Tag Archives: inverter

Going Solar Part 2

Last Updated 2024-05-02

Before we jump back into solar, I’m going to tell you that wherever you see *aig* on an image caption, it means it is an Artificially Intelligence Generated image. Generally using gemini.google.com. Had to dip my toe in the AI thing to help break up the longish paragraphs…

A typical winter day with a 13.6 kWh FranklinWH AC coupled battery, 18 Rec Alpha 410 Watt solar panels (7.36 kW) and Enphase IQ8 microinverters looks like Chart 1, below. Positive values are USAGE and negative values are EXPORTS except for the yellow (solar) production. Just before 8:00 AM you’ll notice a little cyan (house utilization) peak. This was partially offset by solar production (yellow) and part offset by import from the grid (purple). After 3:30 PM another little cyan spike is partially offset by solar, and part battery discharge. See the legend for more details.

Chart 1: A typical winter day production with a battery system.
(1) Insufficient battery so drawing from the grid (purple).
(2) Using energy while the sun shines (cyan)
(3) Riding out the PEAK and night with battery (green)
(4) Charging battery (green) and exporting to the grid (purple)
(5) Battery charge percentage

System Quirks and “Gotcha’s”

There are a number of things that can go wrong or be wrong with your solar setup. Many revolve around what is observable/measured and what is not.
In particular, if your system is unable to see the big-picture consumption it may not behave as you expect, and you might, for example, get surprises from your utility company when they bill you for much more than you thought you’d be billed.

We’ve already thrown around the terms “kilowatts” (kW) and “kilowatt-hours” (kWh) like a pro, but let’s give them quick definitions. A kilowatt is a thousand watts and is a measure of power. An apt analogy to watts is the pressure in a water hose. A kilowatt-hour is a measure of the AMOUNT of that power used in a given time. It is a measure of energy. In our hose analogy it would be the number of gallons or liters that are used from the hose over a given time. And you might also remember that a watt is equal to 1 amp at 1 volt – but that’s not important here.

Excess Energy for Sale

Your utility may reject YOUR solar energy and may in fact try hard to prevent you from installing your own power production. *aig*

Many people, especially those with solar-only systems assume that the best use of the excess energy is to feed it back to the utility company and get credit for it. But that’s less true every day. Typically there is an incentive for exporting excess but as solar energy becomes more prevalent, there is less incentive for a utility to WANT your energy, and they typically will not want to pay you for it. Some utilities won’t pay you for any energy you export as we described in Going Solar Part 1. Some utilities see your energy generation as competitive and are therefore hostile to having to pay anything for that energy (even though every kWh that you export they don’t have to generate and can sell to your neighbor at a hefty profit). In fact, at “grid scale” (very large) solar farms can produce energy at a cost of about $0.025 per kWh and they’d much rather charge you 33 cents for their 2.5 cent energy! As solar becomes more prevalent, a utility company is much more likely to incent you to send them energy when they are not able to produce it themselves (i.e. when the cheap wind and solar systems are NOT producing).

There are currently four major billing/reimbursement models:

Net Energy Metering – in the early versions of this model there is often a one-to-one correspondence between energy you use and energy you export. But the value of the energy is usually NOT a one-to-one credit, and can vary dramatically.

Net Energy Metering 2.0 – again, the specifics vary from location to location, but generally version 2.0 means you get SOME credit, less “non-bypassable” charges for energy you export. The 2.0 implies that the value of the export is roughly equivalent to the value of energy you import. Some utility companies (with the blessing of state public utility commissions) charge you the full rate for the electricity you consume and only periodically (once annually) credit you for the energy you export. The credit may be dollars or kWh and usually cannot be carried over to the next period.

Net Energy Metering 3.0 – generally this model is similar to 2.0, except it implies that the exported energy is credited at the “wholesale” rate and that rate is typically miniscule. Most 3.0 plans also run on monthly billing cycles, not the protracted cycles of 2.0.

