Hi,

Over in the PriusChat "Technical" forum we recently discussed using an inverter in a 2001-03 Prius versus a Honda generator. Putting a 12VDC inverter in an NHW11 Prius can give up to 1kW, about 9 A. of AC power, enough to go through a power outage and 'camp out at home.' This led to a survey of Honda generator specifications looking at their efficiency including the 1kW Prius hack:

1. 6.1kW hr/gal@5000W, 5.4hrs - EX5500K2A
2. 5.9kW hr/gal@2800W, 7.2hrs - EU3000isA
3. 5.8kW hr/gal@1600W, 4hrs - EU2000iA
4. 5.7kW hr/gal@900W, 3.8hrs - EU1000iA2
5. 5.6kW hr/gal@4000W, 7hrs - EX4500SXK1A
6. 5.4kW hr/gal@400W, 15hrs - EU2000iA
7. 4.4kW hr/gal@2500W, 7.9hrs - EX5500K2A
8. 4.2kW hr/gal@2000W, 10.5hrs - EX4500SXK1A
9. 4.1kW hr/gal@700W, 20hrs - EU3000isA
10. 4.0kW hr/gal@1000W, 44hrs - Prius inverter
11. 3.1kW hr/gal@225W, 8.3hrs - EU1000iA2

Source for Honda generator performance:
http://bagleytractor.com/honda/gen/quiet/quiet_spcs.htm

Unfortunately, sorting can make it difficult to match the full and partial power values for the five generators. The chart below has connector lines between the full and partial power values for each generator and labeled for each. We can now plot the efficiency, the kWh per gallon, for these generators and include an indication of how long they run before refueling:

From this we can conclude:

• dedicated generators can be 50% more efficient - 6kWhr vs 4kWhr per gallon
• many generators at partial power match Prius efficiency, 4kWhr per gallon
• The Prius run time, 44 hrs, is more than twice that of the next unit that is also at ~4kWhr per gallon
• Prius efficiency was measured in 30-40F temperatures, much lower than a standard day temperature

Recently, I began looking at inverters and found 1kW units run around $99-$130. The wiring and mounting cost an additional $20. This brings the Prius installation cost up to ~$150. The least expensive, standalone Honda generator, eu1000, runs about $350 (Ebay listing 'Buy it now'.)

If a Honda generator were equipped with a continuous refueling system, the more efficient ones would only need ~7 gallons to match the 11 gallons needed by the Prius. This approach would also have the convenience, the constant power, so the owner doesn't have to respond to 'out of gas' by going outside to refuel the unit. I notice some Honda units have a power sync option so two generators could work like one. This might provide a way to stagger refueling.

Note, 11 gallons is the usable fuel from a Prius but I have put in 11.2-11.6 gallons in the past. A runtime of 44 hours presumes filling up the tank prior to the potential weather showing up.

For partial power, three generators use 25% load and the two others use 50% load. The Prius, no power, fuel burn is 0.06 gal./hr., which gives a maximum run time of 183 hours, just over a week. If we split the differences for run time and power, we get 500W over 103.5 hours, 51.7kW hr. For 11 gallons this would be 4.7 kW hr/gallon but that would be a calculated, not a measured efficiency . . . a paper exercise. At these low values, very small errors can multiply.

A Prius warranty can easily be worth several thousands of dollars, so it makes sense to be aware of the risks an inverter poses to a Prius, 12VDC system. Integration of an inverter has to be done carefully to avoid blowing the 100A fusible link, a repair that could cost the price of a low-end Honda generator.

Inverters have an under voltage protection system that trips it off if the battery voltage is too low, typically 10 VDC. At 90A, my Prius inverter voltage had dropped to 11.5 VDC. A 0.0166 ohm shunt resistor would be enough to drop the additional 1.5 volts and lead to inverter shutdown but at the cost of a steady power loss, ~100W at 70A. This limits the inverter power to 900W.

A better solution would be an 80A circuit breaker, common in RV electronics that will trip out well before the 100A fusible link is at risk. This allows using an over-sized inverter and makes possible the addition of a surge energy source, a lawn tractor or motorcycle battery. Also, oversized inverters often have higher efficiencies than smaller units operating at their power limits.

A Prius using 12VDC inverter is not a universal solution and power limited to 1kW. It has to be driven to where the power is needed and parked with the keys running. A generator can be loaned and when generating power, the Prius can be driven off for other errands . . . like fetching gas for the generator. But a Prius power, 12V inverter is better than cursing the darkness.

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
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