Willem Post: Grid-connected and off-the-grid houses – a comparison

Editor’s note: This commentary is by Willem Post, a retired engineer, who now writes about energy issues, currently specializing in energy efficiency of buildings and building systems. He is a founding member of the Coalition for Energy Solutions.

This article describes two energy, CO2 emission, and cost reduction alternatives. The first alternative is having a standard, code-designed house to which is added a grid-connected PV solar system with sufficient capacity to charge a plug-in vehicle. The second alternative is having a very energy-efficient house to which is added a PV solar system with sufficient capacity to charge a plug-in vehicle, plus an electrical energy storage system and a thermal energy storage system.

Standard House, With Grid-connected PV Solar System and Plug-in Vehicle

An average standard house uses about 6,000 kWh/yr and one plug-in vehicle consumes about 12,000 mi x 0.30 kWh/mi = 3,600 kWh/yr. In New England, the PV solar system capacity needs to be about 10 kW to produce 10 kW x 8,760 hr/yr x 0.14 = 12,264 kWh/yr. It would produce energy during the day and feed any excess into the grid, to be withdrawn at night to charge one or two plug-in vehicles.

Investments and Energy Cost Savings: The cost of the PV system would be about $40,000 less subsidies. Bills for electricity and gasoline would be minimal, but bills for space heating and domestic hot water, DHW, about $3,000/yr. (about $4,000 before tax), would remain.

Energy Efficient House, Off the Grid, With PV Solar System and Plug-in Vehicle

The off the grid concept can readily be applied to freestanding houses, or housing developments; the latter could have PV solar systems on each building roof, or have a parking area with a roof covered with PV solar panels. Bills for electricity, gasoline, space heating and DHW would be minimal. Here is how this would work for a freestanding house.

Investments and Energy Cost Savings: An absorbed glass mat, AGM, battery system costs about $200/100 Ah. A 4,000 Ah system, sufficient for about six days, would cost about $8,000. A PV solar system costs about $4,000/kW of panels. An 8 kW system would cost about $32,000 less subsidies. On the grid, in a standard, code-designed house, no PV solar system, bills for electricity $1,200, space heating + DHW $3,000, and gasoline $1,500, would total about $5,700/yr. ($7,500 before tax). Off the grid, in an energy-efficient house, they would be minimal.

Off-The-Grid: My starting point is a freestanding house, similar to a Passivhaus, NOT grid-connected, with passive solar features, and using about 80% less energy per square foot for heating, cooling, and electricity than a standard, code-designed house.

In winter it would be challenging, as several days may pass with near-zero electrical and thermal energy generation. About a week’s consumption of electrical energy and domestic hot water storage would be required in less sunny areas, such as New England.

For living off the grid, in a near-zero-CO2 mode, the house would need to be equipped with:

• A roof-mounted, PV solar system + a 12 V, AGM battery system, with charge controller, wired for 12, 24, or 48 V output + a hot water storage tank with DC electric heater + a system with DC pump and water-to-air heat exchanger.

• A gasoline-powered, 2 – 4 kW AC generator with 50-gallon fuel tank to periodically charge the batteries to about 90%, in case of too little PV solar energy during winter, due to fog, ice, snow, clouds, etc.

• Any excess electricity would bypass the already full batteries and go to the electric heater in the DHW tank. Any excess thermal energy would be exhausted from the DHW tank to the outdoors.

• A whole house duct system to supply and return warm and cool air, with an air-to-air heat exchanger to take in fresh, filtered air and exhaust stale air at a minimum of 0.5 air changes per hour, ACH, per HVAC code.

• For space cooling, a small capacity, high-efficiency AC unit would be required on only the warmest days, as the house will warm up very slowly.

• For space heating, a DC electric heater, about 1.5 kW (about the same capacity as a hairdryer) for a 2,000 square foot house, in the air supply duct, would be required on only the coldest days.