Demand/Response Billing – this is a “wild ride” where the value of energy you consume (import) and the value of the energy you export are driven by current loads, time of day and the market forces. The rate can swing dramatically at 15 minute intervals. And curiously there are some times of day when you can be PAID to consume energy (because there is excess load on the grid and the power needs to go somewhere). This method of billing is not very common for residential customers in the US, but it is “behind the curtain” of all of the utility companies. The California Independent System Operator {CAISO}, for example has fascinating graphs of the costs of energy. I am aware that in the UK Octopus Energy has several plans including a demand/response plan that they call Agile. The US Department of Energy has details on other Demand/Response plans, but I notice the majority of them are only available to large energy-using businesses.

Chart 2: The wild world of wholesale energy pricing. The “LMP” (Local Marginal Prices per Megawatt-hour) can be both positive (e.g. $14 for Kirkwood, and New Spice), and negative as shown here for San Mateo, Curtis and Morgan Hill). I’ve filtered out the higher costs, but at this same time the highest energy costs in this area were $67 for Hollister. The average cost is $10.97 across the region. ($0.011 per kWh!)

Not for-profit utilities will prefer to get their energy at the lowest cost so they can charge a lower cost. There are a few utilities that offer plans to those with batteries that allows them to discharge your battery to the grid to help with load imbalances. These arrangements are referred to as Virtual Power Plants (VPP) and as battery adoption and capacity ramps up, this strategy could save utilities BILLIONS of dollars in cost – because, well, customers have borne the cost of the batteries themselves. OhmConnect was formed to be a VPP of a different sort. I participated aggressively with them until my solar system made it impractical. OhmConnect is a large base of people who agree to reduce energy usage at specific times in order to gain incentives (like cash and prizes). A utility that can coax the batteries of hundreds or thousands of customers into helping them out is a powerful tool. Of course such plans usually come with monetary incentives, too, below is one example.

Enrolled battery systems will be directed to discharge every day from 7 p.m. to 9 p.m. during the months of August through October, a critical window when energy needs are highest in California. In exchange, customers will receive an upfront payment of $750 and a free smart thermostat for participating. 
~ Sunrun announcement

The utility company taking kilowatts out of your battery 🙂 . *aig*

Would I trust my utility company to use my battery and energy wisely? NO! However some installers, a FEW battery manufactures and a FEW utility companies are adopting virtual power plants (VPPs). Tesla, Sunrun, and sonnen are some of the battery manufactures with such arrangements. FranklinWH and Enphase have announced plans, but have not yet rolled out this potentially game changing technology as best I can tell. Feel free to leave a comment if you know otherwise!

_______________________________________________________________________

Electricity is like apples. Your utility company can only count what you send them, not how many you pick. *aig*

How Do Utilities Determine My Net Usage?

Why doesn’t your utility company credit you for all the energy you generate LESS the amount that you use? Unless you have multiple meters the utility company only sees your exported energy AFTER your house consumes what it wants. The analogy I like is this: you pick a basketful of apples from your tree. You use some of those apples to make pies, some to make applesauce and you give the remaining apples to your neighbor. Your neighbor is grateful for the apples, but doesn’t know how many apples you originally picked, only that they got half a basketful. The utility is like your neighbor, only less friendly – and the apples in this analogy are kilowatt-hours of electricity.

By the way this same “net usage” principle is why a partial backup battery solar system may only see SOME of your usage as we mentioned in Quirks, above.

The Kilowatts are Stacked Against You

The ratios may be stacked against you. *aig*

Before assuming that selling electricity back to the utility is your best option, pay attention to the rates. In Northern California and many areas of the country your electricity cost has at least two components: the cost of generation of the energy (free in the case of solar – discounting amortization and maintenance of the equipment), and the cost of distributing that energy over the poles and towers, under the ground, through the transformers and wires that comprise the grid. As explained earlier, some utilities will give a FULL credit for what you export, some will give full credit less “Non-bypassable” charges – including “minimum delivery charges”. Unpopular plans, like NEM (Net Energy Metering) 3.0 in California mean that excess energy is reimbursed at a paltry $0.05 per kilowatt hour. But the minimum cost to import that energy is about $0.35 per kWh. Under NEM 3.0 to break even you have to export 6 to 12 times as much energy as you import. Ah, but it is costly and not allowed by most utilities to generate that much EXTRA energy, and nor is it practical to do so, especially in the winter. It’s also worth noting that if you are connected to the grid there is some cost to maintain that grid and the utility needs to recoup that cost in order to provide you a service.

