Instal solar/battery system during house construction?

System · September 2011

Looking like I may get the chance to finally build a new house on our land. Have been dreaming of going off grid and reading about solar power for five years. Found this place this week and have been reading alot. After reading here i am starting to reconsider the possibilityof going off grid due to system cost and battery bank life. So the big question is should I install a solar/battery system during construction?

Yes I know this question is kinda like asking you guys for winning lottery numbers, but I need to have some hard facts and a warm fuzzy if I am going to make this type of investment.

The new house will be approx. 2500 square feet, built with ICF type construction, have a gas stove, gas furnace and water heater, 2.5 ton airconditioner with a shallow well.

I know I have no idea of the electrical loads, and this would be the first bit of information I would need to plan a system, but I was hoping someone could give me a few tips to help me determine if all of this is even feasable. Thanks for all the information I have been able to learn here already. I hope this question is not too far out there.

Cariboocoot · September 2011

Re: Instal solar/battery system during house construction?

Welcome to the forum.

I can tell you last week's lottery numbers; no problem! Next week's?

As for putting in solar electric ... if you have grid, use it. Chances are very good it will be cheaper by far. If you can go grid-tie you might want to; it all depends on what sort of incentives are available where you are to help off-set the cost.

On the whole, conservation will return more for your investment than any type of solar. That probably sounds weird for a solar forum to say, but it's true. Around here we don't like unrealistic hype because it leads to unhappy solar users and that's a blemish on the whole industry.

No matter what you decide to do, get a Kill-A-Watt meter (about $30) and start measuring the power usage of everything you've got. Finding what the power hogs are will help you decide what to change.

It always amazes me the number of people who are willing to spend $20,000 on solar electric but balk at the idea of replacing their 'frige or air conditioner - which would cost less and save more. :roll:

niel · September 2011

Re: Instal solar/battery system during house construction?

welcome to the forum,
often times it'll boil down to costs as to what is best to do. if your utility wants $30,000 for instance to install poles to run electric to you before you even get to use the electric they sell to you it may be better to go off grid. now if it is a matter of just tapping a pole that is already nearby then you may want the option of the utility as you can then have a grid tied solar system offset or eliminate much of your usage depending on how much $ you wish to sink into solar. there will be areas of gray that it's hard to decide too. talk to the neighbors about the utility reliability and service record as this could also influence your decision. don't forget that when a failure occurs in an off grid arrangement, it is totally up to you to fix the problem and that could entail some large expenses at inopportune times with long wait times for equipment to either be fixed or replaced making you powerless for long times.

BB. · September 2011

Re: Instal solar/battery system during house construction?

A/C is the wild card here...

Just to give you some ideas, 100 kWH per month (3.3 kWH per day) is a good aim point for a reasonably priced/easy to maintain off-grid system... If you are reasonably conservation minded, you could run an entire home (using propane/solar thermal for everything you can) on 200-300 kWH per month.

Using PV Watts, we can rough out your system size. Say Montgomery Alabama is similar in weather/sun to your region in Georgia... Pick 1 kW (1,000 watts) of solar panels, 0.52 end to end system efficiency, fixed mount, and see what we get:

"Station Identification"
"City:","Montgomery"
"State:","Alabama"
"Lat (deg N):", 32.30
"Long (deg W):", 86.40
"Elev (m): ", 62
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 32.3"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.6 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.76, 57, 4.33
2, 4.49, 62, 4.71
3, 5.29, 77, 5.85
4, 5.90, 82, 6.23
5, 5.86, 81, 6.16
6, 5.63, 74, 5.62
7, 5.60, 76, 5.78
8, 5.73, 78, 5.93
9, 5.59, 75, 5.70
10, 5.24, 76, 5.78
11, 4.27, 61, 4.64
12, 3.84, 58, 4.41
"Year", 5.10, 858, 65.21

Plan for running 9 months of the year on solar only, 100 kWH per month. February at 62 kWH per 1,000 watts of solar array for 28 days:

62 kWH per Feb / 28 days per month per 1kW of panels = 2.2 kWH per 1kW of panels

Say you estimate you will use 300 kWH per month or 10 kWH per day:

10 kWH per day / 2.2 kWH per 1kW of panels = 4.55 kW = 4,550 Watts of solar panels

Battery bank size, assume 1-3 days of "no sun" (use 2 days here), 50% maximum discharge, 48 volt battery bank (large system), 85% efficient inverter, and 10 kWH per day of power:

10,000 WH * 1/48 volt batt * 1/0.85 inverter * 2 days of no sun * 1/0.50 max battery discharge = 980 AH @ 48 volt battery bank

