Before sizing any generation or storage system, you need an accurate picture of what you actually consume — how much, when, and which loads dominate. This chapter covers the tools and concepts for measuring and understanding your household’s electricity use.
Every appliance has a rated power printed on its label, typically in watts. This is the maximum draw under specific test conditions. Real consumption is almost always lower — and often much lower — for two reasons:
Key insight: To estimate annual energy consumption, you need both rated power AND average daily hours of effective operation, not nominal hours of presence.
Effective hours of operation = hours powered on × duty cycle fraction
Household loads fall into three behavioral categories:
Always on. Examples: refrigerator, freezer, router, network equipment, standby power of all devices, aquarium pump.
These are the “silent killers” of energy budgets. A refrigerator at 40 W average draw × 8,760 hours = 350 kWh/year. All the standby loads in a home (TV standby, phone chargers idle, set-top boxes) can together add up to 300–700 kWh/year.
Run at specific times, often user-controlled. Examples: washing machine, dishwasher, oven, electric vehicle charging.
These are the most valuable loads to shift to solar production hours or off-peak tariff windows.
Driven by weather and setpoints. Examples: electric space heating, heat pump, air conditioning, electric hot water tank.
These are typically the largest loads in the house, often accounting for 40–60% of annual consumption when present. Their magnitude varies significantly with climate, building envelope, and setpoint.
A standard residential electricity bill includes:
If you receive monthly bills, sum 12 months of kWh. Alternatively, note the meter reading difference over a full year. This is your annual baseline for all sizing calculations.
Typical annual consumption ranges by household type (Europe, temperate climate):
| Household | No elec. heating | With elec. heating |
|---|---|---|
| 1 person, apartment | 1,200–1,800 kWh | 3,000–5,000 kWh |
| 2 persons, apartment | 2,000–3,000 kWh | 4,500–7,000 kWh |
| 3–4 persons, house | 3,500–5,500 kWh | 7,000–14,000 kWh |
| 4+ persons, large house | 5,000–8,000 kWh | 12,000–20,000 kWh |
Note: “With electric heating” here means resistive electric radiators or underfloor heating. A heat pump heating system is typically 2.5–4× more efficient and falls in a separate range.
Most EU countries are deploying smart meters that record consumption in 30-minute or 15-minute intervals. Access your data via your utility’s online portal. This gives you:
A plug-in monitor (e.g., an energy monitoring socket) measures voltage, current, power factor, and kWh for a single outlet. Cost: €10–30. Use them to:
Clip-on current transformers installed on your main incoming cables give whole-house real-time power draw. More expensive (€150–500) but provide a complete picture without metering each outlet.
A load profile is a plot of power demand (in kW or W) over time, typically averaged over a representative day. For sizing purposes, you need profiles for:
Power
(kW)
3.5 | ████
3.0 | ███████████
2.5 | ██ ████████████
2.0 | ███ ██████████████
1.5 | ███████████ ████████████████████
1.0 | ██████████████████████████████████████████
0.5 | ████████████████████████████████████████████
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
0 2 4 6 8 10 12 14 16 18 20 22 (hour)
Key features:
This shape is important because standard grid electricity is most expensive during the evening peak — exactly when consumption is highest.
In temperate climates, consumption varies by a factor of 2–4 between summer and winter for households with electric heating. Even without heating, winter consumption is ~20–30% higher due to:
For solar sizing, this seasonal asymmetry is critical: the period of highest consumption (winter) coincides with lowest solar production.
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