These factors create a problem just as with roadways that are funded with gasoline taxes. Less gasoline consumption equals less road funding = deteriorating roads for all drivers. Less electric energy consumption from the utility results in less funds for maintenance of the grid. It’s also a bit unfair to those in apartments or with insufficient income to afford their own solar generation – because those folks are “left behind” to pay for those grid maintenance costs.

The one thing I will leave you with in this discussion, however, is that you almost always will get more value out of USING that energy than it sending it back to the utility. Some ways to do that are described in “Energy Hacks”, below.

Battery Or No Battery?

Chart 1 again…

A close look at Chart 1, you’ll notice that the battery system (green graph) is providing all of the energy to run the house after the sun gives out around 3:30 PM in the winter.
Moreover, the battery is being charged from about 8:20 AM until the battery is fully charged at about 1 PM. Thereafter excess energy is then sent back to the grid. You can even tell from the graph that the energy sent back to the grid on this relatively productive winter day is less than the amount of energy imported in the morning (1).
Another take away – the battery didn’t discharge from midnight on because it was set to maintain a minimum 25% charge for emergencies. In this case setting that minimum lower (e.g. 5%) may have avoided ALL of the grid imports on this day!

One obvious takeaway is that the battery is charged “for free” and is used to cover the cost of energy from 3 PM through midnight. Indeed for our home system that energy will generally last until about 2:30 PM the following day. That means for this winter cycle we use ALMOST *NO* energy from the utility at all! One reason energy was imported (other than the 25% reserve) is that the prior day production was low. Notice how the battery level at the end of this day was 60%, but it was only 25% in the morning.

Even if you have a great energy export rate, a well sized battery can prevent almost all import costs!

Some assume that with JUST a battery they can MAKE money by buying it at off-peak rates and selling it back at peak rates (provided that option exists). And yes, that CAN work, but usually won’t unless these things are true:

  1. The rate differential is at least 15% (charging and discharging the battery incurs an average 11 to 14% LOSS). Don’t believe the 6% or 11% figures – those are under IDEAL conditions.
  2. You can actually GET a higher export value for exported energy (or a higher value than the import cost).
  3. You do not have to then also IMPORT energy during the peak period, or a significant amount of off-peak energy.
  4. You are able to export energy while also supplying your home. Remember the apple analogy? If your home load is high, both the battery discharge capacity, and the home load consumption will prevent you from discharging. For example, if your battery discharge rate is 5 kW and you’re running a 4 kW oven, the MOST you’ll be able to discharge is 1 kW.

Let’s do some quick math on the these points to make it clear. Let’s divide the day up between off-peak, generation time (the sun is shining and producing ample energy to supply the home), and peak (non-generation) periods a typical home may use 10 kWh during generation time (all of which is offset by solar production), 7 kWh during PEAK non-generation time, and another 10 kWh during the remaining time (which is usually also off-peak like late night and early morning where there is no generation). Let’s further assume peak rates are $0.50, off peak $0.30. The energy you would need for the non-generation period totals 17 kWh: $3.50 is peak, $3.00 is off peak.

We have these three choices:

(A) Use the energy in self consumption mode only (don’t discharge to the grid unless the battery is fully charged);
(B) FULLY (as much as possible) discharge the battery to the grid during the peak and earn per kWh exported;
(C) Limit exports so that you only discharge as much energy to the grid as is NOT needed to offset off-peak usage.

The last strategy isn’t easy to implement, because we have to guess how much energy will be required through the peak and off peak periods until generation resumes. The best strategy depends on the USABLE battery size and the export rate.

If the export rate is less than the off peak import rate, exporting energy only makes sense after there is no non solar peak or off-peak energy to offset.

Compare Table 1 with a favorable export rate and Table 2 with an unfavorable export rate below. Both are daily comparisons at different battery sizes, utilization and rates. Two obvious takeaways are that a battery CAN save if there is a good export rate (Table 1). The size of the battery relative to non-solar use dictates how much savings are possible. In both cases the maximum savings/earnings are achieved if there is sufficient battery to cover all PEAK and off peak usage. In Table 1, Exporting All possible energy during Peak is clearly the winner, but a Limited Export strategy also works – but no strategy has a positive cashflow unless the battery is > 18 kWh. That’s not to say a smaller battery is of no help, compare the 0 sized battery to the others and all battery options reduce the net cost by about $2 to $5 per day.

