Estimating usage for PV system and choosing panels
Hello!
I’m remodeling a home and have received two proposals for a rooftop pv system. Having never lived in the home, and likewise having rented for the last ten years with my utilities covered by the landlord, I don’t have a good reference point to provide for my normal usage of electricity. Both estimators assured me they could estimate based on the information provided below:
The house is 1300 sq-ft, with a 300 sq-ft attached woodshop (which will be used 4-5 days a week). There is no AC system or hot tub. The house is being rewired entirely and the lighting will be LED. The domestic hot water and radiant heat boiler system will be supplied with gas, as will the stove. The dryer is electric, but there is access to gas should a replacement be needed. Only two people will be living in the house for now, both of us fairly conscientious of usage.
Given that info we received two fairly different estimates for usage: one design is for a 2.78kw system providing 4,600kwh of annual production. This is Sunpower (with all panels having microinverters). The second design is for 4.3kw system providing 7,700kwh of annual production. This would be with LG 360w panels and an SMA inverter. The Sunpower system is less $, given that it’s smaller. What I wonder is if it will be enough. I’m tempted to go somewhere in the middle by asking Sunpower to add a few more panels, bringing it to 3.5kw or so…which I think would raise the price up somewhere close to the other, still-larger system. Overall it seems like the LG panels might be more efficient in terms of production? But the warranty is better for Sunpower, esp. when taking the lack of inverter into account. And (according to Sunpower) their panels have a lower rate of failure and don’t lose as much production power each year as their competitors.
The system will be grid-tied to PNM, and the arrangement with the utility is that all excess generated power is banked indefinitely, meaning if we got an electric car in a few years we could draw on anything built up now. All feedback much appreciated!!!
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Replies
User 69etc.,
First of all, can you tell us your name?
I don't think it's possible to determine your energy use from the information you've given us. Two families in identical homes can have widely varying electric bills, because occupant behavior matters.
Perhaps the biggest unknown -- at least to us, but probably not to you -- is what type of equipment is in the workshop, and how energy-intensive your work is.
If you actually intend to drive an electric vehicle at some point in the next several years even a 4.3 kw array probably would not cover the total use. If PNM really lets you carry the balance forward year on year forever a bigger array is probably the better bet.
A typical battery-EV driving 15,000 miles/year would use about 4500-5500 kwh, essentially ALL of the output of the 2.8kw array. In fewer than 10 years most new cars are likely to come with a plug, even if many/most are still hybrids. (There are good deals to be had on used plug-in hybrids even now.) So if you see an EV in your future, rather than splitting the difference in size between the two, boosting the array size to 6-8kw might be in order.
You can probably figure out roughly how many miles you expect drive in a year. Figure on roughly 3-3.5 kwh for every 100 miles of battery-only driving, and figure you'll probably do at least half (and maybe all) your charging at home.
I searched for PNM and looks like you live in New Mexico?
As Martin said, it's tough to estimate your electric usage because of occupant behavior and the woodshop details. One thought...if you're planning on solar PV, have you considered transitioning more of the house systems to electric instead of gas?
We built a house in 2013/2014, added solar PV in 2015, banked a ton of kWh, and started to use it up in 2017 with our Nissan Leaf. If you are considering an electric car(s) and PNM lets you bank kWhs, I would go with the larger system and (as Dana said) consider taking advantage of your roof space for an even larger system.
Not sure if this makes sense with your remodel details, but you could also consider prepping the house for solar PV during the remodel and then add the system once you have 6-12 months of electric use. You could talk to the solar provider for details, but for prep: make sure roof is all set to accept solar hardware, run conduit, carve out space for the inverter, etc.
First off, is the roof area being used have any shading? If so, then you really need the micro inverters. Sunpower panels are made in California, Mexico, Europe and Malasia but are very strong and efficient. LG mostly comes from China but the SMA inverter is top notch. Micro inverters aren't long term tested as they are only about 10 years old but do have good warranties and it is nice to have less wiring with the AC coming directly off the roof (as opposed to DC).
With a wood shop being used all week, you will want more power so go with the maximum number of panels you can fit. You won't regret it. Most electric dryers are minimally 5KW/hrs so will eat up a lot of electricity depending on usage. Any gas is horrible. One flight over N Dakota or northern Canada will show you what's it's done to the environment. So destructive.
After 30 years with solar, I'd vote for the Sunpower set up.
PK