• A plug-in EV, such as a Nissan, or plug-in hybrid, such as a Chevy Volt, would be charged with DC energy from the house batteries by bypassing the vehicle AC to DC converter, provided the house batteries have adequate remaining storage energy, kWh, for other electricity usages. During some winter days, this may not be feasible, as not enough PV solar energy would be available; public chargers would be needed.

Household Energy Management: To determine the capacity of the energy systems, list all the energy users on a spreadsheet, how much they use (amp-hours/day) and what time periods they are on and off. The sum will give the hour-to-hour energy consumption per day, or per week. Subtract the hour-to-hour PV energy generation to yield the hour-to-hour surplus (charges the batteries) or deficit (discharges the batteries). Energy consuming items can be scheduled on and off to manage the energy flows. If there is a prolonged period of no sun, the engine-generator supplies the energy. Having as many DC devices as possible reduces DC-AC conversion losses.

NOTE: If an EV travels 12,000 m/yr. at 0.30 kWh/mile, about 3,600 kWh/yr. would be required, equivalent to the production of a 3 kW PV solar system in New England. Gasoline cost avoided = 12,000 mi/yr. x 1 gal/28 mi x $3.50/gal = $1,500/yr.

NOTE: Because PV solar systems have become much less costly, it would be less complicated and lower in O&M costs to increase the capacity of the PV solar system to also provide electricity for DHW, instead of having an $8,000 roof-mounted solar thermal system for DHW; no tube leaks, freeze-ups, less moving parts, etc. With a properly insulated, large capacity DHW tank, say 250+ gallons, there would be enough DHW for 5 – 7 days.

NOTE: A maximum of about 70% of battery nameplate rating is available. To prolong the useful service life well beyond eight years, batteries should typically be charged to a maximum of 90% and discharged to not less than 70%; shallow cycling. Very rarely should they be discharged to a minimum of 20%; deep cycling reduces life. Also, life is prolonged if charging and especially discharging is slow; a few amps for many hours is much better than many amps for a few hours.

NOTE: Battery charging loss is about 10% and discharging loss is about 10%, i.e., in 100 kWh, store 90 kWh, out 81 kWh. Inverter DC to AC efficiency, low at low outputs, increases to about 90% at rated output (at which it almost never operates); i.e., using multiple inverters and minimizing DC to AC conversion by using DC devices (fans, pumps, heaters, etc.) avoids losses.

Example of required battery capacity = 10 kWh/d x 6 d x 1.4 DOD factor x 1.2 loss factor = 100.8 kWh, or (1000 x 100.8) Wh/24 V system = 4,200 Ah.

House low energy usage = 0.5 kW x 1 h x 1/0.5 inverter eff x 1/0.9, battery loss = 1.11 kWh from battery, or (1000 x 1.11) Wh/24 V = 46.3 Ah

House high energy usage = 2.0 kW x 1 h x 1/0.8 x 1/0.9 = 2.78 kWh from battery, or (1000 x 2.78) Wh/24 V = 115.7 Ah

PV solar energy to battery; overcast winter day = 8 kW x 4 h x 0.16 CF x 0.9 battery loss = 4.61 kWh; generator is needed for a few hours.

PV solar energy to battery, sunny summer day = 8 kW x 6 h x 0.70 CF x 0.9 battery loss = 30.24 kWh; excess energy for charging plug-in.

NOTE: As space heating and cooling would be required for just a few days of the year, an air-source heat pump would be overkill and too expensive in this case.

NOTE: The PV solar system needs to be oversized to ensure adequate electrical and thermal energy during winter when the monthly minimum winter irradiance is about 1/4 – 1/6 of the monthly maximum summer irradiance. See below URL of monthly output from two monitored solar systems in Munich; 1/6 is about right in South Germany.

NOTE: Whereas, the daily or weekly maximum solar output of Germany may be up to 60% of installed capacity, kW, during a very sunny period, it may be near zero, due to fog, ice, snow, clouds, etc. As a result, Germany’s mix of PV solar systems (old and new, dusty or not, partially shaded or not, facing true south or not, correctly angled or not) has a low nationwide capacity factor of about 0.10. This compares with a New England CF of about 0.12; the theoretical CFs are about 0.12 for Germany, about 0.143 for New England.

