Battery backup for emergency's

Hi I am new to the forum . Thank you in advance, I need some advice. I have a home on grid system with a big 20 panel system on the roof I have two 5 K inverters that feed power to the grid. The inverters are "Sunny Boy" from Sun Power. I would like to build a power storage system to use in case of power failure. I know I could purchase a commercially available system from sun power that would interface with their inverters however the cost of the system is a bit prohibitive. So I have set out to build a small system that could run two refrigerators my heating system and perhaps some LED lights around the house. I've already configured the power bypass to hook up a generator. However instead of the generator I would like to have a solar powered battery back up. I have purchased a 4000 W pure sine wave inverter and at this point I am running it with 2-12 V 100 amp hour AGM batteries in parallel. My two refrigerators pull about 6 A apiece. I am going to add two more batteries to the system giving me 4800 watt hrs. The other day I ran one refrigerator and some lights for about six hours on my two batteries, everything seemed to work OK but I believe I ran my battery down pretty far. I went back to grid power before anything stoped working. It took a 25 amp charger four or five hours to recharge the batteries. I am using a battery charger instead of a charge controller. One question I have is the panels are wired to deliver almost 450 V unloaded and they run at 335 V when connected to the inverters so I don't believe I could find a charge controller to work directly off the panels am I right or am I missing something? In a grid down situation I can pull 1500watts directly from my "sun power" inverters which I will use to power the battery charger. So my question, is this configuration OK? using a charger instead of a charge controller? What should the charger amperage be rated at? it seems 25 amps is far to small to charge 2 100amp hr batteries effectively let alone 4 which I plan to ad shortly. Me second question is are 4 AGM batteries enough? If I have a prolonged Grid Down situation how do I charge the next day after drawing a 1200 watt load all night, wile continuing to draw that load, And in the winter when the panels are not nearly as efficient how will that affect my charging capabilities?

Thanks so much
Len

A random model of sine wave inverter could be used to implement a separate PV/battery UPS if you buy suitable charge controllers and rewire some of the PV panels to it. The power from those panels will go to waste if the batteries are full and the loads on that inverter don't use the electricity at that moment. This is probably a good way to make battery backed communication equipment, but maybe not to run larger appliances such as the refrigerators.

If you're willing to spend about $15k you can build a system that uses the grid-tie inverters and PV panels you have.

Exactly what model Sunny Boy inverters? Some have a "Secure Power" feature which lets you get a couple kW of AC even when the grid is down, directly from PV but only while the sun shines.

"In a grid down situation I can pull 1500watts directly from my "sun power" inverters which I will use to power the battery charger."

This sounds like you do have Secure Power, so while the sun shines but the grid is down your Sunny Boy will provide AC to the charger which will provide DC to the battery and sine wave inverter to the refrigerator. If all the wattages and watt hours add up that will work. You probably need a transfer switch (or manually plug and unplug cords) to change from grid power to PV/battery power. I would guess your battery charger isn't heavy enough to do the job however, putting 1500W = 125A x 12V into the batteries, and the batteries may not like that fast a charge.

Actually, with 200 Ah of battery, 25A is a nice charge rate, spread over 8 hours of daylight. The AGM can take a bit more. It just doesn't use all your available power, and doesn't power your loads for very long. Instead of separate sine wave inverter and battery charger, a better setup would be a hybrid inverter that can pass through the "grid" (your Secure Power) to loads and charge batteries at suitable rate.

Since these are SMA Sunny Boy inverters they should play nicely with Sunny Island battery inverters or with Sunny Boy Storage high voltage battery inverters.

List price for Sunny Island is $5000 but these days you can probably find SI6048US-10 brand new for around $2000. You will need two for 120/240V split phase, and to carry the amperage of your 10 kW Sunny Boy. In addition you need a suitable 48V battery and a "load shed" relay which disconnects all loads such as refrigerator in case battery gets low. When the sun comes back up the battery recharges. Also various circuit breakers, wiring, etc. This is the system I have build for myself.

An alternative is the newer Sunny Boy Storage paired with SMA's ABU, automatic backup unit (separate transfer switch.) This uses up to three 10 kWh 400V lithium batteries.

Batteries are going to cost more than everything else. As you've noted a modest battery bank isn't enough. I started with the size you mentioned, four 12V 100Ah batteries. Mine was in series, 48V 100 Ah. What I have now is eight, 6V 400 Ah AGM batteries, 19 kWh with about 14 kWh usable at 70% DOD. That cost $5000. I can only draw about 1 kW during the night, which requires me to turn off a few non-critical loads like outdoor lights and electronics. An inefficient refrigerator and a furnace fan periodically getting turned on by gremlins contribute to the issue. What I would like to do is turn off the refrigerators at sundown and let them coast until sunup unless they rise above some critical temperature (blue ice or salt water ice to keep freezer cold).

My usage model is I deliver 55 kWh or more to the grid when it is up. During a failure I can run any appliances including air conditioner, but at night only a few loads. If too much the house gets disconnected around 3:00 AM but things come back online after the sun comes back up.

If you want to get through extended grid outages and bad weather, the rule of thumb is battery capacity equal to 3 days consumption. You will probably find that would cost $30k. Since you are doing backup for grid failure rather than off-grid living, you should just get a portable generator for those rare cases, $300 instead of $30,000.

As for winter, the PV panels will be more efficient but will have fewer hours of sunshine. If you were starting from scratch you would have your panels tilted toward winter sun or summer sun depending on anticipated needs, but you're just going with whatever your roof orientation happens to be. With 10 kW of PV (inverters at least; what are the PV panel ratings?) you get about 55 kWh per day. Just scale down consumption to weather blackouts.