Usable Battery SizeSelf ConsumeExport ALL PeakLIMIT Export
0-6.50-6.50-6.50
5-4.30-4.30-4.30
10-2.46-1.92-2.46
2003.360.36
Table 1: Battery Usage Options at $0.60 Export rate;
7 kWh @ 0.50 peak and 10 kWh @$0.30 off peak

If the export rate is unfavorable (e.g. $0.05 in the case of NEM 3.0), then limited export or self consumption are the best choice, – if the battery is sufficiently large. Trying to export energy is a money losing strategy. However as in the prior case ANY battery will reduce the costs!

Usable Battery SizeSelf ConsumeExport AllLimit Export
0 -4.90 -4.90 -4.90
5 -2.70 -2.70 -2.70
10 -1.26 -2.21 -1.26
20 0 -1.77 0.23
Table 2: NEM 3.0 ($0.05 Export rate);
5 kWh @$0.50 peak and 8 kWh @ $0.30 off

Partial Or Full Backup?

When installing a battery system there are a number of constraints. Most them were described in Part 1, but briefly: there are three important specifications: total capacity in kWh, peak discharge rate in kW, and sustained discharge rate in kW. Because of these limitations, it’s often recommended to install a PARTIAL back up system where some number of circuits (essential circuits) are covered by the battery should the grid go down, and the other (non-essential) circuits lose power. The reasons to NOT backup things include: a very high starting or sustained energy draw (e.g. an AC, Heat Pump or pool pump motor) vs a desire to protect things like refrigerators, lights and comfort power like TVs, routers and commonly used lights.



But there is an often UNDOCUMENTED sinister side to this arrangement. Specifically a battery system can normally see all the solar inputs, the battery charge and discharge and the use of the essential circuits. But unless monitoring devices called “CTs” (current transformers) are installed, the system cannot see the rest of the circuits. This creates three problems:

1. Your system won’t know how much energy you are actually using… so what your system says you imported or exported from the grid is NOT what the utility company sees and this can be confusing. If you refer to Chart 3 below, you can see that energy coming from the grid that does NOT go through the solar/battery panel isn’t observable by the components that come after the solar/battery breaker.
2. Normally battery systems are configured to charge the battery with excess solar production (meaning energy would flow back to the grid). However because the system doesn’t see the non-essential consumption, it may elect to charge the battery instead of using the solar energy to run your air conditioner or space heater.
3. You won’t be able to know how much energy you are using at any given time with the system app. This makes it harder to chase down energy hogs.

Chart 3: A Line diagram of a solar system with a battery backup that can run when the grid is down.
1. The main meter/grid power.
2. The breaker that ties the grid to the solar/battery system. Other breakers/circuits in this panel are NOT backed up.
3. The internal disconnect. If there is no power coming in, the FranklinWH aGate (brains) throws a “switch” disconnecting the solar and battery from the grid.
4. The “essential loads” panel which is backed up and will continue to run if there is solar power OR battery power (or both).
5. The solar panel “combiner” box – attaches to the panels on the roof.
6. The FranklinWH aPower (battery).
NOTE: I elected NOT to connect the AC to a smart circuit and instead plan to connect an EV charger.

In my home system we elected to NOT include a radiant under-tile floor heater, the AC and 8 or so other circuits, unfortunately including what we thought was an “unused” bathroom plug that is the source of the power to our security cameras. One take away here… make sure you know what ALL your circuits control before making choices for a
partial backup system.

As you might guess, the floor heating system uses some significant power when it’s running for 5-6 hours a day and 850 watts per hour is up to 5.1 kWh per day or 153 kWh per month ($51 at the best off-peak rates)! It was more than a little disappointing when we got our electricity bill for the first full month and saw a LOT more imported energy than we expected. Mind you the bill was about 10% of what we had paid the previous year, so we weren’t complaining much. We plan to move this circuit to the backup panel to avoid the “gotcha” since we clearly have sufficient battery to make it possible.

