Load calculations for hybrid pv/battery system

[tehan]

I am researching and designing a solar/battery backup system for my home. I have a couple questions about NEC code for this type of hybrid setup. I envision using a sub-panel for my critical loads (refrigerator, some LED lighting, a few outlets for electronics) with an automatic transfer switch supplying this sub-panel from either the grid or a hybrid pv/battery inverter. My understanding is the load for this sub-panel should be calculated using the normal procedure (3W per sq ft plus rated load of large appliances). How should the supply to the sub-panel be calculated: the pv power, the battery power, the inverter power, or some combination of these? May the calculated load be more than the supply (however calculated) if the panel supports automatic load shedding (e.g. the Reliance Controls products)? If I instead use a manual transfer switch (but no load shedding), may the calculated load be greater than the supply (assuming that the supply is at least greater than each individual load)?

[inetdog]

I am researching and designing a solar/battery backup system for my home. I have a couple questions about NEC code for this type of hybrid setup. I envision using a sub-panel for my critical loads (refrigerator, some LED lighting, a few outlets for electronics) with an automatic transfer switch supplying this sub-panel from either the grid or a hybrid pv/battery inverter. My understanding is the load for this sub-panel should be calculated using the normal procedure (3W per sq ft plus rated load of large appliances). How should the supply to the sub-panel be calculated: the pv power, the battery power, the inverter power, or some combination of these? May the calculated load be more than the supply (however calculated) if the panel supports automatic load shedding (e.g. the Reliance Controls products)? If I instead use a manual transfer switch (but no load shedding), may the calculated load be greater than the supply (assuming that the supply is at least greater than each individual load)?

As long as the transfer process is manual AND the wiring is large enough for the maximum output of the PV system, then the calculated load can be greater than the maximum output of your hybrid inverter.
With an automatic transfer, on the other hand, the NEC requires that you include load shedding if the calculated load exceeds capacity.

I am curious about why you are putting in a transfer switch though, since a hybrid inverter will usually include a transfer switch and use of that internal switch is essential to getting the full functionality of the hybrid (both grid tied and off grid operation, with sell back when appropriate.)

[tehan]

Thanks. Do you have any thoughts on the question of how to calculate the capacity of the supply?

Regarding the automatic transfer switch, I am considering both existing hybrid inverters (where as you say the automatic transfer is normally included) and Powerwall (where it is not yet clear if the automatic transfer will be included).

[tehan]

Maybe I'll get an answer if I put some numbers around this. Say I have a 4kW PV system and a 2kW battery attached to a 6kW hybrid inverter. Best case (day time, full battery) this delivers 6kW, worst case (night time, empty battery) it delivers 0kW. At what power is this rated as a supply for NEC code purposes?

[Amy@altE]

Maybe I'll get an answer if I put some numbers around this. Say I have a 4kW PV system and a 2kW battery attached to a 6kW hybrid inverter. Best case (day time, full battery) this delivers 6kW, worst case (night time, empty battery) it delivers 0kW. At what power is this rated as a supply for NEC code purposes?

A couple of questions. What state are you in, what version NEC are you on?

You need to get your terminology correct. The battery bank is rated in kWh, not kW. kWh is a quantity, kW is a rate. A 2kWh 48V battery bank is 41Ah, so your example is not a good one, that's far too small for a 6000W inverter. That aside, if you have a 6000W 48V inverter, that's your answer, it is supplying 6000W to the load. The capacity in Ah or kWh of the battery bank is moot.

[Mike90250]

I am researching and designing a solar/battery backup system for my home. I have a couple questions about NEC code for this type of hybrid setup. I envision using a sub-panel for my critical loads (refrigerator, some LED lighting, a few outlets for electronics) with an automatic transfer switch supplying this sub-panel from either the grid or a hybrid pv/battery inverter. My understanding is the load for this sub-panel should be calculated using the normal procedure (3W per sq ft plus rated load of large appliances).

Whoa. You have a house already ? You have an electric bill now ? There is your current usage and loads.

How should the supply to the sub-panel be calculated: the pv power, the battery power, the inverter power, or some combination of these? May the calculated load be more than the supply (however calculated) if the panel supports automatic load shedding (e.g. the Reliance Controls products)? If I instead use a manual transfer switch (but no load shedding), may the calculated load be greater than the supply (assuming that the supply is at least greater than each individual load)?

Your battery backed loads should be calculated just on their own. You should get a Kill-a-watt plug in meter, and plug each appliance in it for a week, read off the total KWh consumed and divide by 7 . That gives you your daily usage for each appliance. Do it and total t for each, and then that is your daily load you design the rest of the system to support.

Are you expecting to ride out 10 hour blackouts 3 months apart, or a 3 month ice storm every 4 years ? Makes a difference on the scale of the system

[tehan]

Thank you Amy and Mike for your replies. Maybe it would help if I provide a little background and clarification. My goal is to provide grid-failure backup for essential circuits (refrigerator, some LED lighting, consumer electronics, network and security equipment). I would like to support up to 12 hours on batteries alone and an extended period of several days using PV/batteries. I have done the math on the equipment I wish to support and I am working to a budget of 500 W average load / 12 kWh per day. I am roughly targeting a system with a 6 kW inverter, 4 kW PV, and a 10 kWh battery.

