Know Your Numbers
This is the section that separates building the right system from buying a bunch of stuff and hoping it works. Everything you decide from here on — how many panels, how big an inverter, how much battery — flows from the numbers you gather now.
It’s not hard. It just takes a little time and a couple of cheap tools. And honestly, it’s kind of fascinating once you start seeing how your home actually uses electricity.
Your Bill Is the Starting Point
Pull out your last 12 months of electric bills. If you don’t have paper copies, your utility almost certainly has detailed usage data online. PGE customers can log in and see monthly and even hourly usage going back over a year.
What you’re looking for:
Monthly kWh consumption. This is the big number — how many kilowatt-hours you used that month. Write down all 12 months. You’ll see a pattern: higher in summer (AC) and winter (heating, lights, shorter days), lower in spring and fall. A Portland home might range from 500 kWh in April to 900+ kWh in August or January.
Your rate structure. Are you on a flat rate, time-of-use (TOU), or tiered pricing? This matters because it affects the financial value of your solar generation. On a TOU plan, a kWh you avoid buying during peak hours is worth more than one you avoid at 2 AM. If you’re not on TOU yet, it’s worth looking into once you have a battery. See the rate calculator to break down your bill structure.
Your average daily usage. Take your monthly kWh and divide by 30. This gives you a rough daily number to work with. If you used 750 kWh last July, that’s about 25 kWh per day. That’s your target — or more accurately, that’s what you’re trying to chip away at.
Measure Your Actual Loads
Your bill tells you the total. Now you need to know where it’s going.
Kill-A-Watt meter (~$35). This is a little plug-in device that sits between an appliance and the wall outlet. It measures watts in real time and tracks cumulative kWh over time. Plug your fridge into it for 24 hours and you’ll know exactly what it draws — not what the nameplate says, not what some website estimates, but what YOUR fridge actually uses in YOUR house.
Emporia Vue energy monitor (~$150). This clips onto your breaker panel and shows you real-time usage per circuit. It’s a bigger investment, but it gives you the whole picture at once — every circuit, 24/7, with historical data. If you’re serious about understanding your home’s energy use, this is the tool that does it.
Nameplate vs. reality. Every appliance has a nameplate rating — the maximum watts it can draw. But most devices don’t run at their nameplate most of the time. A fridge rated at 800 watts might draw 65 watts most of the day. That 800-watt number is the compressor startup spike plus the defrost heater running. It’s the max potential, not the typical draw.
Devices aren’t constant. Your fridge hums along at 65 watts, then the compressor kicks on and it spikes to 200 watts, then twice a day the defrost cycle fires a heating element and it jumps to 800 watts for 20 minutes. A furnace fan runs at 250 watts but surges to 500+ on startup. A window AC unit draws 400 watts steady but needs 1,200 watts for the first half-second when the compressor engages.
These spikes are brief, but they’re real, and your inverter has to handle them.
Build Your Load Profile
Now put it all together. Make a list — a spreadsheet, a notebook, whatever works for you — with every circuit or device you plan to power from your solar system.
For each one, write down:
Continuous watts — what it draws while running normally. This is what your Kill-A-Watt shows you most of the time.
Daily watt-hours — continuous watts multiplied by hours per day it runs. A fridge that draws 65 watts and runs 24 hours uses about 1,560 Wh (1.56 kWh) per day. A furnace fan at 250 watts that runs 8 hours a day uses 2,000 Wh (2 kWh).
Peak spike watts — the maximum it can demand at any instant. Startup surges, defrost cycles, compressor kicks.
Think about combinations. Your peak load isn’t every device at maximum at the same instant — that almost never happens. It’s the realistic worst case of what’s actually on at the same time. A teakettle and a microwave might both be running at breakfast — that’s a realistic combination, maybe 2,800 watts together. A washing machine and a teakettle? Probably not at the same time.