Wow thanks so much, so it seams for the amount of money it will cost verses the amount of times I lose power I can't seam to justify this just for backup. I have not lost grid power for decades. So I Have one question about changing. I will build my system to a 4 battery 12 volt 100 amp hr and only run small things if I need to, so should I step up the charger to 50 or 75 amps from my current 25 amp charger?

Yup, anything with battery backup is hard to build for less than the cost of grid electricity. Even if electricity costs me $0.45/kWh at peak time of use and $0.15 off-peak, my math tells me that by the time batteries wear out they cost me more than $0.30/kWh.

You probably do get brief power failures and it is likely to get worse in the future unless your utility is making investments. What you've got should be able to run internet, laptop, cell phone charger 24/7, and run refrigerator during the day. So you're good.

"In a grid down situation I can pull 1500watts directly from my "sun power" inverters"

Does that mean they have the SMA "Secure Power" outlet, which lets you manually switch on an AC outlet during power failures, while the sun shines? If you have a 5 kW PV array, it can probably drive that 1500W outlet for at least 300W (25A x 12V) for 12 hours, 3600 Wh per day. If you put in a 4800 Wh AGM battery bank and drain it to 70% DOD, that's 3360 WH usable. Your present 25A charger would fully charge that during the day if no loads present. If your only loads are lights at night, no need for bigger charger. If you run things during the day off that inverter, then sure, add charging capacity.

The two refrigerators are the important loads during a power failure. 6A or 720W is a pretty heavy load. Icemaker much? I was monitoring mine, realized I was making ice with no water hooked up, so I kept dehydrating the air then thawing things, which explained the puddle on the floor. Once I turned off the icemaker that refrigerator dropped to around 2A and cycled less frequently, and my others are around 1A.

You can probably run one refrigerator off each Sunny Boy secure power outlet while the sun shines, so long as they provides enough surge to start. You already have an ideal economical system.

I only built what I did because I had money to burn, I found a bargain on equipment, had time on my hands since getting canned, and I always wanted to implement this system.

"I've already configured the power bypass to hook up a generator."

I hope that includes a proper interlock, so it isn't possible to connect the generator (or inverter) to your panel same time as the grid. Otherwise you could blow up your electronics or possibly electrocute utility workers. It's easy enough to do safely, cost me $50 or so.

Thanks so much seems I can get by with the two 1500w outlets for day time and let the refrigerators ride out the night time. Without worry of food spoilage. I can use my back up for some lights and the electronics of the heating system. Im not sure what the draw of the 4 circulators are so I will need to look that up. I do have my backup transfer isolating my grid power. would not want ruin anyones day out on the utility pole LOL

@Hedges, really nice info.

It's tough getting a grid-tie system to work in an off-grid scenario because they have to be designed to switch off when the grid fails, but it looks like you have a clever system that does offer at least some off-grid capacity.

The AltE YouTube channel has done a few videos on AC and DC coupling grid-tie systems, for example, here:


But seems to me this "Secure Power" feature will probably suit your needs - and remember even if you don't have sufficient storage capacity to continually run your fridge/freezer, there's no way your food is going to spoil just over night, unless you keep the doors wide open!

Thanks for you insite. I have another question about battery protection. Right now I have no fuses or low voltage cut components. So can any one help me size the appropriate components. I have 4 12volt 100 amp hr AMG batteries in parallel with a 4000 watt pure sine wave inverter. I’m charging with a 55 amp 3 stage charger powered by the grid for stand by and then off my SunnyBoy on grid inverter running a 1500 Watt “Secure power “outlet which becomes energized in a grade down situation.

In general you fuse circuits to protect from short circuit failures, you do not generally fuse devices, they have their own protection built in. In general, you would fuse every circuit coming from a battery or other sources of power (PV arrays are nuanced). You fuse a circuit based on the LOWER of (1) the expected load +25% and (2) the ampacity rating of the cable.

Much more info on fusing circuits is available here.

Thank you very much. Very helpful

If this is truly for emergency a less than ideal solution might be worth considering.
1. You can put in an emergency standby battery and inverter that is charged off of AC, to be ready for an emergency. This would run "as is" until your batteries depleted. So for a power outage of a few hours, this might be just fine.
2. In the event of a longer emergency, connect some of your panels to a charge controller to charge the batteries.

It would be possible (do this ahead of time) to put a Y connector between the 3rd and 4th panel of each of your two series strings of panels (assuming 2 strings of 10 panels). Using the Y, add a new wire to each string. Bring those two wires down to your wiring closet, 6 wires total now off of the panels. The negative of your existing string and one of these new wires would give you about 120v DC, which can be handled by many MPPT charge controllers. You could do the same with the other string. You could even parallel the two sub-strings in your wiring closet. I would not connect any of this up without first disconnecting the grid tie inverter, just to protect them from anything you might do that they wouldn't like. So with just two more wires, you should be able to bring about 1500w of solar down for charging. If you don't understand exactly how this would all work, you should not be trying it, as you might put your existing system at risk. But this would be an easy way to bring down some of that power.

I have 32 panels on 8, quad micro-inverters with 240vAC running out to the array. In the event of an emergency that would last for unknown days or weeks, I have a plan that would include disconnecting my inverters and back feeding the DC from the panels back on my 240v run, which would be disconnected in the house and connected to a charge controller. I have 4 panels that are closer to the house and dedicated for emergency charging, but if things were bad for a long time, I would want more power, so would tap into my large array.