However the other catch to this situation is that WHEN one uses power matters. (See Energy Hacks, below).

Energy Hacks

I see lots of commercials about doing your laundry, running your dishwasher and charging your EV at late night or morning. That makes sense if you have to pay to import energy because it moves those activities to the lower rate period. However the LOWEST rate period for a solar system is when it’s producing enough energy to cover the consumption… that is DURING DAYLIGHT hours. So that proverb about making hay while the sun shines, applies also to electricity usage.

If you have solar energy… use it while you have it! *aig*

The MOST cost effective way to use generated energy is use it when it’s sufficient to run your appliancesMost of the time that will be from about 11:30 AM to about 2:30 PM and even later in the summer months. You’ll have no conversion losses (except from the DC power on the roof to the AC power for your house). However its a good idea to make sure there is sufficient energy to also charge the battery system in case you need it for an emergency outage, and to avoid the peak rates which start right about sunset. For a bigger battery, it’s optimal to gain enough charge to see your home through all the peak and off peak hours until generation begins again. Another trick is to stage your heavy appliance usage such as doing the washer and dryer loads before or after you may need that power for your air conditioner. Indeed, tracking when you have excess solar production can be quite helpful to saving energy.

When Do I Have Excess Solar?

Ideally there would be an easy to use automation that does useful things like charge the EV, run the AC, and more WHEN there is more energy available from the sun than is needed. Figuring that out manually is doable IF you can see the actual energy from the viewpoint of your utility in real time. I ended up using Emporia Utility Connect, and Emporia VUE energy monitors to supplement my monitoring regimen because my partial-backup FranklinWH system doesn’t see the full picture. Either or both of those tools paired with Emporia Smart Plugs allow me to do some automation. One of those is a “Greener Hack” I describe below.

Optimizing Payback (Getting Paid for Excess)

We touched on the “export” option, but the gist is pretty simple. Unless you can export energy at a greater profit than the cost to import it, it doesn’t make sense to try to export energy to offset your bill. Under NEM 3.0 a strategy of extreme self consumption is usually most cost effective, and the least prone to rate changes.

Getting a Little Greener

Our house has a natural gas furnace. It is not “green” by any means, but it runs well, and because our blower/thermostat IS battery backed up, we can heat our home even if there is no grid power. However watching electricity flowing back to the utility made me realize I could do even better!

I bought three 750/1000W space heaters with mechanical thermostats and NO remote control. Notice the maximum energy usage is 1000 Watts which is 8.3 amps at 120 Volts. I paired each of them with Emporia Smart Plugs that have a maximum sustained usage of 10 amps (1200 Watts). Why mechanical thermostats? Because most of the fancy heaters these days – especially those with remote controls will NOT come on when the power comes on. Those with mechanical thermostats can be set up so that when the power is on, they will run and produce heat up to the maximum mechanical thermostat setting WITHOUT needing intervention.

*aig*

IMPORTANT Safety NOTE: Your smart plugs generally can NOT handle a typical space heater load. Nor should you use extension cords or power strips. The typical extension cord and power strip is undersized for the current used by a space heater and may overheat and catch fire.
BEWARE!

Using the Emporia App, I set up those smart plugs in the “Excess Solar” option. I’m not entirely sure how their algorithm works, and I had a few surprises, but mostly it seems to work.

Here are some screenshots from the Emporia app.

A smart plug configured to turn on when there is excess solar. Notice the Cyan color indicating it’s on. This smart plug just turns a lamp on to let us know when we have energy to “burn” on things like laundry and space heater.

The configuration of “Excess Solar Management” looks like this. It is under the “Connect” device which watches what our meter is reporting to our utility company in real time.

This is how the smart plugs are configured. “Excess Solar Indicator” is turned on if there is excess solar followed by each of the items listed and “on”. It can also change the living room thermostat.
The “meter” status with one of the space heater plugs overlaid on it. The app, unfortunately doesn’t let you plot more than one thing at a time.

