If your solar power system is grid-tied, loss of grid power will also cause your house to lose power. It’s because it’s dangerous to send power to the grid when people are working on it. If you want to have power in your home during a grid outage, whatever remains powered must stay isolated from the grid. Also, the main circuits usually stay connected to the grid. This means that what stays powered must also be separate from the main circuits. Isolating the powered loads from the grid is one function of a hybrid inverter.
We call the powered loads critical loads because they affect the function of the house. They also influence the inverter, battery bank, and solar array. So, we will look at what to put on the critical loads.
These should be on the critical loads if you have them.
The following items make your house liveable.Electrical components of the house’s climate control
Here is why you need it:Having the lights on is pointless if the house is uninhabitable. If you live in an extreme climate, you will need climate control. Also, if the climate control uses electricity, then it needs to be on the critical loads.
Here is what to consider:
An air conditioner uses a lot of power when operating at a steady-state. During startup, it can use over 4 times as much for short periods. I recommend using a compressor hard-start kit or soft-start kit. This kit can reduce the startup power by a lot. Here is an example:
A 2 ton* air conditioner can use over 3,400 watts (about 15 amps at 230 volts) at continuous power. During startup, it can use around 60 amps. A compressor hard-start kit can reduce the start-up amperage to around 30 amps. Check the power consumption for your A/C unit if you plan to put the air conditioner on the critical loads.
*Note: A 2 ton air conditioner can pump heat at the rate necessary to freeze 2 tons of water in 24 hours. This uses about 24,000 BTU (12,000 BTU per ton).
Refrigerator and freezer
Here is why you need it:It prevents your perishable food from spoiling during a prolonged grid blackout. This is crucial if the cause of the blackout (such as a disaster) makes it hard to buy food at the store.
Here is what to consider:
A refrigerator can use from 300 watts to over 600 watts when the compressor is running steady. When the compressor starts up, it can peak to over 4 times the steady-state power consumption.
The means to cook food (if electric)
Here is why you need it:Not being able to cook limits your food options. This can be a problem if the only food available needs to be cooked. If you need electricity to cook, then add some kitchen sockets to the critical loads.
Here is what to consider:
Anything that produces a lot of heat will use a lot of energy. These include cooking appliances. Here are some examples:
The wattages listed below are for when the heating element is on. This is often higher than the average wattage used over the time used to cook the food.
| Cooking Appliance | Approximate Wattage |
|---|---|
| Small heating element of electric stove | 1,200 watts |
| Large heating element of electric stove | 3,000 watts |
| Hot plate | 1,200 watts |
| Induction stove | 1,200-1,800 watts |
| Electric Skillet | 1,000-1,800 watts |
| Microwave oven | 1,200 watts |
| Conventional Oven | 2,000-5,000 watts |
| Toaster oven | 1,200 watts |
| Crockpot | 200 watts |
As stated before, the wattages listed are only for when the heating element is on. The heating element will cycle off and on to maintain a certain temperature. So, the load is not continuous. Also, these are typical power usages, so you should check the power consumption for what you have.
You should determine how many of what you are likely to use during a grid power outage. An oven range will probably use a 240 volt outlet. Otherwise, a 120 volt/15 amp outlet should work for most other cooking appliances. It helps to have at least two 120 volt sockets in the kitchen on the critical loads.
The Rapid Shutdown System (if you have one)
Here is why you need it:High voltage solar arrays mounted on a house must have a rapid shutdown system. The rapid shutdown system exists to protect firefighters from electrocution. It isolates the solar panels and shuts off the power to the inverter. Loss of electrical power (along with a switch or temperature) can trigger a rapid shutdown.
So, grid power loss without the rapid shutdown on the critical loads will stop battery charging by the solar panels. If this happens, you will have a limited time on battery power. After that, your house will lose power like a grid-tied home. But if the rapid shutdown system is on the critical loads, then the panels can charge the batteries. This will allow the battery to power the critical loads indefinitely.
Here is what to consider:
The power used by the rapid shutdown system is very low.
The following are nice to have on the critical loads.
You don’t need these, but they will make things more comfortable during a grid outage.Garage door opener (if you have one)
Here is why it is nice to have:If you enter and leave your home through the garage, this will help.
Here is what to consider:
A 1/2 horsepower garage door opener uses about 373 watts when running steady-state. It only runs for about 10 seconds at a time. When it starts up, it uses about 1,400 watts for a brief moment.
Internet access equipment
Here is why it is nice to have:It is good to have access to the internet for news and information. If you don’t want to use your cellphone as a Wi-Fi hotspot, the internet access equipment is good to have.