Note, should charge a battery bank at ~5% to 13% rate of charge (minimum charging for battery heath; maximum rate more or less maximum cost effective solar array). Use 10% here (tall flooded cell batteries like 10% rate of charge):

980 AH * 59 volts charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 7,509 Watt Array "optimum charging"

Also, it would be good to size your backup genset to recharge the battery bank... So, using the 5% to ~25% range of charging--again picking 10% rate of charge and a 50% derating of genset to charger output, roughly the "optimum genset" would be:

980 AH * 59 volts * 1/0.80 charger eff * 1/0.50 genset derating = 14,455 Watt Genset

So, you would be looking at a 15 kW genset... Not small, and would consume around 1 GPH of fuel when running at ~50% load (propane more, diesel probably less)...

The above is based on a back of the envelope calculation for a 300 kWH per month (February) sized system...

Air Conditioning--The new (to the US) mini-splits are really worth looking at. They can be very off-grid power system friendly.

Sanyo mini split AC (inverter/variable speed)
Sanyo Mini-Split AC

Note, there are a lot more threads on A/C systems here... You can use Google and the "site:" tag to give you a lot more to read:

mini-split site:wind-sun.com

Everything comes back to your estimates (or actual) load usage... Solar is very expensive--and building out a small system for now (lights, radio, etc.) and using a genset during the day for building/heavy loads may be a good starting point.

In the end, everything you can do to reduce your power usage will (usually) be a much better investment than just building a bigger Solar PV system.

Here is a thread with a lot of different links/projects/sources of information that you should read through too:

Working Thread for Solar Beginner Post/FAQ

-Bill

bmet · September 2011

Re: Instal solar/battery system during house construction?

I don't know the code for cabling new homes where you are, but I wished I'd been around when my folks built their home 10 years ago. It didn't include any network, data, or RF wiring, and I've had to spend quite alot to provide entertaiment for them in all the rooms they've asked. If you are building, don't forget infracture for the digital age, roof access, and attic electricity.

mike95490 · September 2011

Re: Instal solar/battery system during house construction?

I ended up looking at my existing house, planning on energy star appliances, down to the point of the ignition system for the stove and clothes dryer.

Came up with battery bank size, and the the solar array size. Of course more power is always nice, but conserve first.

System2 · September 2011

Re: Instal solar/battery system during house construction?

Wow, thanks for the replys. The land already has power, a well and a septic tank that was installed twenty years ago for a singlewide trailer that used to be there. I thought about using usage number from my existing house for my calculations, however my current house would never be efficient enough for running A/C (15 foot ceilings, and 3000 square feet), also my new house will be built from scratch to be as efficient as possible. I have already throttled back my usage at my current house, and plan on using much less at the new one. My main reason for wanting to go off grid is to be self sufficient, and because i am tired of being robbed by greedy corporations and goverment. This reason is also why I would just rather buy from the grid than go grid tied solar and sell power back at half price, only to buy it for full price when my demand peaks. Add in the fact that our current leaders are saying that electricity will most likely tripple in price in the next five to ten years and if that does not make you want to be self sufficient nothing will. Just want to be clear that I am NOT a conspiricy kinda guy, just one that wants to be responsible for my own needs and not really have to rely on anyone else.

I do have a 6/1 Listeroid mated to an ST-8 that I have almost perfected over the last two years. Max sustained output is 5300-5400 watts turning 748 RPM at a rediculously low fuel burn. I was under the impression that this would be plenty to charge a battery bank. Did not know I would need a 15-20 KW genset.

westbranch · September 2011

Re: Instal solar/battery system during house construction?

I think you have answered your own question.... start with your existing house and figure out how to get down to the 5300W that your gen provides...

have fun, it is worth it in the end.

ej

mike95490 · September 2011

Re: Instal solar/battery system during house construction?

You don't have to get the 15KW genset, there are ways to work around it, Use the genset in the early am to bulk the batteries up, and after the sun is good on the array, you use both sources to perform an EQ cycle if needed.

But if your loads exceed your recharge, you will have trouble....

System2 · September 2011

Re: Instal solar/battery system during house construction?

Thanks Mike, that makes sense. My current genset will carry all loads except the A/C in my current house. With the new house I may just be able to squeak by with everything including the A/C if I use a gas water heater.

techntrek · September 2011

Re: Instal solar/battery system during house construction?

You won't be able to run a 2.5 ton A/C on 5300 watts. If it has a high enough SEER you might be able to run it, but not start it. Take a look at the mini-splits talked about a lot here, they don't have any startup surge. Get a few of them to replace the 2.5 ton unit and then you would be OK. Just don't plan on charging your batteries while you run the A/C.

Instal solar/battery system during house construction?

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