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  • Writer's pictureJeff Mathias

Batteries or a Generator!

Updated: Feb 20

Comparing the 15-year costs of energy storage and a generator.


Back in 2017 when I was co-owner of Synergy Solar, we began designing our first Energy Storage Systems (ESS) for back-up.  Some of these systems were new installs combined with solar and others add-on ESS to our existing solar customers.  We began installing the Sonnen Eco system in 2017 and by 2018 we were also installing the Enphase ESS.  One of the most common questions I was asked when meeting with prospects, what was more cost effective, batteries with solar or a generator.  In 2019 I wrote a blog, comparing a 20 KW generator to a Sonnen Eco 12.5 and in most case the Battery system made the most sense.


In 2019 I preferred the Sonnen system as it had more power in the entry level systems. For example the Sonnen Eco 10, had an 8,000-watt inverter (power) and 10 KWH of storage compared to Enphase Energy 10, with 3,800 KW of inverters and 10 KWH.  But the main reason I liked Sonnen was it had a much better surge rating which is important for homes that have pumps and motors.  Surge is the electrical requirement to start up a pump or motor.  For example, a 2 HP well pump may run at 2,000 watts but could surge to almost 6,000-watts when starting.  So in some country properties, a simple toilet flush can activate 3 pumps, a well pump, a booster pump and a septic pump. With surge wattage being 2-5 times greater than running wattage, it can often be the determining factor when sizing a system .  In a nutshell the Sonnen Eco 10 can surge to 17,000- watts while the Enphase 10 surged to 5,700-watts. And while multiple Enphase 10’s can be tied together; it took 3 to match the surge of the Sonnen at considerably more cost and space.   This is the primary reason I installed a Sonnen Eco at my home in April of 2020.


In 2020, I expressed this concern to Enphase senior management and Enphase co-founder and Chief Product Office, Raghu Belur, not only heard my concern but told me he would be working on doubling the power of the Enphase system in future releases. In the summer of 2023 Raghu and team delivered the new Enphase 5P battery, with a 3.8 KW continuous power and 7.68 surge.  In addition, with this new design, reliability was improved, and the limited warranty was increased 50% to 15-years.  The cost of a pair 5P batteries is about the same as the previous generation the 10T, and offers double the continuous power, 3 times the peak/surge power and the same amount of storage at 10 KWH.  In addition, up to 8 5P’s could be couple together for an industry leading 30.72 KW with surge of 61.44 KW.  Sonnen went the other way, offering the Sonnen Core product with less power than previous models.  While I love my Sonnen and it has performed well for me over the past 4 years, it is time to update my blog comparing Generators to new Enphase 5P ESS.


First what is new with Generators since I wrote my blog in 2019?  The biggest change I have seen over the past 5 years has been in monitoring.  The generators now can send you information on when they need maintenance, when they run their exercise cycles ( a weekly-monthly test for ten minutes to verify everything is working and maintain battery charge) and track usage of generator.  Most Generators do not offer much head room for surge, so one rule of thumb is to find the desired wattage needed and increase it by at least 25%.  So if you need 16,000-watts OF POWER, you would install a 20-22 KW generator, (16 KW * 1.25 = 20 KW) to included estimated surge.  Also to keep this simple, I will assume adding a generator or ESS to an existing solar system.  This is a worst-case option for the ESS option as usually we use solar savings to help justify the storage system.


Operationally these two systems have both pros and cons.  The Generator system can put out 22 KW of power continuously, which is great if you are pumping water 7/24, but overkill if all you need is your router and refrigerator to run while you sleep.  Generators are limited to the fuel source on the property, thus if you have a 200-gallon propane tank, this will last about 2.5 days when running the generator without being refilled.  One concern some have expressed is that in a major disaster, they were not sure they could count on getting propane resupplied, (road/infrastructure closures).  Also, many propane and natural gas systems have safety valves designed to shut off gas in a major earthquake.   Finally, we saw in the Tubs, Kincade and Paradise fires, one of the first things PG&E does in fire affected areas is turn off natural gas, no generator.


ESS is well designed for most homes to not continuously put out power but to balance it

over the homes usage.  So, if the only load is a 10-watt night light, we do not need to run a 22,000-watt generator to supply the need.  And if we need water for a shower we can handle those surge loads. And if power is needed during the evening, the ESS system is always ready, but not discharging till it is needed..  When the grid is down, during daytime operations the Enphase system first draws power from the solar system minimizing the requirement for storage.  One nice thing about the Enphase system design is it can provide power from both the solar and ESS allowing more power to be available during the day.  When the grid is down, the primary method of charging batteries is the sun.  While this can be limited by overcast days or smoke, solar will still produce some power in low light conditions.  The sun provides an endless supply of power for the ESS system, recharging the batteries daily.  One option not included in this blog is a generator can be added to a storage system if needed, I have never needed this option. 