Here is what to consider:
The internet access equipment uses a tiny amount of power (about 6 watts).
Bathroom outlets
Here is why it is nice to have:It helps if someone in your house needs to use a hairdryer. It also lets you plug in a portable lamp for lighting.
Here is what to consider:
The average hairdryer uses about 1,500 watts. It is a continuous load.
At least one outlet in each room that is not already covered
Here is why it is nice to have:They provide convenient places to charge laptops and cell phones. They also allow you to plug in low-wattage LED lamps. These will provide comfort and convenience for a tiny amount of power usage.
Here is what to consider:
If outlets in the room are already going to be on the critical loads, you don’t need to add another. Also, some circuits include outlets in more than one room.
More to consider:
Having these circuits on your critical loads will affect your solar power system. Here are some of the components that they affect:Inverter
The inverter should be powerful enough to handle the continuous loads. It should also be able to handle the transient loads. Now, many of the high loads on the critical loads are transient, so this helps. You should add up the continuous and transient loads separately. You should then compare each of them to the inverter’s capabilities. If in doubt, oversize your inverter.
Batteries
The batteries should be able to discharge at the rate necessary to power the inverter. Otherwise, they will become the limiting factor. Also, this discharge rate should account for any inefficiencies in the inverter.
The batteries should also store enough energy to power the critical loads for at least 18 hours. You can use your electric bill to help with this. You start by looking at the electric bill from a high usage month. Divide the total kilowatt-hours in the month by the number of days in the month. This will give you the kilowatt-hours per day. You multiply this by 75 and divide it by the percentage efficiency of the inverter.
KWh/day = KWh in the month/Days in the month
Battery bank size = KWh/day x 75/Inverter’s percentage efficiency
This should give you enough battery capacity to last over 18 hours. It’s because your critical loads will use less electricity than the entire house. But if you are in doubt, then oversize your battery bank.
Solar Array
You should size this for the entire house, which will demand more than the critical loads. You can start by dividing the KWh/day by the number of Sun hours (based on location). But then you will need to account for solar panel orientation and shading. You will also need to account for inverter efficiency. So if you are in doubt, then oversize your solar array. Also, a larger array means quicker recharging of the battery bank.
Here is a sample house that uses about 14 KWh/day and gets about 6 sun hours of sunshine per day on average. Here are the items on the critical loads:
| Equipment | Continuous Wattage | Peak Wattage |
|---|---|---|
| 2 ton air conditioner with a hard start kit | 3,500 | 7,200 |
| Refrigerator | 300 | 1,500 |
| 2 kitchen outlets to share between a microwave oven, 2 hot plates, a toaster oven, and a crockpot (Use the 2 highest wattages.) | 1,200 | 1,200 |
| Rapid shutdown system (estimated) | 10 | 10 |
| Garage door opener | 373 | 1,400 |
| 2 bathroom outlets used for rechargable toothbrush and a LED lamp (No hair dryers) | 12 | 12 |
| Outlet for internet router and LED lamp | 12 | 12 |
| 3 more outlets for LED lamps | 18 | 18 |
| Totals | 5,425 | 11,352 |
So, the inverter needs to be powerful enough to supply 5,425 watts continuous and 11,352 watts peak power.
Let’s assume that the inverter that satisfies this need has a 95% efficiency. The battery bank should be able to discharge at over 5,710 watts continuous and over 11,949 watts peak. It should also be able to store over 11.05 KWh of energy.
The solar should be able to deliver no less than 2,456 watts. This assumes optimum orientation and no shading. Most likely, it will need more.
In summary, the critical loads contain the things you need to remain powered during a grid outage. These are the things needed to make your home habitable and comfortable. They include the things you should have and the things that are nice to have on the critical loads.
These are things that you should have on the critical loads:
Electrical components of the house’s climate control
Refrigerator and freezer
The means to cook food (if electric)
The rapid shutdown system (if you have one)
These things are nice to have on the critical loads:
Garage door opener (if you have one)
Internet access equipment
Bathroom outlets
At least one outlet in each room that is not already covered
What you have on your critical loads will influence your solar power components. Your hybrid inverter should be able to handle all continuous and transient loads. Also, size your battery bank to provide at least 18 hours of power to your critical loads. It should also be able to discharge at a rate that meets the inverter’s demands. You should already size your solar array for your entire house. So it should be more than enough for your critical loads panel. But if you have any doubts about any of these components, then oversize it. Otherwise, they keep your house comfortable even during a grid power outage.