# Net zero is not expensive?

| Posted in Mechanicals on

I’m having a home built and am trying to go net zero with PV and an all electric home. The solar guy and some contractors are very surprised when I say it can be done with 4 kw of panels and fairly standard construction and heating system. I’d like to run my numbers by the wise advisers here and you can tell me if I’m wrong.

The new home will be about 2200 SF, conditioned crawlspace, sun tempered. 24000 btu/h heat load. Sunny colorado, zone 5B, 7000 heating degree days. Heating will be by heat pump forced air, details TBD). HPWH will be in suntempered insulated shop, possibly ducted cold exhaust to garage. For comparison, current home is 2000 SF + basement, slightly sun tempered, insulation and windows about what you’d expect in a 22 yr old tract home, gas furnace & WH.

I’ve put together a model in energyplus and am getting pretty similar numbers to what I’ve come up with in a simple spreadsheet based on current usage.
Current annual usage:
Elec: 3650 kwh/y ( I assume no change here even tho we will have new appliances)
Ngas: 9317 kwh (318 therms)
Splitting up the Ngas usage into DWH and heating I estimate
DHW: 2000 kwh
Heating 7317 kwh
Assuming current water heater is 75% efficient and new HPWH will have a COP of 2.5
DHW: 600 kwh (2000*0.75/2.5)
Assuming current furnace is 75% efficient, new home will have heat loss and solar gain giving 30% reduction in heat load, and a 33% increase in heat load due to increased heating degree days for location change, and an effective COP of 1.75 for the heat pump. I’m basing this COP from a HSPF of 10 and a 40% reduction for the colder climate (http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-413-04/)
Heating: 2919 kwh (7317*0.75*0.7*1.33/1.75). Pretty close to what I get from the eplus model

Total, 7169 kwh (3650+600+2919)

Using nrel pvwatts, this means I need a 4.0 kw PV array.

I’m looking for feedback on any errors I may be making since I don’t see many net zero houses this size with an array this small.

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### Replies

1. | | #1

Brad- My only source of comparison is my own house in Maine, zone 6. About 1650 square feet of conditioned space. Heat load calculated at 12000 btu/h at 0F design temp.

Our 6.6kw solar array will produce about 8200 kwh for the year 10/15 to 9/16. CO is sunnier, but your projected output seems high.

Our use will be about 10,000 kwh, but we have an electric resistance water heater and a hot tub. without them, we'd be at about 8,000.

2. GBA Editor
| | #2

It's possible to design and build a net-zero-energy house that requires only a 4-kW PV array.

In fact, the first net-zero house in the U.S. (that is, the first to document 12 months of net-zero energy use), the Habitat for Humanity house in Wheat Ridge, Colorado, has a 4-kW PV array. (That milestone was reached at the end of September 2005.)

The Wheat Ridge house measures 1,196 square feet.

3. | | #3

Thanks,
Stephen, my projected output came from nrel's PVwatts online calculator.
Martin, thanks for reminding me of that house, I read about it a while back & just reread some articles on it. That interesting, but it isn't a good apple-apples comparison since it is half the size, they have solar thermal for DHW and gas furnace for heat. For today's tech & costs it doesn't seem like a good cost-effective design, but maybe it made sense back then. 4 KW seems like a lot of panel with the solar DHW, but they did have a surplus, and probably have some shading being in the city.

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