The single most useful thing a homeowner can do before sizing a solar system is read their own utility bill correctly. Installers and online calculators will ask for your monthly bill amount or your average usage, but the bill itself contains four specific numbers that determine whether and how solar pays back for your specific household.
Number 1: 12-month average kWh consumption
Find this on the bar chart at the top of most residential bills, which shows the last 12-13 months of usage. Sum and divide by 12. This is the load a solar system has to displace.
The trap: a single month, especially a summer or winter bill, can be 2-3× the annual average. Sizing a system to your July bill produces a system 2× too large and saddles you with exported energy at the lower export rate the rest of the year. Sizing to the 12-month average produces a system that approximately matches your annual consumption — the right baseline.
Typical US household: 800-1100 kWh/month, or roughly 10,000-13,000 kWh/year. Heat-pump households, electric-vehicle households, and Sun Belt cooling-heavy households can run 1,500-3,000 kWh/month.
Number 2: effective per-kWh rate
Add every charge on the bill — generation, transmission, distribution, customer charge, taxes, rider fees, demand fees, late fees — divide by the kWh consumed. This is the effective rate.
It is almost always higher than the "rate" the utility quotes on its rate-schedule page. The customer charge alone (typically $5-25/month) is amortized over your kWh and shows up in the effective rate even though it is technically a fixed charge.
Why this matters: when solar displaces a kWh of grid purchase, it offsets the variable (generation + transmission + distribution + variable taxes) portion of the bill. The fixed customer charge usually does not go away even after solar. So your displaceable rate — the number to put in a payback calculator — is the effective rate minus the customer-charge contribution.
For most US residential customers, the difference is 1-3¢/kWh. Not catastrophic, but real, and the installer's calculator usually does not subtract it.
Number 3: time-of-use or tiered structure
Some utilities charge a flat rate per kWh. Some charge tiered rates (more expensive after you cross 500 or 1,000 kWh/month). Some charge time-of-use rates (cheaper overnight, more expensive 4-9 PM, sometimes by season).
This matters for two reasons. First, solar produces during the day, so its value depends on what the daytime rate is, not the all-hours average. Second, if you might add a battery later, the spread between peak and off-peak rates determines whether storage pencils out.
Where to find it: the rate schedule name printed near the kWh detail, then cross-reference on your utility's tariff page (search "[utility name] residential tariff"). PG&E, SCE, SDG&E in California, ConEd in NY, Eversource in MA all default residential customers to TOU rates as of 2026.
Number 4: residential demand charge (if any)
A demand charge bills you for the highest single kW your home pulled during the month, not just total kWh consumed. Most US residential customers are not on demand-charge rates, but exceptions are growing: Salt River Project (Arizona) charges residential demand on the E-27 plan, and Black Hills Energy in parts of the Mountain West has experimented with residential demand.
If you see a line item like "Demand $/kW" on your bill, solar alone does not directly displace it — your peak draw is usually a hot afternoon when AC kicks on at the same moment the system is producing, but a single brief peak (an oven and dryer at the same time) can spike the monthly demand reading. A battery can shave demand. A naked PV system cannot reliably do so.
Net usage if you already have solar
If you are sizing an expansion or a replacement system, your current bill shows net usage (imports minus exports). Net usage is not your true household load — true load is net + solar production. Pull your last 12 months of production from your inverter app and add to the bill kWh to get the load number for sizing.
What to ignore
Most of the bill is noise for sizing purposes. The graph showing average daily kWh is useful. The bar chart of last 12 months is useful. The line items on the back of the bill — riders, surcharges, environmental adjustments, fuel adjustments — sum into the effective rate and individually do not matter.
Putting it together
Walk into a solar quote with three numbers and a question:
- Annual kWh consumption (12-month average × 12).
- Effective rate (total $ / total kWh, averaged across 12 months).
- Rate structure (flat, tiered, or time-of-use; if TOU, the peak hours).
- What is the new-customer net-metering or export tariff for my utility?
You will spot bad quotes immediately. Any installer whose proposal does not engage with your actual rate structure is selling you a system sized for someone else.