | 2-5 | Gaming PC under load 300-600 W |
| Laptop | 30-70 | 4-8 h | 0.5 | |
| Wi-Fi router + modem | 8-20 | 24 h | - | Always on: list as running load |
| Set-top box / streaming box | 10-20 | 4 h | 5-15 | |
| Space heater (fan / oil-filled) | 1000-2000 | 2-6 h in winter, 50-80% duty | 0 | Use months_per_year |
| Portable air conditioner | 800-1500 | 4-8 h in summer | 1-2 | |
| Dehumidifier | 200-500 | 6-12 h | 1 | |
| Electric water heater (tank) | 2000-3000 | 2-4 h | - | Often the hidden big load |
| Electric shower | 7000-10000 | 10-20 min | 0 | |
| EV charger (home, 7 kW) | 7000-7400 | per km driven: ~0.15-0.2 kWh/km | 2-5 | Use kWh/year = km/year x 0.17 |
| Pool pump | 750-1500 | 6-10 h in season | 0 | Variable-speed pumps save about half |
| Aquarium (heater + filter + light) | 50-200 | 24 h at 30-60% | - | |

Sources for ranges: manufacturer data sheets and energy labels, plus common national energy agency appliance calculators. Check the user's model when a value drives a decision.
FILE:templates/energy-report.md
# Home electricity report: <household name or address>

**Tariff:** <EUR 0.32/kWh, off-peak 0.21 (23:00-07:00), standing charge 0.40/day>   **Bill check:** <estimate 373 kWh/month vs bill 330 (-13%)>
**Summary:** <one sentence: e.g. "Lighting and laundry are 40% of the bill; LED spots and 30 C washes save about EUR 200 a year for EUR 40.">

## Where the money goes
| Appliance | kWh/year | Cost/year | Share | Assumption |
|---|---|---|---|---|
| <Halogen spotlights x6> | <548> | <175.20> | <12%> | <50 W, 5 h/day> |
| <...> | | | | |

Total: <4479 kWh/year, EUR 1420.54 energy + EUR 146 standing charge>

## Standby (always paid, nothing gained)
<319 kWh/year, about EUR 102. Biggest: game console 15 W, TV 12 W.>

## Action plan
### Free habits
| Action | Saving/year |
|---|---|
| <Wash at 30 C, full loads> | <EUR 48> |
| <Run laundry and dishwasher off-peak> | <EUR 70-110> |

### Cheap fixes
| Action | Cost | Saving/year | Payback |
|---|---|---|---|
| <6 LED spots> | <EUR 30> | <EUR 149> | <2-3 months> |
| <Smart plug for TV + console> | <EUR 15> | <EUR 68> | <3 months> |

### Bigger purchases (only when replacing anyway)
| Action | Cost | Saving/year | Payback |
|---|---|---|---|
| <Heat-pump dryer> | <EUR 600> | <EUR 63> | <~10 years> |

## Next steps
- <Measure the space heater and fridge with a plug-in meter for one week>
- <Re-run the estimate after the LED swap and compare the next two bills>

Numbers are estimates from rated watts and typical use; savings overlap and are not additive.
FI

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