Add it all up. You should end up with three numbers:
- Average load (watts): the typical continuous draw of everything combined during active hours
- Peak load (watts): the realistic maximum when spikes overlap with other running loads
- Daily energy (kWh): total watt-hours across 24 hours for everything on your list
Know Your Sun
You know what you need. Now you need to know what the sun can give you.
Peak Sun Hours (PSH). This is the most important solar planning number and it’s widely misunderstood. PSH is not how many hours the sun is in the sky. It’s the total solar energy delivered to your location, expressed as equivalent hours of perfect, full-intensity sun. A day with five PSH could be five hours of blazing direct sun, or it could be ten hours of hazy, partly cloudy sky that adds up to the same total energy.
PVWatts (pvwatts.nrel.gov). Your best friend for this is a free tool from NREL. Plug in your address, and it gives you monthly solar resource estimates specific to your location. It accounts for your latitude, local weather patterns, and even typical cloud cover. Don’t guess at your solar potential when you can look it up for free.
Portland’s solar reality, roughly:
- June: 5-6 PSH — peak production, long days, high sun angle
- July-August: 5-6 PSH — still strong
- April-May, September: 3-4 PSH — solid shoulder months
- March, October: 2-3 PSH — noticeable drop, still contributing
- November-February: 1-2 PSH — the valley. Don’t design around these months.
See the reference table below for month-by-month Portland numbers. For your specific address, use PVWatts.
Walk your property. Go outside on a sunny day and look at where you’d put panels. Watch where shadows fall — from trees, neighboring houses, chimneys, fences. Do this at different times of day if you can. Morning shade from an east-side tree might not matter if your panels face south, but a tree to the south is a serious problem.
If you have shade you can’t avoid, that’s not a dealbreaker. You can group shaded panels on a separate string connected to a separate MPPT input on your inverter, so one shaded panel doesn’t drag down the rest of your array. More on this in String Design.
Orientation. South-facing is ideal. East or west works — expect roughly 80% of south-facing output, which means you might need an extra panel or two to hit the same numbers.
Can I Shrink This?
You’ve got your load profile and your sun data. Before you start sizing equipment, take one more pass at the load side.
This is where “Insulate Before You Generate” becomes concrete. Look at your load profile and ask: what can come down?
- That always-on gaming PC drawing 150 watts around the clock — does it need to be on solar, or can it stay on grid?
- Could better attic insulation reduce your AC runtime by an hour a day? That’s 400-600 Wh saved.
- Would air-drying clothes instead of using the dryer take a monster load off the list entirely?
- Is there a circuit on your list that’s more “nice to have” than essential?
Iterate here. Adjust your load profile, recalculate your daily kWh, and see how it changes the system you’d need. This loop is free and it’s the most cost-effective part of the entire project.
Napkin Math
You now have three things: a daily kWh target, a peak load number, and your local PSH data. Time for some quick math that gets you in the ballpark.
Panel sizing:
Daily kWh target ÷ Peak Sun Hours ÷ 0.70 = total panel watts neededThe 0.70 is an efficiency factor that accounts for real-world losses — heat, wiring, inverter conversion, dust, non-optimal angles. It’s conservative on purpose.
Example: 10 kWh/day ÷ 5 PSH ÷ 0.70 = about 2,860 watts of panels. That’s seven or eight 400-watt panels.
Note: this is sized for summer production. In December with 1.5 PSH, those same panels produce a fraction of that. This is expected — design for the productive months.
Battery sizing:
Overnight load (watts) × hours of coverage ÷ 0.85 = minimum battery WhThe 0.85 accounts for depth of discharge and efficiency losses.
Example: 500W overnight load × 10 hours = 5,000 Wh. Divide by 0.85 = about 5,900 Wh. Call it 6 kWh of battery to get through a summer night.
Inverter sizing:
Peak simultaneous load × 1.25 = minimum inverter continuous ratingThe 1.25 gives you headroom. You don’t want your inverter running at 100% capacity as its baseline.
