Solar Panel Calculator: Size, Batteries, Payback Made Simple

My neighbor put panels on his roof last summer and I'll be honest I was jealous. His electric bill dropped from 180 bucks a month to basically nothing. So I started digging into the numbers myself, reading forums at 2am, trying to make sense of all the conflicting advice.

Took me about two weeks of reading and three installer quotes before I actually understood how this all works.

And tbh the math isn't that complicated once someone explains it without the sales pitch. Three formulas. That's really all there is to it.

Here's everything I figured out. The formulas, the stuff installers don't tell you, and what actually matters when you're sizing a system. No fluff. No upselling. Just what I wish someone had told me before I started.

Sizing Your System

Your solar system size comes down to two numbers. How much power you use and how much sun your roof gets.

That's really it.

First find your daily kWh usage. Grab your annual electric bill and divide total kWh by 365. If you use 10000 kWh per year that's about 27.4 kWh per day. Pretty straightforward.

Peak sun hours vary a ton by location. Phoenix gets about 6.5 hours daily. Seattle maybe 3.5. Most of the US sits somewhere in the middle, probably around 4.5 to 5.5 depending on exactly where you are. Check your region's average on the NREL site.

The formula: System size in kW = daily kWh divided by peak sun hours.

So 27.4 kWh divided by 6.5 hours gives you a 4.2 kW system.

But panels have losses. Inverter efficiency, a bit of shading here and there, dust buildup, birds leaving surprises on your panels, you know how it goes. Add a 20 percent buffer and you're at about 5 kW.

Now panels. A typical residential panel these days is 400 watts. So 5000 watts divided by 400 watts equals 12.5 panels.

Round up to 13. Easy.

My neighbor in Tucson has a 6.6 kW system with 16 panels at 415 watts each. In summer he cranks out 30 to 35 kWh per day. Winter drops to about 20 kWh. So always plan for your worst month not the annual average. Learned that one the hard way from watching his January numbers tank while his December numbers were fine.

Battery Storage

Batteries are expensive.

Like 8000 to 15000 bucks installed for a 10 to 13 kWh unit. The Tesla Powerwall, LG Chem, Enphase, etc. Here's how to figure out what you actually need without blowing your budget on storage you'll never fully use.

Most homes use 30 to 50 percent of daily energy after sunset. If your daily usage is 27 kWh your overnight consumption might be around 10 to 12 kWh. That's your battery target.

But there's a catch. Depth of discharge. Lithium ion batteries have about 90 percent DoD so a 10 kWh battery only gives you 9 kWh of usable power. To get 12 kWh usable you'd need a 13.3 kWh battery. Round up to 14 kWh.

Honestly if you have net metering you might not need a battery at all. In California with NEM 3.0 batteries start making sense because you lose credit value exporting to the grid at those new lower rates. In Texas where there's no net metering a battery is more of a luxury than a necessity. Unless you get blackouts a lot. Then it's probably worth the peace of mind.

I've talked to people who bought batteries and regretted it. And I've talked to people who skipped batteries and then sat through a three day power outage eating canned soup by candlelight. Depends on your situation, your budget, your tolerance for sitting in the dark, all of that.

ROI and Payback

Payback period is how long until your savings cover what you paid.

The formula: Payback in years = (total system cost - incentives) divided by annual electricity savings.

Let's run an example. Say your system costs 15000 dollars after the 30 percent federal tax credit. Annual savings are 1800 based on 12 cents per kWh and 15000 kWh per year generation. Payback is 15000 divided by 1800 equals 8.3 years.

Not amazing. Not terrible.

Throw in state incentives and it drops further. New York has a 5000 dollar rebate that knocks payback down to about 5.5 years.

Now that's solid.

High electricity rates speed things up massively. California at 30 cents per kWh gives you a 5 year payback, maybe less if you oversized a bit. Low electricity rates like 10 cents in some parts of the Midwest push it past 12 years. Still worth it over a 25 year panel life but not exactly a quick win.

Shading kills production. Even 10 percent shade can cut output by 20 percent, I've seen it happen with a single chimney shadow. And if you finance with a 6 percent interest loan that adds 2 or 3 years to the payback. Something installers don't always mention upfront when they're showing you the shiny brochure.

My advice. Run the numbers with your actual utility rate not the national average. A friend in Chicago with 11 cents per kWh and 4 peak sun hours gets a 12 year payback. Still positive over 25 years but nothing like the 5 year numbers you see in solar ads. And those ads, man. They make it sound like you'll be printing money by year three.

Using a Calculator

A good online calculator like PVWatts from NREL does the heavy lifting for you. You plug in your address, monthly bill, roof pitch, azimuth, shading, electricity rate, and net metering policy. It spits out system size, panel count, and estimated savings.

I always recommend trying 3 or 4 calculators.

Seriously.

They disagree by 10 to 20 percent all the time because different tools use different weather databases and different assumptions about panel degradation and inverter efficiency and a dozen other little variables. Average the results and you'll be close enough.

Don't just trust one number from one website. I compared four calculators for my own house and got system sizes ranging from 4.8 kW to 6.2 kW. That's a 2500 dollar difference in equipment cost. Kinda important to get this right.

Random Things Worth Knowing

A 2000 square foot home with average efficiency burns about 30 kWh per day. With 5 peak sun hours you need a 6 kW system which is about 15 panels at 400 watts each. But if you have electric heating or an EV you probably want to bump that to 8 kW. Maybe 9 if you drive a lot or run the AC constantly in summer.

Panels last 25 to 30 years and lose about half a percent efficiency per year. After 25 years they still produce 85 to 90 percent of original output. No need to replace them. The inverter is what dies first. Budget about 1000 to 2000 bucks for a replacement around year 10 to 15. Factor that in.

And yes solar is still worth it without net metering. Payback is slower though. Without net metering you export excess at low wholesale rates like 2 to 4 cents per kWh which is basically nothing. To maximize savings size your system to cover 80 to 90 percent of annual usage. Avoid overproduction. Batteries help store excess but add 10 grand plus to the price tag. Run the math for your situation. For most people it's still positive over 20 years. Barely sometimes but positive. Who knows, in ten years you might be glad you did it.