Battery Sizing

Going with a heat pump is a good move, especially if your furnace is electric. Key is avoiding the higher rate brackets for your house.

Be careful about adding batteries as something like the Tesla Powerwall is a real fire hazard and when this type of battery bank catches fire it puts out toxic fumes. Best to install it outside the building enclosure and not attached to a wall of the house or the garage.

Some inverters, as with the 5G Solis inverters are designed to work with battery banks. For most inverters the linkage is a bit more complicated and you need to spec the entire new system and how it will get wired together properly and to code. The electrician hours and charges is considerable and get a quote before making your final decision.

I have a natural gas powered standby generator that I installed instead of batteries as the cost was the same and the batteries could power my house for at most 2 days whereas the generator can run indefinitely. I had a power outage last month that lasted for 3 days and one the prior year that lasted for 6 days. Avoiding lost food in the fridge and the freezer and not having to spend nights at a motel and eat in restaurants for several days was part of the consideration in chosing a standby generator over a battery bank.

Battery costs are decreasing so I would look at how to have 24 hours of power and being able to later increase the capacity of the bank with minimal effort.

I used a Sense and an Emporia energy monitor to determine what my peaks and baselines were over a given period of time. That was helpful in designing my solar and batteries. I found that my 5 ton AC/Heat Pump was spiking 80 amps on startup, which would be difficult to find batteries, or generator for that matter, to handle. I went with a Soft Start device that allows the fan to start first, then a few seconds later kicks on the compressor. It reduced the spike down to high 40's amps, significant difference. The $350 device + a few hours to install saved me having to buy several more batteries or a far larger generator ($$thousands)

Your average and peak watts would determine how many batteries you need to run your house effectively. The number of batteries affects length of run, and ability to handle spike wattage. I have four PW+, so I can peak around 20k watts total. In the event I need more than that, it would attempt to pull from the grid to offset the difference. What if the grid was offline? Well, not sure what would happen, but in those situations I alter my loads (turn on and off) to ensure I don't go over that and to extend the length of time I can run. I'd only come close if my AC, fridge, water heater, stove and a few more heavy hitters started up around the same time. That's where you can get an idea of those numbers from Sense or Emporia.

I don't think there is any ROI on batteries, in my case, I bought them for peace of mind.

Hope this helps!

Could you please clarify why you needed both the Sense and Emporia energy monitors? Was Emporia monitor alone not sufficient to find the peak and baseline load?

When we had our Generac system designed, we opted for the full 18kW battery backup instead of an external propane generator. Like you, we replaced our oil burning technology with a Mitsubishi electric heat-pump for HVAC and separate heat-pump hot water. We went online in May of last year. Our system can handle a total of 60amps of power on the fail-over, and we have the HVAC, refrigerators, and a few outlets on that circuit. We only had one power outage since the installation and it only lasted a few minutes. The system worked as designed.

That said, our inverter offers the option for "Self Supply" where the house will pull power from the solar array first, then the batteries after sunset, and when the batteries reach a low-charge threshold (between 20 and 50% depending on the time of year - I am able to adjust this setting), it then swaps over to grid power. Last summer, from the end of June through the first week of August. our home, with the whole-house AC running, never pulled from the grid at night. The nights were short enough that just as the batteries reached low charge, the morning sun would begin to hit the panels and provide power to both the house and to start recharging the batteries.

AC is one thing, and actually much more economical than I had expected, but holy cow... the ELECTRIC HEAT this winter was a killer. That sucked down our power credits in no time.

Where you live will make all the difference. The amount of daylight you get in San Diego should more or less keep you comfortable and off grid.