NOTE: The above example shows to provide off-the-grid standard (energy-hog) houses with PV solar systems and electrical and thermal energy storage, they would need to be of such large capacity the costs would be prohibitive, if “zero-energy and near-zero CO2” is the goal.

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  • Carl Marcinkowski

    I appreciate reading these factors. Thanks to Willem Post and Vtdigger for making this available this morning.

  • William, thanks for the post. Appreciate the information and the link you provided above, all a great read!

  • Matt Fisken

    Willem lays out some ideas here, but it’s hard to understand what the conclusion is. As a numbers guy, he likes to crunch them, but often misses the big picture.

    What percentage of Vermonters will ever be in a position of building a home from the ground up with tens of thousands of add-on tech which may or may not provide a pay back?

    Does anyone in Vermont currently use an off-grid PV system to run a 1.5 kW DC water heater, or any such high wattage heater for that matter? The long-standing rule is that you don’t use batteries (or DC straight from panels) to generate heat.

    I know Willem chooses not to burn wood in his home for personal reasons, but I think any discussion of sustainable, low-carbon, off-the-grid living in Vermont must consider that in a modestly sized, passive-style house (maximizing thermal mass and solar gain while minimizing expensive tech), burning wood in the colder months is the most logical way to supplement the sun and keep your living space comfortable. A new, 80% efficient woodstove that can be fueled by wood from your own or nearby land for many decades is the most economical and elegant alternative to coating one’s property with silicon, metal and glass.

  • Moshe Braner

    One rather large number not included in the examples is the cost of the electric car. In particular, the high cost of the vehicle’s batteries, which is due to embedded energy used to manufacture them. That, and the limited life (in years) of those batteries make an electric car a lot less “green” than it first seems. Just like Willem’s motto of “efficiency first, renewables second”, which I fully agree with, one should also apply a similar concept to transportation: reduce your needs first. Does the average person in Vermont really need to drive 15,000 miles a year? I drive about half of that, and I commute mostly by car, 8 miles each way. Similarly, the amount of domestic hot water needed can be greatly reduced by switching to, e.g., an efficient front-loading washing machine – and of course a low-flow showerhead.

    • Willem Post


      I also drive about 7000 miles per year, but the average for the US is much greater. Just google it.

  • Willem Post


    Both of you made good comments.

    Burning wood is about as bad as burning coal regarding CO2 and particulates. It takes about 100 years of bio growth to offset the CO2 from burning wood. Just google it. The more we can do with PV solar, the better.

    In Germany, the Passivhaus cost adder is 8 – 10 percent, added to that is the PV solar system and battery system.

    If a person lives in a house for 30 years, then his REAL energy costs 30 years from now will be about 1.025 to the power 30 = 2.1 times present costs.

    I doubt real household incomes will increase by that much, as they have decreased for the past 14 years!!

    Here are the REAL (inflation-adjusted) MEAN household income declines of the bottom 60% of US households:

    3rd quintile -8.4% since peak year in 2000
    4th quintile -11.1% since peak year in 2000
    5th quintile -15.9% since peak year in 1999

    Note: Taxes are excluded from the cost of living index.

    This website also shows how the top 5% of households has faired so much better than the rest.

    I am well aware EVs are not ready for prime time, but they will be eventually.

    But Passivhaus level construction IS ready for prime time, as all technologies have been developed and proven over the past 30 years!! The state should just donate a 10 kW system to any household building a new Passivhaus, plus using a plug-in EV or hybrid; we shall call them “Pioneer Households”, with an official badge of honor on the front door.

    It would be so much less costly than subsidizing RE a la SPEED, etc.

    It is best to start such construction now, and when batteries and EVs are ready for prime, all will fall in place.