top of page

Goal-Zero: How to calculate your Solar Panel needs in Portland, Oregon

Well howdy! (Yes - I'm from a small population western state, and truly - we do use the word 'howdy' - laugh it up!)

What we have available today for a discussion is the question all perspective solar purchasers ask:

"HOW MANY SOLAR PANELS DO I NEED?" Read on to learn how to calculate your solar panel needs.

You'd think that a company that's been doing the solar thing for over a decade would just be able to drop a number on you and it would be on the mark, right?

(if I had a camera recording my face while I'm writing this - you'd have seen me look up and to the right - remembering all the times I've been asked this question - and then you'd see me make the "yikes!" face where you see genuine concern/surprise on my face and a look that says: "how do I answer this without sounding like a completely self-serving salesman?")

Yup. This is THAT kind of question - it's tough.

Yes - there are some general (and I mean VERY general) "truths" that we can apply to MOST property owners that might allow us to get "in the ballpark" of a guess as to how many solar panels you "need" - and that's the real word I want to discuss here: NEED

If you were to ask me how many solar panels any property owner NEEDS, I'd tell you that you NEED enough to offset 100% of their electric bill, per net metering year.

And there's that all-to-common term you'll find pop up in blog after blog (that I write): Net Metering (here's the link if you're not familiar:

A net metering year is the 12 month cycle from when the electric company starts allowing you to roll-over your excess power generation to the following months. Around here, it starts on April 1 and ends on March 31 of the following year.

If I were to look at 12 of your electric bills, I'd be able to figure out (based on your current electrical usage) how many kilowatt-hours you've used during that span of time.

Our goal - when designing a solar array for your property is to provide a design that generates 100% of this power (measured in kilowatt-hours - or kWh - and NOT in Dollars ($)).

Because you live in or operate your property in a unique (to you) way, your usage over the course of a year is different than someone else with a similar property. Why? Well if we look at residential solar, it's pretty easy to see how this could be different:

1) Do you have multiple people living in the home 24/7? (a family)

2) Are you retired and spend most of your days at home (rather than at a 9-5 job)?

3) Do you have anything special about your home? (a hot tub, an electric sauna, an

outdoor water feature that runs 24/7)?

4) Do you hobby in Indoor Gardening (that uses a TON of lights and electricity)?

5) Do you have an EV (electric vehicle)?

6) And on, and on, and on...

If you look at the above list, these are just a few items that can change your electric bill (for better or for worse). Some of these things are permanent (like family) - and others can come and go (like an EV or a water feature) - and yet even others might only see seasonal spikes - like air conditioning and electric heating. What if you have a well for drinking water? Well, you pay to operate that well - as opposed to urban homes that typically have city water and thus no costs associated with a well to pump the water. Heck - even the amount of miles per year you drive your EV can make a decent difference in your electric bill as compared to other "standard" users.

The point I'm trying to make here is that there's no one specific formula that lets us accurately estimate how many solar panels you NEED. ( (are the words "accurately estimate" oxymorons when used this way? Pretty sure they are. Ironic, huh?)

You're saying: "All that I told the other companies was my average monthly electric bill," right? Wanna know a secret? I can count on less than 10 fingers (and/or toes) how many customers send me a true average electric bill. Wanna know something else? Some of you are saying: "Well, I'm on "equal pay" so that my bill is already averaged by the electric company, so I know that the average is spot-on" - but (shocker!) the Electric company errs on the side of caution when they estimate your annual equal-pay amount. What I mean by this is that they (the electric company) EXPECT you to use more power than they're selling you with that equal-pay bill. Because of this, they actually build in a "buffer" of cost for the "equal pay" amount so that they never get LESS money than they expect to sell you (in kWh) per year. When the calendar year ends, some of you (who've used less power than they factored in for your payment) get a refund or credit on your January bill. This is because they are required to audit your bills and make sure they didn't overcharge you on equal pay.

What's my point with all of this electric bill mumbo-jumbo? It's this: even with a high degree of certainty on your monthly expense for your electric bill, using the dollar ($) amount from your electric bill won't give us an accurate accounting of how many KILOWATT-HOURS you use per month or year. This is due to the litany of other charges (and sometimes credits) that are spelled out on the second (back) page of your electric bill. From the basic charge that is standard per month just to have the electric meter on your home to the extra charges for transmission and distribution that aren't linear in their calculation - things change on every single bill we see - and because of that, there's honestly no one single, guaranteed fool-proof method to use anything other than the actual kilowatt-hours (shown on each of your electric bills) to figure out how many kilowatt-hours you're buying per year with from your electric company.

And this brings us full circle to the question of NEED. And as previously stated, your need is based on how many kilowatt-hours you use per year currently.


There are many OTHER factors that come into play with coming up with an accurate design that will generate how many panels you NEED to offset 100% of your electric bill:

1) The tilt of your solar panels (anything other than 32° is going to be less than ideal here

in NW Oregon) - your roof faces each have their own specific slope/tilt - which can't be


2) The orientation (to south) that the solar panels face (180° is considered ideal in the

norther hemisphere) - your roof faces each have their own specific orientation - which

can't be changed

3) The shading on the solar panels (at their proposed locations on your property)

If you have the perfect tilt, orientation, and shading profile that makes your solar panels 99.9% effective at seeing the sun (on your property) then you'll need LESS solar panels than someone else whose home was built with roofs facing different directions than yours. If we assume that the two homes in this paragraph have exactly the same amount of usage, it doesn't mean that they both need the exact same amount of solar panels. In fact - the home with the "perfect" roof space for solar will use NEED less solar panels to generate the same amount of power that the other home does in the same net metering year.

So...we've now explored just a few things that change the number of "How many solar panels do I NEED" - right?

We now know that the first data point we need is your usage - in kilowatt-hours. We also know that the next data point we need is an accurate performance model of the roof spaces you want your solar panels on. With these two sets of data points, DPI Solar sales advisors can give you a REAL accounting of how many solar panels you NEED - and a price to boot. This is why we want to visit your property in person before we throw numbers around: maybe a tree that once shaded the solar array location is no longer there? Maybe you have an addition to your property that has a better roof space than what is shown on the standard mapping programs. Maybe you want a ground mount after realizing that you don't have enough roof space to meet your "needs". There's a lot of maybe's here. What we aim to do is turn those maybe's into answers that give you a true, transparent view of your solar project, provided by our trained sales team. You'll find (after visiting with one of them) that they generally go much deeper and ask more detailed questions during their visit (than you had with one of the other solar companies out there). Our team is great and we want to show you.

Is there a way we can get an idea of how many panels you NEED? Sure - but it's a "best guess" and if you were to ask me (when I'm considering spending thousands of dollars on an investment) if I'm ok with a "best guess" - I'm not. And I don't expect you to be either. So - we live by the age old wisdom: good information in will give us good answers; bad information in will give us bad answers. And we don't like giving bad answers.

And you shouldn't accept anything less than a detailed analysis with the data points to back it up!

Let us know if your ready to figure out how many solar panels you need?

Thanks for reading!


DPI Solar


Recent Posts

See All


bottom of page