Whole House Planning

[dishmael]

We are at the very early stages of building a farmhouse on 20 acres in central Florida. There's grid access but we are hoping to only use grid power when necessary, relying solely on solar and batteries as much as possible. We will be utilizing a ground mount system since we have the space. We have a few initial estimates noted below for power needs. Based on my initial reading and the size of our system, I believe we are looking at using 48v for the system. Looking for any initial comments and suggestions on what we should be thinking about during this planning period. All help is greatly appreciated.

Goal: Off-Grid Whole House Power using PV and Batteries (if I can afford it)
Kilowatt Hours per Month: 2,795 kWh
Peak Load: 31,424 watts
Sun Hours: 4.58 hours/day
System Size: 24.41 kW (or should we over engineer with more solar?)

Given the cost of implementing the system to provide whole house coverage, I'm hoping to do this in stages.

Stage 1 (Grid Tied)
I'd like to implement a system that uses solar during the day and grid power at night. Is that possible? Are there inverters that will switch between PV and grid when the PV system is not generating enough power? This isn't an ideal end state as it does not protect against grid outages. I can always add a generator, which is likely cheaper than batteries - but I'm thinking that's more stage 2 expansion.

Stage 2 (Grid Tied with Battery Backup or Generator)
I'd like to augment the system with batteries to provide power at night and during cloudy weather. The system will continue to have access to the grid to prevent brownout from lack of energy, switching over to grid as necessary. Is this possible? Another option that may be cheaper is a generator. While I would prefer not to draw from grid at all, I do recognize the high cost of batteries. Perhaps we can use a generator as an in-between option.

 

[DaGoose13]

I would spend a lot of time trying to get energy consumption down to the minimum that you can.

For example, add in extra insulation to walls, roof, under slab, etc as it's cheaper to do during construction and will have a greater payoff over the long haul. If you spend any $5000 extra for insulation, will that allow you to go with a smaller HVAC unit which might pay for itself in a few years.

You could also look at solar water heaters to reduce electricity usage.

Spend money on good windows as they may be a major drain for cooling/heating or look into overhangs for the windows, so direct sunlight doesn't enter the house.

 

[jimbob420]

That's a whole lot of energy!

Quick rough calculations - 2800kWh over 30 days is about 93/94kWh a day.

94kWh over say 4 hours a day minimum is 23.5kWh worth of panels.

But.. in Florida they will run hot, and you might not get the maximum output all the time for a number of other reasons too, so it would be wise to build in some extra capacity.

The more panels you have, the bigger battery you will need to be able to keep the possible charge rate below 1C, but ideally more like 0.5C, others will weigh in on this for sure. How many days would you want/need to last without any sun? 3 days means around a 100kWh battery, if you use the middle 70-75% of the capacity (not the top 10% and not the bottom 15-20%).

You might find there's much more milage in reducing your consumption, if you can, as this will be cheaper than adding extra capacity by far.

 

[dishmael]

@DaGoose13 - we are absolutely looking to implement an energy efficient building. Block construction. Spray foam. Double pane low-e windows. And so on. Doing as much as we can on the efficiency, now we need to determine the right size and components we need for the usage.

@jimbob420 - setting aside the batteries for a moment, I am thinking we're going to want around 24kWh for PV. the good news is that we have plenty of space for lots of PV panels. I'm lost on what inverter I should be looking at to ensure sustained power from the PVs to the house during the day and then using grid power at night. that's really our first step; PV during the day, grid for night. the next step would be to use enough battery power to get us through the night. then we can switch to grid if we absolutely need during long periods without sun. an extensible system is the goal; build what I can now and then add to it over time. my concern is not buying something I have to replace later.