The problem I have is when I sum the rated loads of the branch circuits I want to connect to the automatic transfer switch it comes to about 6 kW. This is partly because some appliances (particularly the refrigerator) have a rated load much higher than their average demand, and partly because the NEC method for calculating rated load of branch circuits (3 W per sq ft) is very conservative in today's world of LED bulbs. My question is can I use automatic transfer without load shedding (if it helps, I am in Texas and I guess I probably should target NEC 2014). Contra Amy's comment, all batteries are rated for both quantity (kWh) and rate (kW) of discharge. This isn't really an issue for traditional solar batteries which are derived from automotive designs and have very high power densities (e.g. a 10 kWH battery would probably provide about 10 kW of power). But newer Lithium products have much lower power densities (e.g. a 10 kWh Powerwall only provides 2 kW of power). Hence the issue and question. If an inspector is willing to just look at the rating of the inverter I am probably fine; but if they also want to look at the rating of the power sources then maybe I have an issue.

I can, as Mike suggests, measure my loads. Would this satisfy an inspector for permitting purposes? What data would I need to collect - average load or peak? If I do go down the load shedding route, are there any nice products out there that would support prioritization and automatic load shedding on say six 120 V / 20 Amp branch circuits?

[inetdog]

Thank you Amy and Mike for your replies. Maybe it would help if I provide a little background and clarification. My goal is to provide grid-failure backup for essential circuits (refrigerator, some LED lighting, consumer electronics, network and security equipment). I would like to support up to 12 hours on batteries alone and an extended period of several days using PV/batteries. I have done the math on the equipment I wish to support and I am working to a budget of 500 W average load / 12 kWh per day. I am roughly targeting a system with a 6 kW inverter, 4 kW PV, and a 10 kWh battery.

The problem I have is when I sum the rated loads of the branch circuits I want to connect to the automatic transfer switch it comes to about 6 kW. This is partly because some appliances (particularly the refrigerator) have a rated load much higher than their average demand, and partly because the NEC method for calculating rated load of branch circuits (3 W per sq ft) is very conservative in today's world of LED bulbs. My question is can I I use automatic transfer without load shedding (if it helps, I am in Texas and I guess I probably should target NEC 2014). Contra Amy's comment, all batteries are rated for both quantity (kWh) and rate (kW) of discharge. This isn't really an issue for traditional solar batteries which are derived from automotive designs and have very high power densities (e.g. a 10 kWH battery would probably provide about 10 kW of power). But newer Lithium products have much lower power densities (e.g. a 10 kWh Powerwall only provides 2 kW of power). Hence the issue and question. If an inspector is willing to just look at the rating of the inverter I am probably fine; but if they also want to look at the rating of the power sources then maybe I have an issue.

I can, as Mike suggests, measure my loads. Would this satisfy an inspector for permitting purposes? What data would I need to collect - average load or peak? If I do go down the load shedding route, are there any nice products out there that would support prioritization and automatic load shedding on say six 120 V / 20 Amp branch circuits?

Often the biggest problem with an ATS is that if it does not transfer instantaneously on failure (as would be the case with a generator which had to start up) you could get all motor loads trying to start at the same time when it finally closed.
That will not be a problem for you, and so I would be happy with a peak load over a 24 hour period (not average load). But the NEC does not make any provision for measured load other than over a one year period. It goes strictly by calculated load, so you would have to find an inspector who is willing to go beyond the letter of the code.

[Amy@altE]

Maybe I'm misunderstanding what you are trying to do. Here's what I think: Grid tied with battery backup using either a Schneider XW, Outback GTFX or Radian, or SMA Sunny Island. If so, they all have an internal transfer switch that would automatically switch your critical loads panel from the grid to the inverter when the grid goes down. The maximum power source to the critical loads panel when the grid is out is based on the inverter size, since the batteries aren't connected to the panel, it doesn't care what the potential output of the batteries are, only the limitation of the inverter. The critical loads panel can't have source breakers that equal over 120% of the bus bar rating, so if you have a 200A breaker box, you can have up to a 40A breaker for the inverter. If I'm misunderstanding your question, I apologize.

[tehan]

You are understanding fine, but you are thinking about a different NEC rule. I'm not concerned about the maximum power that will be supplied to the critical load panel, which as you say does not depend on battery specifications. I'm concerned about the minimum power that will be supplied. NEC requires an automatically switched standby power system to supply sufficient power to support the loads in the critical panel. The questions then is how to calculate the minimum supply from a hybrid solar/battery system.

[inetdog]

You are understanding fine, but you are thinking about a different NEC rule. I'm not concerned about the maximum power that will be supplied to the critical load panel, which as you say does not depend on battery specifications. I'm concerned about the minimum power that will be supplied. NEC requires an automatically switched standby power system to supply sufficient power to support the loads in the critical panel. The questions then is how to calculate the minimum supply from a hybrid solar/battery system.

Short term (i.e. when you transfer and before you manually shed loads) the possible output of the hybrid system will be simply the rated surge output of the inverter. You can assume no PV contribution (if the transfer happens at night) and assume that the battery bank will not be a limiting factor in the short term.

Calculate your load on the critical panel, worst case, and then get a hybrid inverter with at least that power rating. That will satisfy the NEC, but still may not be a good design.