System sizing

Panel count for 110% offset

Planning ahead on purpose? Enter your annual usage and see the panel count that covers 110% of it, room for panel degradation and whatever you add to the house next.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your annual usage
Add up 12 months of bills, or check your utility's online usage history.
Defaults to 110%, adjust if your state's net metering cap allows a different ceiling.
3 Your location and equipment
Check NREL's PVWatts for your address. Most of the US ranges from 3.5 to 6 hours.
Most home panels quoted in late 2025 were 400 to 460 watts, according to EnergySage's marketplace data.
4 Derate factor
Accounts for inverter loss, wiring loss, dust, and temperature derate combined.

How it works

Three steps to your 110% target

Step 1

Confirm your state allows this offset percentage

Net metering size limits vary widely: some states cap systems at a percentage of annual usage, others use a flat kW limit. Confirm your specific state and utility before committing to 110%.

Step 2

Identify what's driving the extra 10%

Panel degradation alone justifies a modest buffer, but a planned EV or heat pump justifies a larger one. Check the oversizing for a future EV calculator if that's the bigger factor for your household.

Step 3

Confirm your roof and inverter have the headroom

A larger target needs more physical panels and possibly a bigger inverter. Check the 100% offset comparison tool to see exactly how many extra panels 110% costs over the baseline.

Why sizing above 100% is a deliberate choice, not overbuilding

Degradation quietly erodes a system sized at exactly 100%

Solar panels lose a little output every year. NREL's compendium of photovoltaic degradation rates found median degradation of 0.5 to 0.6% a year for crystalline silicon modules, a slow, steady decline built into how photovoltaic cells age. A system sized to produce exactly 100% of today's usage on day one produces about 95% of that after a decade, while household usage typically holds steady or grows. A 110% target roughly absorbs 20 years of that median decline.

Future electrification changes the baseline entirely

Adding an electric vehicle commonly raises annual usage by 3,000 to 4,000 kWh, a load the EV charging load addition calculator breaks down for a specific vehicle and the future EV oversizing calculator turns into a panel count. Converting to a heat pump can add thousands of kWh too, depending on climate and the system it replaces. Sizing against today's usage alone ignores these foreseeable additions, and building in room for them is cheaper than planning a second installation later.

Where the extra production goes in the meantime

Before an EV or heat pump arrives, the surplus from a 110% system exports to the grid. Under traditional net metering it earns credits month to month, but what happens to a leftover balance at the annual true-up varies by utility, and under net billing exports earn less than retail from the start, a distinction the net billing vs net metering comparison covers in more depth. If a home battery is part of the same plan, the future battery calculator shows that its round-trip losses add only a small amount of production, so the extra 10% does not need to cover the battery.

Why state net metering caps matter here specifically

Net metering size limits vary widely. DSIRE's entry for Maryland shows a limit of 200% of baseline annual usage, with credits reconciled each April at the commodity energy supply rate, while other states cap systems at a lower percentage or use a flat kW limit. A 110% target fits many percentage caps but not every one, which makes confirming yours before finalizing a design worthwhile. The net metering cap tool works backward from a hard limit, and the 100% offset comparison tool shows exactly how many panels separate 110% from the baseline.

Questions

Frequently asked

Two reasons compound over time: panels degrade at roughly 0.5% a year, so a system sized at exactly 100% today produces closer to 95% after a decade, and many homeowners plan to add an EV or heat pump that raises usage well above today's baseline. Sizing to 110% builds in room for both.

It depends on your state and utility, and the limits vary widely. Some states cap systems at a percentage of annual consumption, from about 100% in stricter programs to 200% in Maryland, while others use a flat kW limit instead. A 110% target fits many percentage caps but not every one, so confirm yours.

It exports to the grid. Under traditional net metering it earns credits month to month, but what happens to a surplus at the annual true-up varies by utility. In Maryland, for example, leftover credits are reconciled each April at the commodity energy supply rate unless the customer elects to keep accruing them, and under net billing exports earn less than retail from the start.

Roughly 10% more, proportional to the increased target. A home needing 20 panels for exactly 100% offset typically needs 2 more, around 22, to reach a 110% target instead.

Related tools

Keep sizing your system

Panel count for 100% offset Panel count for 90% offset Oversizing for a future EV Oversizing for a future battery Solar panel calculator Panel count for a net-zero home System size for a net metering cap limit All system sizing tools