Typical last mile battery runtime (protecting against power cuts)

John van Oppen john at vanoppen.com
Sun Feb 5 07:32:02 UTC 2023

20 KW should easily cover the 9KW you could max draw with your strip heat.   It is super uncommon to have even peak loads over 20 KW in a house.   Even your peak day was only an average of 6 KW.

You might need some load shedding just to keep the big stuff from coming on all at once but that is pretty easy.   If you have instant hot water that also could be a problem those are huge, typically 15-20 KW by themselves. 

-----Original Message-----
From: NANOG <nanog-bounces+john=vanoppen.com at nanog.org> On Behalf Of Roy
Sent: Saturday, February 4, 2023 10:56 PM
To: Mark Tinka <mark at tinka.africa>; nanog at nanog.org
Subject: Re: Typical last mile battery runtime (protecting against power cuts)

On 2/4/2023 9:31 PM, Mark Tinka wrote:
> On 2/5/23 07:02, Roy wrote:
>> My all electric house is in a rural area.  The generator that came 
>> with the place is a 20KW Onan,  The bad news is in can't handle the 
>> house.  I think it is the Aux Heat on the heat pump that is the 
>> problem.  I have to also power the well pump and the septic pump.
> Is your house single or 3-phase?

Single phase.  The house is 200A service and the barn is another 200A service

> I'd be curious how much horsepower your well and septic pumps require. 
> The most I've seen is 15hp @ 11kW, but that is pretty massive for an 
> average home, even an off-grid one. Typical requirements would be in 
> 0.75kW - 5kW range, which is a wide range.
> Do you know how much power the heat pump requires?

I don't know how much the pumps require.  The water well is about 100 feet from the house and the pressure tank.

The septic pump has to pump uphill to the drainage field.  Distance is about 250 feet and elevation gain of 100 feet or so.

The heat pump doesn't seem to be a problem but the aux heat is on two 20amp 220v circuits.   There is a switch on the fan enclosure to disable the aux heat.

Another biggie is the electric hot water heater.

On 1/30 it never broke 32 degrees and the house used 145KWHR (average was 6KWH).  Thank goodness I am not far from the Columbia River and the BPA has a major substation about 5 miles away so I pay less than 10 cents per KWH

Over 2022, I lost power about 8 times.  The longest outage was 15 hours.

> I'd struggle to see how a 20kW generator struggles to to run a home, 
> unless you've also got heated floors, saunas, steam baths, water and 
> space heaters, electric stoves and ovens all running at the same time 
> :-).
> Mark.

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