cheap/simple device to measure actual power used by HVAC- building solar just to run AC

[hertfordnc]

I'm recently retired and thinking about solar for my home. I don't want to partner with Dominion power. I just want to run my heat pump. This solves two problems. In North Carolina we tend to have bad hurricanes in August. Isabel in 2003 we lost power for 8 fairly hot days. We were lucky it was a mild September.

So, I want to measure the actual KWH used by the heatpump alone during AC season. I'm not concerned about heat in the winter.

I don't want to augment the grid power. I want to close a transfer switch in the spring and get all my cooling from the sun until Labor Day. Or, i want to measure the actual usage and calculate what it would take.

thanks

 

[Hedges]

You can build a system to start and run the compressor with PV, but that is more involved than grid-tie. Starting surge is typically 5x operating current. See if an inexpensive AIO hybrid has ratings to do it. You'll need a battery. Some inverters have a battery charger so they can draw extra power from grid as needed, while never backfeeding.

Consider a zero-export inverter, which adds AC power to the wires of your house but uses current transformer to avoid export.
Consider a simple grid-tie inverter connected after switch for heatpump, sized to be slightly less than consumption.
Either of those probably is supposed to involve notification to your utility company, but if proper UL-1741 equipment and never exporting, don't think they'd ever know.
Older model GT PV inverters might not have zero-export but could be used after the load relay. Have PV strings with two orientations so production during the day is more flat.

 

[hertfordnc]

You can build a system to start and run the compressor with PV, but that is more involved than grid-tie. Starting surge is typically 5x operating current. See if an inexpensive AIO hybrid has ratings to do it. You'll need a battery. Some inverters have a battery charger so they can draw extra power from grid as needed, while never backfeeding.

Consider a zero-export inverter, which adds AC power to the wires of your house but uses current transformer to avoid export.
Consider a simple grid-tie inverter connected after switch for heatpump, sized to be slightly less than consumption.
Either of those probably is supposed to involve notification to your utility company, but if proper UL-1741 equipment and never exporting, don't think they'd ever know.
Older model GT PV inverters might not have zero-export but could be used after the load relay. Have PV strings with two orientations so production during the day is more flat.

All of that is good but I want to completely sever the grid-tie for the HVAC. And i want to start by figuring out how much power needs to be replaced.

Part of my rationale is that i want this to keep me cool for a week without power in august. And the other part is that as this stuff gets cheaper and cheaper utility companies will bribe elected officials for more power and control and the consumer will get screwed.

So i wan't my entire HVAC to be just like a solar patio tiki light.

I have 12 SEER 1.5 ton heat pump (small for the house but the previous owner insulated the heck out of it) Once I figure out the load for the season i can go from there. I don't care about the startup load either. If there is not a good solution (big battery/big inverter/capacitor) for the surge then I'll eventually swap out the unit for something inverter based.

 

[BentleyJ]

Simple inexpensive energy monitor Emporia Vue 2 has seconds, minutes, hours, days, weeks and month data logging of Watts and Amps. You will need this data to determine the battery capacity needed to run overnight.

Start up surge on the compressor can be cut by up to 70% with a soft starter. Mico-Air or HyperEngineering are two brands I know of.

 

[Hedges]

Read label on AC unit or compressor motor.
If it has "LRA", that is the surge power your inverter needs to supply for a couple seconds. Full-load it has to supply continuously.
If LRA not quoted, assume 5x FLA. A soft-starter can reduce the surge required.

My system uses Sunny Island and has oversize PV array, runs A/C while keeping batteries floating.
Schneider has low-frequency inverters that would work too.
Outback used to do low-frequency, now has high-frequency.
For a non-grid-interactive system, check out the new Midnight Rosie the Inverter. High frequency but lots of surge power.
There are inexpensive Chinese AIO inverters, and Midnight DIY line is one of those rebranded.
SolArk is a large high frequency AIO, pretty popular.

Some of the mini-splits are inverter based. Lots of models in the US may have poor power factor (more stressful current waveform drawn from your battery inverter), but some models, especially sold for Europe, are PF corrected and may work better. Not sure poor PF will be a problem for you but it is a problem for some of my SMA equipment.

 

[Quattrohead]

Simple inexpensive energy monitor Emporia Vue 2 has seconds, minutes, hours, days, weeks and month data logging of Watts and Amps. You will need this data to determine the battery capacity needed to run overnight.

