Will Solar + Battery Backup run my whole home? OR: What can I run when I'm on battery backup?

We get this question a LOT...


Wait...actually...We DON'T get this question as often as we should.


Having "been there, done that" in my industry for 15 years (23 as a licensed electrician), I will tell you that privately, you could definitely find me (at least once a week) holding my head in my hands, staring frustratingly down at my desk as I listen to one of our latest, sadly mis-informed potential customers tell me how "solar company x" says that they can install one battery and they'll never know that the power went out!


Gee...I wish it was that simple.


But, it's not. It REALLY REALLY isn't just so simple as to add a battery and you'll never know the power fails at your house, ever again.


So kiddos, here we go:


If you haven't read my previous blog about how battery backup with solar works (https://www.dpisolar.com/post/solar-power-with-battery-backup), now might be a good time to start there.


You're back so soon? Wow - let me say that what you just read in seven minutes took me a week of edits to trim it down enough so it MIGHT make sense!


Ok, so you're up to speed on how it works, so the next question is, "what can I run when I'm on battery backup?"


If you DID read the previous blog post, then this should all flow (if not, I hope it still makes sense to you as well!)


ALL currently offered solar installations that include a battery backup have the pieces we discussed in the previous post (some products have it all in one "box" while other products have multiple pieces that attach to your wall). We are going to consider two main pieces here for your question:


1) the output of the system when it's in battery backup mode

2) the size of the battery bank.


Let's start with what is really the most critical spec on the whole system:


The output of your system when it is in battery backup mode.


With several manufacturers out there, it should be kind of evident that each brand will probably have a different output, and... they do. So for this blog, I'm going to use the Q.Cells Q.Home+ ESS HYB G1 (gen 1) system that we currently offer (we also offer a more robust product for our rural customers, but in general the brand we are focusing on in this blog comprises about 80% of our installations). The product has Li-Ion Batteries with two available sized battery banks.


The Q.Home+ product has an off-grid output of 7,500 watts, and can offer a short-time surge (less than 30 seconds) of 8,200 watts.


When we consider what you can / should / should not run on your batteries when the grid fails, we need to consider two different aspects of your proposed loads:


1) start-up (ie: Locked Rotor Current) demand (in watts)

2) running demand (in watts)


As you might be guessing, a lot of your appliances run at much lower demand (watts) than they do when they initially start up. The science here is a bit complex, but the National Electric Code gives us some ideas on this in their tables in section 430.

*For the nerds, look up NEC Table 430.248. This table lists out various horse-power loads and shows how many amps they will draw at the specified voltages. A lot of designers will "cheat" and convert each horse-power rating into watts so we can quickly figure out loads (when we're used to working in watts, this is much easier). The other table you'll want to look at is table 430.251(A). This gives us "locked rotor currents" that we might expect to see when the respective loads start up.


So, why do we need to take these things into account? Well, because we want to make sure that when we install and test your system, it works like it should and doesn't "fault-out" under "overload".


So, am I dancing around your question? Or...am I laying the groundwork necessary to make sure you get what you want out of your system? (hint: it's the second one!)


Let's start a list of things we want to run when the power goes out:


1) 1 hp water well pump with "soft-start" or "VFD" - we want water to drink (and maybe just as important, to flush the toilet)

2) refrigerator - don't want our food to spoil

3) freezer - don't want all that stuff we bought at Costco to thaw and spoil

4) microwave - easily the fastest way to heat/reheat things

5) LED lighting in the home - if you haven't converted, do so before you go with a battery backup

6) various outlets to plug things into (phone chargers, laptop chargers, gaming systems)

7) TV - gotta have that news to update us (or our weekday "stories" aka: soap operas)

8) Internet modem/router - keeps your hardline phone running, your internet on (so long as the lines are still in tact to your home)


Now - let's list out the average start-up or running demand for this list:


1) Well Pump with VFD/Soft-Start: 4,080 watts to start

2) refrigerator: 600 watts to start the compressor

3) freezer: 600 watts to start the compressor

4) microwave - no start-up surge - 1,100 watts

5) LED Lights - no startup surge - 10 lights = 200 watts total

6) Various Outlets - usually 1,000 watts or less total

7) TV - typically pulls about 250 watts when on

8) Internet Modem/Router - up to 100 watts running


So, what are our loads (if EVERYTHING on this list is running at the same time):

4,080+600+600+1,100+200+1,000+250+100 = 7,880 watts when EVERYTHING is running at the same time


Are we good? Well, if you scroll back up, you'll see I put the output of the Q.Home+ system in bold (and that number was 7,500 watts)


But Josh - 7,880 watts is more than 7,500 watts, right? YES! But...this system is designed to offer a short-time SURGE to allow larger loads (like a well) to start up (surge power of the Q.Home+ is 8,200 watts).


So, this list works. You would be able to start and run everything on the above list without issue. But, what's missing? What did I NOT put on that list?


1) Central Air Conditioner (9,000 watts typical)

2) Central Heating (13,500 watts typical)

3) Hot Water Heater (4,500 watts)

4) Electric Range/Cook Top (8,200 watts typical)

5) Clothes Dryer (5,000 watts typical)

6) Washing Machine (1,800 watts typical)


The reason these loads aren't included in the above list is because each one of them on their own would likely use up ALL of the 7,500 watts of power to run. If you were to try to start a central air conditioner or central heater (heat pump) with a backup capable of only 7,500 watts, you'll overload the system and it will show the overload as a fault, asking you to reset the system and "try again".


