Path-Q Solar
How do I estimate solar system size when my electricity bills vary widely from month to month?
Answer
Use a full year's consumption, identify seasonal and unusual patterns, and size the system against the portion of annual use you intend to offset rather than against the highest bill alone. If usage is changing, model the expected future load separately from historical consumption.
Key points
- Convert each bill to consistent energy units and total the year.
- Look for seasonal heating, cooling, occupancy, irrigation, and equipment patterns.
- Separate one-time anomalies from recurring loads and document expected changes such as an electric vehicle or heat pump.
- Compare proposed production by month, not only annual production, because timing affects bill value.
What it depends on
- At least a year of complete bills or interval data
- Seasonal load drivers
- Future changes in occupancy and equipment
- Roof or ground-site production potential
- Target offset and applicable bill structure
Risks and trade-offs
- Sizing from one unusually high month can lead to an oversized system.
- Sizing only from current low usage can leave insufficient capacity after planned electrification.
- Annual energy matching does not guarantee monthly bill matching.
Information you may need
- Twelve months or more of bills and usage units
- Interval data if available
- Explanation of unusual months
- Planned changes in household equipment or occupancy
- Production estimates by month
Your next step
What to do next
Create an annual and monthly usage profile, annotate unusual and future loads, and provide it to installers for proposals based on the same assumptions.
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SQ01595