 

[RCinFLA]

You're not too far off reality however the peak load power is really excessive. You are going to need a lot of batteries which you did not include in your inventory. I would plan on two days purely running of batteries. At 90 kWH/day at 90% efficiency that requires over 4200 AH's of useable capacity from 48v batteries for two day reserve capacity. Just the batteries is in $50k range. You can trade battery capacity reserve for grid power if not knocked out by a storm. Central Florida does not have as high a hurricane risk. In high storm risk area PV panels can be a liability. Summer rainy season can really put a hurt on average solar production in Fla.

You need to do peak load management. First is not to allow air conditioner, hot water heater, range, and water pump to run simultaneously. Super easy to knock off water heater whenever A/C is running. Little more inconvience on A/C vs. range. Water pump if you have one can be helped by larger bladder tank then managed like hot water heater.

You would be well served to put in direct solar hot water heater with 200-500 gallon reserve tank depending on number of people in house. That is usually the second largest electric consumer after A/C. Good thing about A/C power consumption is it is usually proportional to solar illumination level.

 

[DaGoose13]

@DaGoose13 - we are absolutely looking to implement an energy efficient building. Block construction. Spray foam. Double pane low-e windows. And so on. Doing as much as we can on the efficiency, now we need to determine the right size and components we need for the usage.

@jimbob420 - setting aside the batteries for a moment, I am thinking we're going to want around 24kWh for PV. the good news is that we have plenty of space for lots of PV panels. I'm lost on what inverter I should be looking at to ensure sustained power from the PVs to the house during the day and then using grid power at night. that's really our first step; PV during the day, grid for night. the next step would be to use enough battery power to get us through the night. then we can switch to grid if we absolutely need during long periods without sun. an extensible system is the goal; build what I can now and then add to it over time. my concern is not buying something I have to replace later.

I would look at Insulated Concrete Forms (ICF) and add in extra insulation to the outside. Also, look into Structural Insulated Panels (SIP) for the roof.

The power consumption you are shooting for is about the same as my house and it's not energy efficient at all (older stick built house), so I would think a newer construction that went towards the super insulated model would have a much reduced energy load.

Think the type of inverter you are wanting is a hybrid one (Sol-Ark and Enphase are the two that are jumping into my mind right now).

 

[pvdude]

Another inverter to look at is the Schneider XW Pro.

XW Pro 120/240V

The Schneider Electric XW Pro 6848 solar inverter provides flexible design with optional generator integration for home and small businesses.

solar.schneider-electric.com

The inverter will take some power from the grid when the PV output is reduced by clouds, or night.
It is smart enough to try to source from the PV first.
The loads are powered by whatever input (grid, pv, batteries) the inverter can use, dynamically.

 

[jimbob420]

an extensible system is the goal; build what I can now and then add to it over time. my concern is not buying something I have to replace later.

This was one of my goals also - I started with one inverter, one charge controller and went from there.

I'm not saying they're the best out there, but I used the Deye 2kw grid tie inverters - they run off solar or batteries and output "2kw" each, tho I limit them to 35A or about 1750w. But by adding more inverters, so long as the supply is big enough, it's easy to add extra capacity, and has the added advantage that if one unit fails then you don't lose all your generation. 2kw is relatively small compared to your overall demand of 32kw, so you'd need a lot of these. It would obviously more economical to investigate a larger inverter in your situation.

 

[dishmael]

@RCinFLA - we've been thinking about having dedicated solar/battery for the water pump. and we considered splitting up critical versus non-critical loads so we can focus backup on the critical components. I will float that by the builder to see what we can do.

@DaGoose13 - thanks, I'll see what the builder can provide to ramp up on efficiency. the farmhouse is 3600 sqft and has dual a/c units. that's undoubtedly causing most of the high electrical needs. I also work from home in the tech sector, so I have a heavy electrical demand for my job. I can drive towards maximum efficiency, but at the end of the day, I am only going to be able to reduce my consumption by so much. I am also envisioning adding more demand in the future as we continue to use more of the land for a pool and other creature comforts.

