I wanted to share my thinking on solar/battery power.
Use Case:
Our power is very reliable with only occasional outages usually lasting only a few hours. This, in itself would not require any backup power. However, we are in an area subject to "Public Safety" power shut offs which can last for days and our area occasionally has multi day outages.
So, I want a battery backup system that can last multiple days and power essential services. I don't want or need to power everything but I want to remain comfortable.
We have solar and net metering with a flat rate so no opportunity for arbitrage (currently... that may change in the future).
The "payback" for this system is "security", not financial. I'm willing to invest what is necessary to provide energy security but don't see any return in oversizing the system.
What size system?
I want to be able to power essential loads (heat, lighting, cooking, house monitoring, communication) for multiple days.
We need to consider both the power (kW) and energy (kWh/day) requirements of our loads.
Lights, house monitoring, Internet, computers and other conveniences.
Fortunately, these do not take much power or energy. From my home energy use monitoring, it appears that all of these loads together use less than 1 kW of power and only about 15 kWh per day.
Heating and hot water. (No A/C required where we live)
We have a heat pump which heats half the house (hydronic) and provides hot water. This draws about 2 kW when running and ranges from 2 kWh in summer (hot water only) to a maximum of 20 kWh in winter.
The rest of the house is heated with a forced air gas furnace. 0.5 kW when running and up to 3 kWh in the winter.
Cooking.
Gas stove.
Hot tub.
Non-essential. It can go for days even in winter without energy use.
Tesla.
This doesn't need to be charged during a power outage. It just doesn't make sense to charge a large car battery from a small home backup battery. If I have not planned ahead, I can drive to someplace there is power and charge. If the Tesla battery is depleted, I can charge at a very low rate (2 kW) for a few hours to get the car drivable.
Solar.
My 9kW solar system generates about 70 kWh/day in summer peak and as low as 30 kWh/day kWh winter. (Cloudy days will decrease both these numbers.)
If I add up the power, I get a maximum load of 3.5 kW for essential loads.
For energy, I can get by with 17 kWh in Summer (probably less) and will need up to 37 kWh/day in winter.
So, my solar can provide all the power I need for extended outages during most of the year; falling short only it the darkest days of winter when we will need to turn the thermostat down (or, just turn off the hydronic heating and let the gas furnace take over).
So, what size Powerwall?
It seems that a single powerwall will handle both the maximum power (3.5 kW) and energy (running on solar during the day, charging the battery for use overnight... a 10-15 kWh battery) should be sufficient.
Footnote on Tesla Powerwall:
I placed a deposit a year ago with Tesla for Powerwalls. They have been silent since. I have called them several times and they give only vague statements "someday... maybe". I've given up on them.
I'm looking at the Outback Skybox which seems to be roughly equivalent to the Powerwall and seems to have much nicer (more flexible) software. Each puts out 5 kW and can be configured to use solar and also supplement grid power from the battery (useful for TOU). Up to 10 units can be chained together if my needs change. I'll need to provide batteries and fortunately have a brother in law who works for a battery company and can get me a good price on LiFePO4 batteries (safer and more durable). I can also install this system myself (with help from my electrician friend). Total cost should be much less than the Tesla Powerwall.
Any comments?
Use Case:
Our power is very reliable with only occasional outages usually lasting only a few hours. This, in itself would not require any backup power. However, we are in an area subject to "Public Safety" power shut offs which can last for days and our area occasionally has multi day outages.
So, I want a battery backup system that can last multiple days and power essential services. I don't want or need to power everything but I want to remain comfortable.
We have solar and net metering with a flat rate so no opportunity for arbitrage (currently... that may change in the future).
The "payback" for this system is "security", not financial. I'm willing to invest what is necessary to provide energy security but don't see any return in oversizing the system.
What size system?
I want to be able to power essential loads (heat, lighting, cooking, house monitoring, communication) for multiple days.
We need to consider both the power (kW) and energy (kWh/day) requirements of our loads.
Lights, house monitoring, Internet, computers and other conveniences.
Fortunately, these do not take much power or energy. From my home energy use monitoring, it appears that all of these loads together use less than 1 kW of power and only about 15 kWh per day.
Heating and hot water. (No A/C required where we live)
We have a heat pump which heats half the house (hydronic) and provides hot water. This draws about 2 kW when running and ranges from 2 kWh in summer (hot water only) to a maximum of 20 kWh in winter.
The rest of the house is heated with a forced air gas furnace. 0.5 kW when running and up to 3 kWh in the winter.
Cooking.
Gas stove.
Hot tub.
Non-essential. It can go for days even in winter without energy use.
Tesla.
This doesn't need to be charged during a power outage. It just doesn't make sense to charge a large car battery from a small home backup battery. If I have not planned ahead, I can drive to someplace there is power and charge. If the Tesla battery is depleted, I can charge at a very low rate (2 kW) for a few hours to get the car drivable.
Solar.
My 9kW solar system generates about 70 kWh/day in summer peak and as low as 30 kWh/day kWh winter. (Cloudy days will decrease both these numbers.)
If I add up the power, I get a maximum load of 3.5 kW for essential loads.
For energy, I can get by with 17 kWh in Summer (probably less) and will need up to 37 kWh/day in winter.
So, my solar can provide all the power I need for extended outages during most of the year; falling short only it the darkest days of winter when we will need to turn the thermostat down (or, just turn off the hydronic heating and let the gas furnace take over).
So, what size Powerwall?
It seems that a single powerwall will handle both the maximum power (3.5 kW) and energy (running on solar during the day, charging the battery for use overnight... a 10-15 kWh battery) should be sufficient.
Footnote on Tesla Powerwall:
I placed a deposit a year ago with Tesla for Powerwalls. They have been silent since. I have called them several times and they give only vague statements "someday... maybe". I've given up on them.
I'm looking at the Outback Skybox which seems to be roughly equivalent to the Powerwall and seems to have much nicer (more flexible) software. Each puts out 5 kW and can be configured to use solar and also supplement grid power from the battery (useful for TOU). Up to 10 units can be chained together if my needs change. I'll need to provide batteries and fortunately have a brother in law who works for a battery company and can get me a good price on LiFePO4 batteries (safer and more durable). I can also install this system myself (with help from my electrician friend). Total cost should be much less than the Tesla Powerwall.
Any comments?