Home solar system: calculating usage for Prius Prime

I'm glad that you consider solar. I also have a 6.4kkW solar system for many years but it's a lease. I only put $2k down and pays 5.7c per kw produced. My local utility company (NJ) charge around $0.20 per kW so I broke even in just around 2 years. Since it is a lease, there is no maintenance cost for me. The inverter did broke once and they have to replace it. My system always over-produce 5 ~ 10% more than the designed 6.4kkW output. My net meter used to be in the -ve zone for a long time before I bought my Prime, I always get some credit pay back from the electric company.

But after I bought 2 Prime, I need to pay extra to the electric company, I wish I have a bigger system initially but it was restricted on my previous 12 month usage when it was submitted for approval.

Is there a lease option? The payback period really depends on how much your electricity co. charged. NJ electricity cost is always above national average so it make sense to me.

Is your prior year usage including the Prime charging? If not, you might better wait for few more months to ensure your prior 12 month consumption includes 100% of your Prime charging. Otherwise, you will end up like me, under estimated the consumption.

It's always greener to use electricity to power your car, even the cost is higher than gasoline, right?

 

You only used 6.3 kWh per day on average last year? If you were to fully charge your Prime daily, your usage would double. Given your average week is 100 miles, if it can be done all electric, you would charge the equivalent of 4 times, not 7.

That is pretty conservative. I think 4.5 miles/kWh and better is common. But it depends on where you live (temperature) and how many of your miles are uphill and/or high speed.

Our usage is around 20 kWh per day year round (stupid pool pump in the summer). The 6.6 KW installation has put out 8 MWH the last 2 years (installed Dec 2015). It generates about 700 kWh more than we use.

My 2 arrays are mounted on posts so I can adjust them seasonally. The company I didn't go with wanted the same amount of money for an installation for the same 6.6 KWs but ground rack mounted and only 7 MWH per year. Pretty much only have to use the credits during Nov, Dec and Jan. Credits expire 1 year after they are earned. The electric company here pays retail for whatever we put into the grid.

 

My ideas:

(1) Buy the Prime now, put off the solar decision for a year until you have a better idea of your vehicle demand (do sub-meter the car charger); or

(2) Install the solar in stages, enough for the house now, with the expectation of adding more capacity for the car later. Make sure the first stage is sized and configured for minimal-fuss expansion to somewhere in the range of 4000 to 5000 kWh/year.

As a retired electrical engineer, I designed and installed my own home PV system. It went up in three stages: a starter system six years ago, then a second stage that autumn to fill out that branch circuit to capacity. Then four years ago, I put in a third stage as a not-quite-filled-up second branch circuit, while also placing the wiring (but not racks) for a future third branch circuit. All three branches combine in a subpanel next to the PV production meter.

This 7kW system currently produces about 6,000 kWh/year in my Seattle-region climate zone. After serious conservation efforts, this is now more than enough for my all-electric house. Before conservation efforts over a decade ago, it burned 10,600 - 11,000 kWh/year. But more efficient appliances, heat pump space and water heat, building envelope insulation and air infiltration improvements, and better use of winter solar heat gain, brought that down to just 5,000 kWh/year. As we get lazy, that has since crept up a bit, but is still Net-Zero with some margin left.

When I get a plug-in car, likely a Prime, I'll simply (if changing electrical codes and permitting can be simple ) snap in the two last PV modules and microinverters (cabling and rack already in place) for Branch 2 and have enough capacity for over 4000 EV miles per year, likely plenty for our household. Most of my PHEV miles will be beyond home-plug-in range, demanding gasoline and some on-the-road charging. The third PV branch won't be needed until a plug-in larger than a Prime, such as a Tesla, is acquired. Or until a larger family moves into this house.

My DIY system is made up of Enphase microinverters and SolarWorld PV modules. All components have been operating between 4 and 6 years, with no hardware failures whatsoever. Just a couple pilot errors , flipping off the wrong breakers when depowering half the house for trips away.

I didn't get caught up in payback-time considerations. As an electrical engineer, this had been on my wish list for two decades (a single 12V PV module for emergency lighting was installed in 1994). Had I been able to do this at modern prices when we bought the house thirty years ago, it would have long since fully paid off. AGW and national security considerations add value too, putting my money where my mouth is, on top of mere home utility cost offsets.

Therefore, while replacing the roofing, I just went ahead and did it. The racks were going up on the south side before the roofers were finished on the north.

 

Here is my data and hope you can find it useful. Before I start, I would let you know that for Prius Prime it needs one kWh to drive 4-5 miles.

My annual usage is about ~7500kWh, before I bought the Prime. I charge Prime daily, and it uses 5-6kWh per day, annual EV 1usage is around 2000kWh. So I was expecting 10000kWh total annual usage.

In December/January/Februrary, the system generates about 500kWh per month due to rain, but in April through September, it generates 1000-1400kWh per month. If it is sunny, the system generates 25kWh in January to 48kWh in May-July. But it could be as low as a few kWh/day if it is rainy.

My system is 6.5kW with LG panel, costs $19825, and after tax credit, total cost is $13877.

It is supposed to generate 10300kWh per year. I have it installed mid May last year, and it is trending to generate 12000kWh/yr. So if I buy a Tesla, I could get even without using too much from PG&E.

As PG&E charges me 22c/kWh for first 350kWh, my annual cost is more than $2300 without solar system. So I can expect to get my investment back in 6 years even if PG&E won't increase their price, which is impossible given their history and current situation.

I am in Sacramento which is hot in summer to make the system less efficient.

Look for $3/w for solar system before tax credit and $2.1/w after. Many installers still charges much more than that.

 

Hmmm, Tesla is very greedy in our area. They want to charge 21c/kWh because our electric company, the famous PG&E charges 22c/kWh for first 350kWh, then 29c for next 350kWh, then >40 cents for rest.

 

With as little electricity as you are using currently for whole house, it would be crazy not to try to cover entire electricity needs in your situation. The question is do you have big enough roof without shade? PRIME takes ~6.3-6.6 kWh to fully charge. Your 100 miles a week commute means ~20 miles a day for 5 days per week, should be covered by your daily charge. With tax credit paying 30% and you may even have some state incentives, I would go for it.

That said, for saving money, buying a new car and solar system is going to add up quite bit of up front cost to you. You may not see a payback immediately or for a long time. How much electric (for house) and gas (for car) are you currently paying?