@pvdude - good info, I checked out the link for Schneider Electric. That sounds like what I want. An inverter that can use multiple sources and smart enough to switch between them. It looks like the XP Pro has a continuous output of 6.8 kW and allows for stacking up to 4 units for a maximum continuous output of 27.2 kW. That seems like it would work, but I would need 4 units. I'm wondering if it makes sense to look for a larger inverter. I did a google search and there are 30 kW inverters on the market. would that work if I have a PV array that only generates 22-26 kW? I'm sure larger inverters are more expensive but I would need fewer inverters and would not need to tie a bunch of units together.

@jimbob420 - thanks for the info about Deye. It looks like Deye has a 10.5 kW inverter (SUN-9/10/10.5K-G | 9-10.5KW | Single Phase | 2 MPPT). That might be a better option if I can string two or three of them together. I'm a noob so trying to wrap my head around what that entails. I'm thinking I should try to find a 30 kW inverter so I only need a single unit. not sure if they make good inverters that size, if a smaller PV array can power that inverter, or how much they cost (can I even afford it).

I did find a few large inverters, but I have no idea if these brands are good or if a smaller PV array can power a larger inverter...

• 33kW SMA Sunny Tripower Core1 Inverter
• 30kW Solis Solar Inverter Three Phase 4 MPPT 480VAC
• 30KW PURE SINE POWER INVERTER CHARGER 300 VDC 240 VAC SPLIT PHASE

 

[pvdude]

Much simpler and probably less $$$ to go with one giant inverter, vs stacking 4 of the XW Pro's.
I went the opposite direction, reduced/managed the loads so as to work on the single inverter.
Less wall space too, here is how much room our single inverter & charge controller takes :

 

[DaGoose13]

@DaGoose13 - thanks, I'll see what the builder can provide to ramp up on efficiency. the farmhouse is 3600 sqft and has dual a/c units. that's undoubtedly causing most of the high electrical needs. I also work from home in the tech sector, so I have a heavy electrical demand for my job. I can drive towards maximum efficiency, but at the end of the day, I am only going to be able to reduce my consumption by so much. I am also envisioning adding more demand in the future as we continue to use more of the land for a pool and other creature comforts.

Your planned size is just a little larger than ours and we have two HVAC units (5 ton and 1.5 ton) with a huge amount of east facing windows.

I would hope they could get your energy needs down from ours as we may have R-13 batts in the walls at best.

Are you running a lab at home or just multiple workstations/monitors as I'm doing the same, but don't have a lab set up.

 

[dishmael]

@pvdude - thanks for the input. if there's a way to power portions of the house or just offset grid consumption and build a modest system at the start that I can expand on later, that's definitely an ideal situation. That's why I was thinking about this in stages. build something small that gets me started, but allows me to add batteries later, and increase the PV size so that eventually I can be off grid or only use grid when I absolutely need to do so.

@DaGoose13 - I do have a lab, mostly low watt raspberries and a few odds and ends. my home office is on all the time. the network equipment is beefy; I run mostly Ubiquity equipment (24 port PoE switch, a couple of APs, dream machine pro, cameras). my biggest vice is tv and movies, so that's almost always on. I've uploaded a breakdown of various electrical systems - it's a guesstimate at the moment. I did not include future things like a pool. my thought is to build a system that I can expand on over time.

 

[Skipr]

I am research a similar sized system, but starting with my office/shop. I installed a Sense Monitor to track usage to size the system. The system peaks around 4kW, so it would seem like a 5kW system would handle it for most of the load. Once the shop is completed, I will size the house which is significantly larger.

For the larger system, I am seriously considering using the new F150 lightening as the battery system (155kW). The night load will not be that high and should be able to recharge during the day. The F150 is actually cheaper than buying the batteries and you can trade it in for a new one after a couple of years!

However, I am doing the shop/office first. The current plan is to use either the MPP or Growatt as the inverter charge controller to support the 240 split phase needed for the AC.

The cost of the Sense system is nothing compared to the information it gives you. IMO

 

[Ralphloren]

.