Time of Use Plans and Battery Systems

Curiously, my experience is that most systems (FranklinWH, Enphase, and more) don’t take FULL advantage of rates. In the FranklinWH system for example, I had to set up a plan to force my system to NOT import during the off-peak time and instead import during a “super-off-peak time” which I created. This rate doesn’t actually exist, but the problem is that off-peak is Midnight to 3 PM. But charging the battery at midnight means burning money by importing from the grid. However if I force it to wait until noon, then my solar system gets a crack at charging the battery for FREE before the Time of Use plan drinks costly juice from my utility. And if the sun has been insufficient to the task, it makes sense to spend a couple of hours charging the battery before the “Mid Peak” and “Peak” rates kick in and drain more from my wallet. But this is true for me because my off-peak rate is $0.334 but the summer peak rate is an appalling $0.724 – that’s more than enough difference in rates to make up for the roundtrip losses using the battery. Setting this up in the current FranklinWH app is a bit too tedious to explain, unfortunately. I’d point you to Reddit where it’s been discussed. When you switch between modes or edit the Time of Use schedule and rates watch it carefully! I made a mistake and instead of playing out my battery at off-peak, it elected to import from the grid to cover my household use instead. My utility company netted an extra $2 a day from me due to that mistake!

Questions To Ask

If you’ve made it this far, you’re probably wondering… what questions should I ask my installer, and what things should I try to discover for myself.

  1. How long have you been in business? When did you do your first residential solar panel installation? When did you do your first do a solar + battery system? How many total installations have you done to date? Is residential or commercial solar systems your primary company focus?
  2. Are you the company that will do the installation? Or are you a broker, or reseller, or sales organization? If the installer, do you have a C-10 Electrical contractors license?
  3. Do you stock equipment like solar panels, inverters and batteries?
  4. What is a typical work-start to system-completion timeframe, currently? What causes the bulk of that time to pass?
  5. Do you do warranty repairs, or monitoring of the systems you install?
  6. Is the crew you use your employees, or do you contract out some or all of the work?
  7. Are your skilled workers paid competitive rates?
    > Why this question? An increasing number of utilities (e.g. Pacific Gas and Electric) will not certify/allow interconnect if the workers are not paid “competitively”.
  8. Can you quote me systems that optimize my return on investment? Please be sure to include the expected total cost over 5, 10, 15 and 20 years. (This is especially important if you are thinking of getting a lease, a Power Purchase Agreement, or financing the purchase with a loan).
  9. Do your estimates and financial models include degradation of components like solar panels, batteries and inverters? What annual energy cost increase (or decrease) is assumed in the model?
    > A model that assumes NO cost increases may be conservative, but might also be more accurate than one that outrageously assumes cost increases.
  10. Tell me how you affix racks and panels to my roof. What methods do you use and what guarantees do you have regarding roof penetrations and roof leaks.
  11. Does the system you propose have tools that will make it easy for me to monitor and track my daily and monthly energy consumption? I.e. can I expect the system to agree with my utility company to within, say 5%?
  12. Why did you propose these panels, inverters (and batteries) over others?
    > It may be familiarity, cost, or availability that leads them in one direction or another. And it might be profit.
  13. Do you have any reference accounts for a system similar to the one you are proposing that I can compare daily and monthly costs with my own? Preferably another house in the same area or neighborhood?

If any of those questions make your proposer/installer hesitate, I’d suggest treating them with circumspection. Also, it’s rather useful to get more than one estimate from more than one installer because two estimates sometimes reveal things you may not think about. For example one estimate might include a larger battery and a smaller number of panels or vice versa.

Finally my $0.02 on the cost: it’s unwise to try to pay the “lowest possible cost”. If you want your installer to stay in business to support you and do repairs under warranty, you need to pay them enough to make a profit that keeps them in business.

Disclaimer of Warranty and Stuff

If you’ve got general questions, I’m a night photography guy who is nerdy enough to dig into the minutia of things like this. I’m not making myself available to answer every (or any) question you may have. But I can recommend some resources worth looking into.

* Reddit subgroups [Solar, FranklinWH, Enphase, Electrical, EmporiaEnergy]
* YouTube Channels: [Gary Does Solar some very practical advice from a Brit, Solar Time with Martyna – some interesting comparisons of panels and the effects of shading)

Related Topics:
* Tech Connections: My Furnace is Too Big; Your HOUSE can Store Energy, too

If you want to ask, feel free to ask. If I see enough interest in a direction I’d like to go, there might be a third article in this series!