How to size your solar system — and why your current electric bill is lying to you

Your bill is a dollar amount. Your system is sized in kilowatt-hours.

When most people think about “how much solar do I need,” they reach for their electric bill — “I pay about $200 a month.” But a dollar amount is the one number a solar design can’t use. Two homes with identical $200 bills can use wildly different amounts of energy, because rates differ by utility, by season, and by rate plan. The number that sizes a system is your usage in kilowatt-hours (kWh) — and it’s on the same bill, usually in smaller print.

Three ways your bill misleads you

None of this is your utility being sneaky — a bill is built to collect payment, not to design a solar system. But if you size from it naively, it misleads you three ways:

• It shows dollars, not energy — part of every bill is fixed fees and delivery charges that have nothing to do with how much you used. The dollar total overstates what solar can offset.
• One month isn’t a year — usage swings hard with seasons (summer AC, winter heat). A system sized off your January bill will be wrong in July. Honest sizing starts from twelve months of usage, not one.
• It shows your past, not your future — the bill says nothing about the EV you’re considering, the heat pump replacing your furnace, or the home office that moved in. A system sized for last year’s life is undersized for next year’s.

What actually determines system size

A real design comes down to three inputs, in this order:

• Your annual usage in kWh — the target the system is trying to cover. Twelve months of real data beats any estimate.
• What your roof can produce — orientation, pitch, shading, and your local sun determine how many kWh each kW of panels actually generates. The same panel produces very different energy on a south-facing Texas roof versus a shaded northern one.
• Your offset target — how much of your usage you want solar to cover, which depends on your utility’s export rules and your budget. This is a choice, not a formula.

Count tomorrow’s loads, not just today’s

This is the step most sizing skips. A solar system is a 25-year asset, and the biggest usage changes of the next decade — an EV in the garage, a heat pump replacing gas heat — each add thousands of kWh a year. Adding panels later is possible but costs more per panel than including them up front. If a new load is genuinely on your horizon, size for it now.

Try it — start from your real monthly usage, then toggle the loads you’re planning:

Why 100% offset isn’t always the target

It’s tempting to aim for covering every kilowatt-hour — but whether that’s smart depends on your utility. Where exported solar is credited well below the retail rate — like California under NEM 3.0 or Illinois under net billing — the extra production you send to the grid earns much less than the power you use yourself, so oversizing pays back slowly unless a battery captures it. Many utilities also cap interconnection near your historical usage.

That’s why we size to an offset target rather than to the biggest array that fits: enough to cover your real (and planned) usage, with headroom — not the most panels the roof can hold.

The honest bottom line

Sizing isn’t a guess off a dollar figure — it’s your twelve-month usage, your roof’s real production, and the loads you’re planning, sized to an offset target that fits your utility’s rules. Our Build Your System tool walks that exact sequence with your actual roof and usage — and if you want a second set of eyes on the result, we’ll walk through it with you.