Start up surge on the compressor can be cut by up to 70% with a soft starter. Mico-Air or HyperEngineering are two brands I know of.

Everything here is perfect. You might not need the soft start, try without.
Emporia is great, everyone should install one.

 

[Shimmy]

All of that is good but I want to completely sever the grid-tie for the HVAC. And i want to start by figuring out how much power needs to be replaced.

I think the words of caution are that the economics for a off-grid system supporting HVAC only aren't great. You likely need a unit that has about 6.5kVA surge capacity minimum, and a random guess is about 15kWh/day of battery and 4kW of PV. Spending a little extra on each component-- maybe $3,000 total (~25%) would likely give you whole-home backup.

 

[hertfordnc]

I think the words of caution are that the economics for a off-grid system supporting HVAC only aren't great. You likely need a unit that has about 6.5kVA surge capacity minimum, and a random guess is about 15kWh/day of battery and 4kW of PV. Spending a little extra on each component-- maybe $3,000 total (~25%) would likely give you whole-home backup.

Surge is pretty much eliminated with inverter heat pumps.

You're right, building it for the HVAC is pretty much building it for the whole house but I am determined to NOT have a relationship with the power company.

So, we start with the HVAC, and once that is off grid maybe we add other big draws, laundry, water heater, etc. but everything is stand alone and the utility only powers stuff that is not connected to solar. Maybe it's a misguided idea, for now i just want to understand the math

 

[Crowz]

Simple inexpensive energy monitor Emporia Vue 2 has seconds, minutes, hours, days, weeks and month data logging of Watts and Amps. You will need this data to determine the battery capacity needed to run overnight.

Start up surge on the compressor can be cut by up to 70% with a soft starter. Mico-Air or HyperEngineering are two brands I know of.

HIGHLY recommend Emporia. I can see every watt used in my home. I monitor my ac units and everything else. Ive used it to decide which circuits to attach to which inverters. Couldn't imagine doing my solar setup without it or something similar (had a different brand before which worked well till it died).

You can plan the loads out on paper from specs but real world usage data is so much more accurate when tuning stuff.

 

[Crowz]

Surge is pretty much eliminated with inverter heat pumps.

You're right, building it for the HVAC is pretty much building it for the whole house but I am determined to NOT have a relationship with the power company.

So, we start with the HVAC, and once that is off grid maybe we add other big draws, laundry, water heater, etc. but everything is stand alone and the utility only powers stuff that is not connected to solar. Maybe it's a misguided idea, for now i just want to understand the math

One thing to keep in mind which the Emporia is a god send for is that what uses the most power in 24 hours isnt going to be what you think it will be.

I figured my ac units and hot water heater etc would be the kill draw. Nope.
My computer room/office and another set of center house outlets account for 85% for my power usage on a normal day.

The ac, hot water heater, stove etc is in that other 15%.

This is cooling a 4000 square foot home too.

Basically its what runs all the time that makes the biggest difference in total kw used in a day. Drove me nuts to start with since I was all over putting the heat pumps and such on solar first and then did an about face half way thru the build.

If I was start over or recommend an approach to someone Id tell them to get an Emporia monitor or something similar and run it for a month or two before buying anything solar wise.

Its the only way to approach the project with enough information to know what you really need to buy in my opinion.

Zeroing in on the data will even let you spot surge loads on ac startup.

 

[Zapper77]

So, I want to measure the actual KWH used by the heatpump alone during AC season. I'm not concerned about heat in the winter.
thanks

I use IotaWatt. Web GUI, standalone, upload to a host site or lock down to intranet only. Lots of options for graphing usage.

 

[sunshine_eggo]

Surge is pretty much eliminated with inverter heat pumps.

You're right, building it for the HVAC is pretty much building it for the whole house but I am determined to NOT have a relationship with the power company.

So, we start with the HVAC, and once that is off grid maybe we add other big draws, laundry, water heater, etc. but everything is stand alone and the utility only powers stuff that is not connected to solar. Maybe it's a misguided idea, for now i just want to understand the math

The aforementioned Emporia Vue 2 monitoring system will give you detailed power and energy usage on 16 circuits. This would be a key first step in your efforts and be a source of continual new information for refinement over the seasons.

 

[hertfordnc]

I think the words of caution are that the economics for a off-grid system supporting HVAC only aren't great.

