Solar panels have gotten a lot cheaper in recent years but can still be a big investment for a homeowner. Before you commit thousands of dollars to the project, why not spend $100 (or less!) to be sure it makes sense with a home energy monitor?
Here’s how to break it down.
Making your own Power
Most of the benefit of installing solar panels on your home will be generating your own electricity instead of buying it from your utility. So, the closer you can get to generating 100% of your own power, the more you can save and the better return you can get on your investment in solar panels and installation.
But- how much of your own power you can generate from the sun depends on how much of your energy you use while the sun is shining. The graphic below shows power generation from solar panels throughout the day, relative to the typical household’s energy usage.
Selling power back to the Utility
When you make extra power, you can sell it to the grid. Great right? But when you need power, you may have to buy it back from the grid at a higher price. The concept of banking your excess power on the grid and getting full credit for it, is called "net metering" and many power providers are starting to push back against it. Depending on where you live, the price your utility will buy excess power from you can range from half, down to a small fraction of the retail rate at which they sell power to you. So, the best way to get value from your panels in the long term is to shift as much of your electricity usage as possible to the hours when your panels will be generating their maximum output.
How feasible is that for you and your family? If you have a long commute to work and leave and return in the dark and leave your home unoccupied all day, it might be more of a challenge for you than for someone who spends their days at home.
But with a little more detailed information, you can find out how close you can get to the 100% mark, it’s kind of a fun game! Here’s how you do it:
How to Optimize your Power Use for Solar
- Pick up a Vue from Emporia (if you live in California and are a customer of PG&E or SCE, you can do this wirelessly with a plug-in Vue: Utility Connect)
- Spend an hour installing the Vue with our easy installation guide and download the Emporia app on your phone from the App Store or the Play store
- Give the system a week to accumulate weekday and weekend usage data
- See what your home’s usage peaks look like and what’s causing them. If you installed the Vue with Expansion Module you know for sure because you’re monitoring individual circuits, but otherwise you should be able to guess (when did I run the dryer? Did it get very cold last night?).
- Spend another week trying to shift your electric usage to peak solar hours and see how it works for your family. Some ideas:
- When you leave the house in the morning, set your dishwasher to run at noon
- Do laundry on the weekends so that your dryer runs at mid-day
- Set your thermostat to turn down your furnace or A/C starting in the evening
- Then, at the end of that week, see how your profile has changed and try to calculate what percentage of your power usage is during the sunny hours.
What System to Buy
Now that you know how much of your power you can generate from your solar panels, it’s time to see what size of system you need to buy and what it will cost, to see what kind of return you can get on your investment.
Size of panels to buy: Check your utility bill and see how many kWh you used in the past year. Let’s say you used 10,000 kWh. And after using the Emporia Vue for 2 weeks, you know that you can use 70% of that during the sunny hours. So what size system do we need to generate 7,000 kWh per year?
Thanks to the National Renewable Energy Laboratory, we can get an "energy production factor" for our area, in this case (say we're in Colorado) it's 1,900 kWh/kW-yr. We divide 7,000 kWh/yr by 1,900 kWh/kW-yr to get 3.7 kW. Is that the system size we need? Maybe, but it's best to round up to account for inefficiencies of the system. If we round up by about 25%, we get to a 5-kW array. Easy!
How much will that system cost, and how much can we save per year?
A rule of thumb is about $2,500 per kW, but you can get a detailed breakdown of costs before and after tax credits in your area here.
So roughly, our cost for a 5 kW system will be about $12,500 for the complete system. But how much will we save?
Well, if we didn't buy 7,000 kWh/yr from our utility, and our power cost was $0.13/kWh (check your bill), we saved $910/yr. Our payback is around 13.7 years, but likely improved by the tax credits in your area to 9.6 years or better.
But what if we’d been able to shift 80% of our usage to the sunny hours? If we can squeeze a little more efficiency out of the same sized system, we can save $1,040 per year and get our money back in 8.4 years. This doesn’t include the fact that utility rates may go up over time, or that you can make a little bit of money every year by selling excess power to the grid during extra sunny times, both of which can improve the economics. If your utility offers it, you can also switch to a time-of-use plan, which will let you buy your off-peak power at a cheaper rate, since you're now generating your own peak power.