You'll also notice that some of the loads are NOT going to take up all of our 7,500 watt capacity (water heater, dryer, washing machine). So, can you run THESE loads when you're on battery backup? Short answer? Yes. Longer answer: carefully, and only recommended during daylight hours when the sunshine is hitting your solar panels. These loads on their own will run just fine, but if the sun is down and your only source of energy is coming from the batteries, you will deplete the batteries in a matter of hours. This is opposed to sunlight hours when the sunshine is getting converted to watts by the solar panels and then directly feeding these loads. During daylight hours, the battery bank merely acts like a shock absorber, delivering instantaneous power to start things up, but then letting the solar panels and the rest of the system take over and run the loads without using your battery.


The other thing to be mindful of is what you're trying to run at the same time:


You could run your water heater, the microwave, your 1,000 watts of outlet loads, and your lights - but not the well.


You could run the clothes dryer, outlets, and microwave, but not the water heater or well at the same time.


You could run the washing machine, water heater, and outlets, but not the well or clothes dryer at the same time.


I could go on and on, but what I'm trying to establish here is: all things in moderation. Don't try to pretend that you have the same amount of unlimited power available to you when you're running on your batteries (like you would when the electric grid is online). The fancy term here for the nerds (like me) is: load-shedding. And it's exactly what it sounds like: you should shed unnecessary loads (ie: turn them off) when you're not using them (with the noted exception of your fridge and freezer - don't turn those off!). And...yes... There's a bit of "magic combination" involved here - meaning you can mix and match loads and run them together, but in general, you won't be able to run everything at the same time.


Now it's time for the reality: How LONG can I run my loads when I'm on battery backup?


Well kids, this math is a LOT easier:


Your battery / battery bank is usually rated in kilowatt-hours (kWh). The Q.Home+ battery bank is either 13.5 kWh or 18.9 kWh.


So, now we need to explore: what IS a kilowatt hour? Simply stated, a kilowatt-hour is 1,000 watts used for 1 hour. Our friends who use the metric system are very aware that "kilo" is the designator for 1,000 - so one kilowatt-hour is (as previously stated) 1,000 watts used for one hour.

Final time: if we refer back to the Solar+Battery Backup blog I wrote previously, I tried to help establish that the battery bank is nothing more than a "gas tank" where we store our fuel. The bigger the gas tank, the longer it lasts, right?

Let's use the above mentioned 13.5 kWh battery bank and convert it into watts:


13.5 kWh battery bank = 13,500 watts of power for ONE hour. Now if we look at the OUTPUT of our inverter when we are running on batteries, we know that it's capable of giving us 7,500 watts of power, but for how long?


Well, if we divide 13,500 watts of stored power (in our battery bank) by the maximum output of the inverter, we see that the battery bank will last for about 1.8 hours! That's not inspiring, is it? Nope - it's not. So how and why is Josh telling me and others that I could make it for days or weeks on batteries? Well, even though you're now nervous about that low number of 1.8 hours, just remember - you sleep at night and don't usually use a lot of power. The fridge and freezer will cycle on and off as needed (usually 15 minutes for every hour). Your cell phone won't use a lot of power overnight to recharge, and the lights are off, so in general, you won't tax your battery too much overnight. You'll usually wake up in the morning, see that the battery has been drawn down overnight, and watch it recharge from your solar panels during the day while those solar panels are also doing double duty to run your loads during the day as well. Additionally, you'll be very surprised to learn that (in general) when you're not doing big things like washing clothes or dishes (or running your hot water heater), your average consumption is usually less than 1,500 watts at any given time. If you have a sufficient amount of solar panels attached to your battery system (don't worry - we'll make sure you've got what you should need), then the solar panels will MORE THAN handle your average loads during the day while also having more than enough power to simultaneously recharge your batteries.


The truth here is that the size of your battery bank becomes less important as your ability to load-shed and manage your loads/do your tasks during daylight hours improves. During the day, your battery bank is a shock-absorber, helping out to start large-demand loads - then handing off the running demand of those loads to the solar panels. At night, the battery bank is your life-boat: it's your only source of power and should be treated with care so you don't wake up with a dead battery (and no way to make coffee!).


The summary here is this:


A battery backup is no different than a similar sized generator. You'll be able to run some loads, but maybe not EVERY load you want (like central AC or central heat). The gas tank on the generator is just like your battery bank - it will last a certain amount of time depending on how much load your putting on it. The more diligent you are with your loads, the longer the gas tank (aka: battery bank) will last.


The other take-away here is that we look at how many things might start and run at the same time to make sure you're not overloading your system when you're on battery backup. BUT! Just because our list above shows 7,880 watts doesn't mean all of those loads are running non-stop. In fact, none of them are. Your well might run for 15 minutes a few times a day. Your microwave? Five minutes a day? Maybe? You get the idea. This means that even though we're trying to set proper expectations, it doesn't mean you're going to only have a few hours of backup when the power-grid fails. Far from it. And like I said, with some diligence and knowledge, you can be running on battery backup for weeks without issue.


The best part of a solar array with battery backup is this: you get to use sunshine and light to refill your battery bank (aka: the gas tank). This means no more messy spills when you're refilling the generator's gas tank, no more exhaust fumes when it's running, and it's SILENT! The benefits of a battery bank system are amazing (and I've found that - after owning one - I'll likely never be without one again - they are that impressive).


So...is battery backup with solar for you? Find out more and give us a call at (503) 857-0099!

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