Yes, but if you add the cost of an emergency generator to the equation it might make sense. I could DIY a whole house generator for aout $2K plus some propane work and It would sit there and rust between hurricanes. So if i spend $10K to make the HVAC stand alone it solves the blackout problem and pays for itself eventually.

Lot's of good stuff in the other comments.

Yes, the Heat pump is not the biggest draw over time but it is the biggest single load. Looking at my power bills the difference between August and October is about 25% - October in NC is usually a free month. Heat and AC don't come on at all. But then we travel a lot in the summer so the AC is often left at a temperature that will not kill the cat. And then we have the pool pump running almost constantly (i apparently suck at pool maintenance)

 

[Robbert]

Assuming that your AC runs 220v, you could order a 220v smartplug (rated with enough watts) and install that behind the ac. After a couple of weeks, you will get a good insight in your usage via an app.

A Shelly plug (rated 3.500 watts) is around 40 dollars.

 

[Crowz]

Yes, but if you add the cost of an emergency generator to the equation it might make sense. I could DIY a whole house generator for aout $2K plus some propane work and It would sit there and rust between hurricanes. So if i spend $10K to make the HVAC stand alone it solves the blackout problem and pays for itself eventually.

Lot's of good stuff in the other comments.

Yes, the Heat pump is not the biggest draw over time but it is the biggest single load. Looking at my power bills the difference between August and October is about 25% - October in NC is usually a free month. Heat and AC don't come on at all. But then we travel a lot in the summer so the AC is often left at a temperature that will not kill the cat. And then we have the pool pump running almost constantly (i apparently suck at pool maintenance)

From my experience the pool pump is probably the worst draw if its running all the time

That thing ate us out of house and home power wise on our pool. Well that and the hot tub.

 

[sunshine_eggo]

From my experience the pool pump is probably the worst draw if its running all the time

That thing ate us out of house and home power wise on our pool. Well that and the hot tub.

+1. A 1hp pool pump 24/7 is going to burn almost as much as the average U.S. household.

 

[hertfordnc]

I'm going to rephrase my question, Ya'll know what I'm trying to accomplish, I figure if it works in August it will work the rest of the year.

So, let's say we have a 3 ton mini split, 23 SEER that pulls 7 amps so it needs 1800 watts 16 hours a day and the sun is out for 14 of those hours. It's running full blast 80% of the time from 10am to 9PM and maybe 50% for the remaining hours. and the panels are at the optimal angle for that latitude in August.

What do i need for solar wattage and battery capacity to meet that load?

 

[zanydroid]

I'm going to rephrase my question, Ya'll know what I'm trying to accomplish, I figure if it works in August it will work the rest of the year.

So, let's say we have a 3 ton mini split, 23 SEER that pulls 7 amps so it needs 1800 watts 16 hours a day and the sun is out for 14 of those hours. It's running full blast 80% of the time from 10am to 9PM and maybe 50% for the remaining hours. and the panels are at the optimal angle for that latitude in August.

What do i need for solar wattage and battery capacity to meet that load?

probably still easier to buy a Vue2 than to learn how to do math from first principles. I used one on my new inverter drive heat pump over the past month and I’m very confident of how much it draws in different conditions.

Also 50% vs 80% means different things depending on cycle duration & number of compressor speeds. If you have variable compressor like an inverter you can get away with a smaller inverter with lower surge.

 

[zanydroid]

Above post is about getting the watt and watt hour demand accurately.

From there you can do the other calculations.

You want to avoid a garbage in garbage out disaster.

 

[Robbert]

I'm going to rephrase my question, Ya'll know what I'm trying to accomplish, I figure if it works in August it will work the rest of the year.

So, let's say we have a 3 ton mini split, 23 SEER that pulls 7 amps so it needs 1800 watts 16 hours a day and the sun is out for 14 of those hours. It's running full blast 80% of the time from 10am to 9PM and maybe 50% for the remaining hours. and the panels are at the optimal angle for that latitude in August.

What do i need for solar wattage and battery capacity to meet that load?

To give you an idea,
I have one mpp solar inverter with 10 panels, that is dedicated to run 2 mini splits and a poolpump. Mini splits are 24x7 and the poolpump 8 hours a day. 10 panels (400W per panel) are just enough to handle that load incl. charging.