The Generator I will use for comparison is a 22 KW Generac Guardian generator, with automatic transfer switch (disconnect PG&E in an outage), a load control panel with 10 electrical feeds for critical loads (well, refrigerator, lights, heater igniter and blower fan, router etc.) and permit.  Since 2019 we have seen slight increases in Generator costs including installation with a range of $15,000 - $18,000 depending on vendor and installation difficulty.  What is excluded from the above quote is hiring a plumber to run the propane line and test, pad for generator to sit on, and trenching if required, estimated at $1,500.  The Generac generator comes with a limited warranty of 5 years and the cost to extend the parts and labor warranty to 10 years is $1,050.  There is no optional warranty for years 11-15, so I will estimate this at $1,500. Annual maintenance is required on a generator adjusting valves, oil, filters, and spark plugs (not covered under warranty).  This can range from DIY who can purchase the kit for $60 or getting an annual service contract at ~$273 per year, from a generator service company.   So, 15-year annual maintenance would be $900-$4,095 assuming no annual increases.  For running the generator, I will be assuming propane costs at $3 per gallon and assume the generator when running will use 3.6 gallons per hour.  Generator also need to be exercised, run weekly-monthly for 10-15 minutes to charge the battery, and ensure all is well.  Propane costs for this is about $4 per event and if we assume bi-monthly exercising the cost would be $1,440 over 15 years assuming no increases.  Finally, there is a cost to run the generator when the grid is out.  I will assume a customer will need to run the generator for 4 days, per year or 345 gallons of propane at $3 equals $1,035 per year or $15,525 over 15 years. 


Total cost of 15-year Generac 22 KW generator:


                                                                                    Low Range                  High Range

Purchase Price:                                                             $15,000                     $18,000        

Plumbing gas, trench and pad :                                  $  1,500                     $  1,500

5-10 year parts and labor warranty:                            $  1,050                     $  1,050

Estimated cost no warranty years 11-15: $ 1,500 $ 1,500

15-year annual maintenance cost:                               $     900                     $  4,095

Bi-monthly exercise propane costs:                             $  1,440                      $  1,440

15-year propane costs, 4 day usage per year:              $15,525                      $15,525


Total                                                                              $36,915                     $43,110


To compare to the 22 KW generator, I will use a significantly larger battery system then I used in 2019.  The design will consist of (4) 5P batteries offering 15.2 KW continues power, with 30.4 KW surge/peak power.  I will also almost double the KWH of storage from 5 years ago to 20 KWH.  This system includes the system controller (automated PG&E Disconnect), a protected load panel with 10 circuits, installation, permits and PG&E interconnection agreements.  The system comes standard with an Enphase replacement warranty of 15-years and a labor warranty of 10-years.  In addition, this ESS system can peak shave by storing power to the ESS system when rates are low, and using power from the ESS system when rates are high (4 pm – 9 pm).  This savings is estimated at roughly $500 per year. If more power or storage is needed, the system can be expanded up to 4 additional 5P batteries.


Total 15-year cost of Enphase 20 KWH Energy Storage System:


Purchase Price:                                    `           $  30,327

Estimated Labor Warranty 10-15 years: $ 850

30% Federal Tax Credit:                                   $<.9,098>

15-year peak shaving savings:                        $<  7,500>


Total 15-year ESS Costs:                                   $  14,579


A few differences that do need to be mentioned that are in the ESS’s favor:


1)    I assumed no price increase in any of the generator annual maintenance and fuel costs.

2)    I assume no increase in utility rates when figuring out the peak shaving benefits of ESS.

3)    ESS is noise free with no fossil fuel requirements or pollution.

4)    ESS allows a homeowner to minimize their transmission and distribution to the grid, by storing their own power and using it when solar is not producing.

5)    ESS allows you to be prepared for PG&E’s current Net Billing Tariff.




 While the initial cost of the Enphase ESS is roughly double that of the Generac, the 15-year cost is less than half.  I have often heard from homeowners who already have a generator, a desire to add an ESS system as running the generator make it difficult to sleep.  A few homeowners have mentioned that they feel bad running their generators during the fires as it just adds to the already bad air quality.  Adding ESS to solar enhances both systems, allowing the solar to continue to run when the grid is down and allowing customers to store their own solar power to be used later.  Energy Storage Systems are a big part of moving our homes and the grid to a more sustainable future.








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