Example: 2,400W peak realistic load × 1.25 = 3,000W minimum continuous inverter rating.
These are napkin numbers. They get you in the right neighborhood — “I’m looking at roughly a 3kW array, 6 kWh of battery, and a 3,000-watt inverter.” That’s enough to start researching specific equipment.
Ready to go deeper? Head to the design pages: Panels, String Design.
Reference: Common Device Wattages
These are typical numbers. Your devices will vary — measure yours with a Kill-A-Watt for real data.
| Device | Typical Draw | Spike / Peak | Typical Daily Usage |
|---|---|---|---|
| Refrigerator | 60-80W | 200-800W (compressor + defrost) | 1.5-2.0 kWh |
| Standing freezer | 80-100W | 200-500W (compressor) | 1.5-2.5 kWh |
| Furnace fan | 250-400W | 500-800W (startup) | 2-4 kWh (depends on run time) |
| Window AC (inverter) | 400-600W | 1,000-1,500W (compressor start) | 3-6 kWh (depends on heat/runtime) |
| Internet router + modem | 15-30W | minimal | 0.4-0.7 kWh |
| LED lights (whole house) | 100-300W | minimal | 1-2 kWh |
| Microwave | 1,000-1,200W | same (resistive load, no spike) | 0.1-0.3 kWh (short use) |
| Electric kettle | 1,200-1,500W | same (resistive) | 0.1-0.2 kWh (short use) |
| Clothes dryer | 4,000-5,500W (240V) | same | 2.5-4.0 kWh per load |
| Electric oven | 2,000-6,000W (240V) | same | varies widely |
| Washing machine | 300-500W | 500-800W (motor start) | 0.3-0.5 kWh per load |
| Laptop charging | 30-65W | minimal | 0.2-0.5 kWh |
| Desktop gaming PC | 150-400W | same | 3.6-9.6 kWh (if always on) |
| Phone charging | 5-20W | minimal | 0.05-0.1 kWh |
| TV (LED, 50-65”) | 50-100W | minimal | 0.3-0.8 kWh |
| CPAP machine | 30-60W | minimal | 0.2-0.5 kWh |
| Sump pump | 300-800W | 1,500-2,500W (startup) | varies (intermittent) |
| Well pump | 750-1,500W (240V) | 2,000-4,500W (startup) | varies |
| EV charger (Level 1) | 1,200-1,400W | minimal | 8-12 kWh (overnight) |
Note: 240V devices (dryer, oven, well pump, EV charger) require a 240V inverter. If you’re building a 120V system, these are off the table — plan alternatives.
Reference: Peak Sun Hours
Monthly averages for the Portland, OR area. Your results depend on your specific location, orientation, and shading.
| Month | PSH (avg) | Notes |
|---|---|---|
| January | 1.2 | Shortest days, lowest sun angle, heavy clouds |
| February | 1.8 | Slightly better, still tough |
| March | 2.8 | Spring begins, noticeable improvement |
| April | 3.8 | Good production starts |
| May | 4.8 | Strong month |
| June | 5.5 | Peak production — longest days, highest sun |
| July | 5.8 | Best month — long days, clearest skies |
| August | 5.3 | Still excellent, days getting shorter |
| September | 4.0 | Solid shoulder month |
| October | 2.5 | Falling off |
| November | 1.5 | Winter begins |
| December | 1.0 | The bottom — plan accordingly |
Use PVWatts (pvwatts.nrel.gov) for numbers tailored to your specific address.
Previous: Define Your Goals | Next: Panels
DATA SOURCED FROM: National Renewable Energy Laboratory (NREL) — PVWatts solar resource data. Portland-area PSH values are approximate regional averages; individual results vary by location, orientation, and shading. Device wattage figures are representative ranges from manufacturer datasheets and field measurements. Measure your own devices for accurate load profiling.