System sizing

System size by monthly kWh usage

Enter your monthly usage, whether it's a typical month, any planned future load, and your rate structure. Get a system size that accounts for net metering, not just a raw kWh conversion.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your usage
Find this on a recent utility bill, usually labeled kWh used.
An atypical month gets corrected toward a more representative baseline.
3 Planning ahead
An EV, heat pump, or pool you're planning to add. Leave blank or 0 if none.
This changes whether a small buffer above your usage is worth adding.

How it works

Three steps to a precise estimate

Step 1

Enter your usage and flag if it's atypical

Pull your monthly usage from a recent bill, then say honestly whether that month ran higher or lower than usual. This one flag keeps an outlier month from skewing your entire system size. If you'd rather work from a full year instead of one month, the annual usage tool smooths seasonal swings automatically.

Step 2

Add any planned future load and your rate structure

If an EV, heat pump, or pool is coming, add its expected monthly load now rather than sizing for today only. Check the net billing vs net metering comparison if you're not sure which rate structure you have. Your rate structure decides how much buffer above your usage is actually worth building.

Step 3

Refine further with the full calculator

This tool still uses typical panel wattage and national average sun hours. If you already know your preferred equipment or local sun hours, the full solar panel calculator uses your own numbers instead.

Understanding this estimate's precision

How net metering changes the recommended size

For 900 kWh a month, close to the national average EIA reports, with full net metering, this tool recommends a 9.6 kW system, a 10% buffer above the 21 panels your raw usage needs, since banked summer credit at retail rate has real value. Without net metering or on time-of-use billing, it sizes tighter at 8.4 kW, since excess production sold back at a low rate or wasted isn't worth paying for.

Why a single month can mislead a sizing calculation

If your entered usage came from an unusually high or low month, sizing directly off it would over or undersize your system. This tool adjusts an atypically high month down by about 15% and an atypically low month up by about 15% before converting to a system size, correcting toward a more representative baseline instead of taking an outlier month at face value.

Why planned future load belongs in the sizing math now

Adding 300 kWh a month for a planned EV on top of an already-high month raises the estimate from 8.4 kW to 11.2 kW, a meaningful difference. Sizing for a future load now is usually cheaper than adding a second array later, since a second installation means a second round of labor, permitting, and interconnection paperwork. The Department of Energy's interconnection roadmap describes the steps and timelines that second round involves.

What this still doesn't capture

This estimate still relies on typical panel wattage and national average sun hours rather than your exact equipment and address. Monthly totals smooth over the seasons, so for a fully custom result using your own panel choice and local sun hours, use the full solar panel calculator instead.

What to do with this number next

Take your estimated system size to the payback period calculator to see what it might save you over time, and check what incentive programs apply to a system of this size before you get quotes.

Questions

Frequently asked

For 900 kWh a month with full net metering, this tool recommends a 9.6 kW system, a 10% buffer above the 21 panels your raw usage needs, since banked summer credit at retail rate has real value. Without net metering or on time-of-use billing, it sizes tighter at 8.4 kW, since excess production is worth much less.

If your entered usage came from an unusually high or low month, sizing directly off it would over or undersize your system. This tool adjusts an atypically high month down by about 15% and an atypically low month up by about 15% before converting to a system size, correcting toward a more representative baseline.

Yes. Adding 300 kWh a month for a planned EV on top of an already-high month raises the estimate from 8.4 kW to 11.2 kW, a meaningful difference. Sizing for a future load now is usually cheaper than adding a second array later.

Check a recent electric bill, usually labeled kWh used or energy consumption. Your rate structure, whether you have full net metering or time-of-use billing, is usually stated on the same bill or your utility's website under net metering or rate plan details.

Related tools

Keep sizing your system

Solar panel calculator System size by annual kWh usage System size by electric bill amount System size by square footage, quick estimator Quick 5-minute system size estimator Oversizing for a future EV Roof space vs output estimator All system sizing tools